Abstract The flotation method of ultra-fine bubbles (UFB) aims to address pollution and has been used for combating the undesirable water conditions of contaminated soils. Hence, water containing UFB is gaining increasing attention for potential agricultural applications. Although certain hypotheses have been proposed, such as the collection of ions in water through the electrical characteristic of UFB, no clear experimental data have been provided. We found that improvement in turbidity may cause the adsorption of fine soil particles in the water by the UFB, thereby improving the quality of the water. The data from the paddy field showed that a decrease in turbidity (below 2 nephelometric turbidity units) occurred over a short period of time (3 days). UFB concentration is directly related to turbidity with a coefficient of determination of 0.93. This phenomenon was also observed through the distribution of bubbles and soil particles, where the average particle size increased because of the aggregation of soil particles and the decrease in turbidity in the paddy field, indicating that UFB collect soil particles and thereby improve water quality. Therefore, UFB are highly effective in cleaning rice field water and will be a preferred method for purifying the environment in the future.

Abstract The extensive and intensive use of herbicides has resulted in the spread of herbicide-resistant weeds in many crop production systems; therefore, it is imperative to devise new organic weed control methods. Recently, the application of spent coffee grounds (SCG) in agricultural fields has been found to inhibit plant growth and germination and is thus considered a potentially effective weed control measure. This study aimed to evaluate the effects of different amounts and methods of SCG application on weed growth through field experiments. The field experiments were conducted in an upland field converted from a paddy in western Japan. The results show that the plow-in application of over 10 kg m⁻² of SCG and mulching application of 20 kg m⁻² decreased the weed dry weight compared with the control. In addition, the growth of weed species of families other than Gramineae, such as wingleaf primrose-willow and horseweed, was not significantly affected by SCG application. Weed species of families other than Gramineae are dominant in some upland fields. Hence, the inhibitory effect of SCG on weeds may be lower in original upland fields than in the upland field converted from paddy field that was investigated in the present study. Overall, this study demonstrated that the plow-in application of 10 kg m⁻² of SCG every 4 mo was effective for weed control in an upland field converted from a paddy field. Because SCG worked against grass weeds under the specific conditions in this study, it would be valuable to explore other potential applications of this novel means of weed control.

The production of pearl millet (Pennisetum glaucum (L.) R.Br.) is important in Namibia, in sub-Saharan Africa, owing to the prevailing low precipitation conditions. Most fields supporting crop production in northern Namibia are located in a network of seasonal wetlands. The aim of the present study was to evaluate the effects of ridging and fertilizer application on the yield and the growth of pearl millet in the seasonal wetlands under different rainfall conditions. The study was conducted for two years (2017–2018) in the experimental fields in northern Namibia, and yield, yield components, and growth parameters were evaluated in relation to the application of different fertilizers (manure and mineral) with and without ridge-furrows. Manure fertilizer application presented the highest yield in 2018, whereas mineral fertilizer application showed the highest yield in 2017. The proportion of rainfall was the highest during the mid-growth period in 2017, and the reproductive stage in 2018. Thus, pearl millet plants under manure fertilization overcame damage resulting from waterlogging stress during the seed setting stage by improving the soil and plant nutrient conditions. In contrast, the plants under mineral fertilization were more tolerant to large amounts of rain during the mid-growth period. In this study, yield was mainly determined by total dry weight, and it was closely related to panicle density in both years. Therefore, we concluded that fertilizer application, including additional fertilizer based on the growth diagnostic, could be important for improving crop production in seasonal wetlands.

We have investigated the effectiveness of a new soybean fertilization technique, named crack fertilization, which involves the application of nodule bacteria on biochar to soybean roots through cracks formed between planting rows during midterm tillage. In the present study, the factors and timing of crack fertilization were investigated in an upland field converted from a paddy. The variables investigated were: 1) crack formation without the application of any agricultural materials, and 2) the application of biochar or 3) nodule bacteria on biochar into cracks. The treatment periods were: 1) before sowing, 2) during midterm tillage, and 3) during both periods. The combination of crack fertilization and reduced tillage was also tested. The method of crack fertilization that increased yield was the combination of crack formation and the application of biochar, and the most effective period for the treatment was before sowing in the reduced tillage field. Seed yields in conventional and reduced tillage fields were comparable in the upland field converted from the paddy. These results suggest that the application of biochar into cracks after scratching the soil surface to remove weeds before sowing is a practical method of increasing soybean yield in upland fields converted from paddies.

In the cultivation of Citrus fruit, pruning in late winter after harvest is important to obtain excellent flower buds in the next spring and a rich harvest of superior fruit in the following winter; a large number of pruned leaves are usually discarded. The purpose of this study was to determine the utility value of pruned leaves of Citrus unshiu. A methanol extract of pruned C. unshiu leaves showed a porcine pancreatic lipase inhibitory activity. Fractionation of C. unshiu leaf extract (CUL-ext) followed by bibliographic and chromatographic analyses revealed that a part of the pancreatic lipase inhibitory activity of CUL-ext was attributable to nobiletin (1), rutin (2), and hesperidin (3). Thus, pruned C. unshiu leaves may be a reasonable natural resource for the preparation of ingredients with lipase inhibitory activity.

Flooding has globally become common, and thus cultivation techniques to adapt to the stress are required. We have developed a technique called “close mixed-planting” using flood-adapted rice (Oryza sativa L.) and upland crops with tangled root systems, showing that this technique can mitigate the damage caused to upland crops by flooding. However, the plant response during the reproductive stage has not yet been examined. In this study, we estimated the alleviative effects of close mixed-planting on the growth inhibition and physiological damage to pearl millet (Pennisetum glaucum) caused by flooding at the reproductive stage in northern Namibia. Pearl millet and NERICA4 (Oryza spp., interspecies of O. sativa and O. glaberrima) were used. Four-week-old single- and mixed-crop pearl millets were transplanted and grown for 22 days, and then flooding treatment was administered for 28 days. The growth and physiological parameters of single pearl millet were significantly decreased by flooding stress, and the parameters did not recover during a 14-day recovery period compared with those of unflooded single and mix-cropped pearl millet. On the other hand, the damage to mix-cropped pearl millet by flooding was suppressed. Thus, the mixed-cropping mitigated the anoxic stress of pearl millet caused by flooding at the reproductive stage and contributed to the improved growth after the recovery period. This result suggested that the close mixed-planting with rice can contribute to the mitigation of flooding damage not only at the vegetative stage but also at the reproductive stage of pearl millet in the semi-arid African country, Namibia.

The objectives of this study were to classify the small seasonal ponds that develop in the Cuvelai Seasonal Wetland System in north-central Namibia based on soil physicochemical properties so that potential agricultural uses of the ponds could be assessed. Satellite imagery was used to examine the relationships between soil characteristics and the probability of water presence in the region. Soil samples were collected from 66 ponds at three sites and their physicochemical properties and levels of salt accumulation were investigated. Soil data were analyzed by using principal component analysis and cluster analysis. The ponds were classified into four types based on soil properties. Type A accounted for 20% of the ponds and was characterized by high levels of clay, silt, total N, and exchangeable Ca, Mg, and K. Type B accounted for 14% of the ponds and was characterized by high values of organic C, C/N ratios, and available P. Type C accounted for 20% of the ponds, and this type featured high levels of salinity and sodicity. The remaining ponds were classified as Type D. If these seasonal ponds are to be considered for crop production, Type C should be avoided and Type D would require soil fertility improvements.

Aims: Rice is often cultivated in drought-prone regions causing growth inhibition. Therefore, we investigated whether close mixed planting of rice with pearl millet would mitigate these effects. Methods: We used pot and lysimeter to evaluate whether close mixed planting with pearl millet is more effective than single-stand planting in suppressing the growth inhibition of rice (NERICA4) under drought. Results: Close mixed planting only slightly alleviated the growth inhibition of rice under drought in the pot experiment and the first year lysimeter experiment, but mitigated this in the second year lysimeter experiment due to the lower levels of competition. Deuterated water applied from the bottom of the pot and lysimeter was present at a higher concentration in the xylem sap of rice under close mixed planting than under single-stand planting, and this tendency was enhanced by drought. Partitioning of the available water between the two crops was also observed under close mixed planting, with the rice and pearl millet depending on water from the soil surface and deep soil, respectively. Conclusions: Close mixed planting with pearl millet effectively mitigates the growth inhibition of rice under drought due to the increased access to deep water and the partitioning of available water between the two crops.

The authors have proposed the close mixed planting technique using mixed seedlings of two different crop species that results in close tangling of their root systems. Especially, the combination of drought-adaptive upland crops (e.g. pearl millet or sorghum) and flood-adaptive lowland crop of rice would be beneficial to overcome the drought and flood conditions and to reduce the risks of crop failure. In our previous studies, we found that upland crop yield losses by flood stress was mitigated by mix-cropped rice, owing to the oxygen gas released from the rice roots into the aqueous rhizosphere. In the present study, we conducted two experiments to assess whether mixed cropping a drought-resistant cereal, pearl millet, would improve the performance of co-growing drought-susceptible crop, rice under drought conditions. In the field experiment, some grains were obtained from the rice plants mix-cropped with pearl millet under drought condition. However, no rice matured in the single cropping system. In the model experiment using deuterium analysis, it was confirmed that water absorbed by pearl millet roots from deep soil layer was utilized by rice, suggesting that mix-cropped rice could withstand drought stress and complete grain filling using water released into the upper soil layer by hydraulic lift.

Broad scale monitoring of inland waters is essential to research on carbon and water cycles, and for application in the monitoring of disasters including floods and droughts on various spatial and temporal scales. Satellite remote sensing using spatiotemporal data fusion (STF) has recently attracted attention as a way of simultaneously describing spatial heterogeneity and tracking the temporal variability of inland waters. However, existing STF approaches have limitations in describing abrupt temporal changes, integrating "dissimilar" datasets (i.e., fusions between microwave and optical data), and compiling long-term, frequent STF datasets. To overcome these limitations, in this study we developed and evaluated a lookup table (LUT)-based STF, termed database unmixing (DBUX), using multiple types of satellite data (AMSR series, MODIS, and Landsat), and applied it to semi-arid seasonal wetlands in Namibia. The results show that DBUX is: 1) flexible in integrating optical data (MODIS or Landsat) with microwave (AMSR series) and seasonal (day of year) information; 2) able to generate long-term, frequent Landsat-like datasets; and 3) more reliable than an existing approach (spatial and temporal adaptive reflectance fusion model; STARFM) for tracking dynamic temporal variations in seasonal wetlands. Water maps retrieved from the resulting STF dataset for the wetlands had a 30-m spatial resolution and a temporal frequency of 1 or 2 days, and the dataset covered from 2002 to 2015. The time series water maps accurately described both seasonal and interannual changes in the wetlands, and could act as a basis for understanding the hydrological features of the region. Further studies are required to enable application of DBUX in other regions, and for other landscapes with different satellite sensor combinations.

In the Cuvelai Seasonal Wetland System (CSWS) of North-central Namibia, there are widespread manifestations of seasonally flooded river and seasonally closed wetland ecosystems (ponds). These wetlands are called oshana (seasonally flooded river wetland) and ondombe (seasonally closed wetland) according to the local language. This study was initiated to find out the soil fertility status of ondombes and whether they could be utilized for agricultural purposes unlike the present situation. Soil salinity and sodicity were determined to find out impact of such adverse conditions on possibility of food production. A total of 70 representative ondombes were identified from three selected villages. A total of 210 soil samples were collected from upper, middle and lower positions adjacent to ondombes, and 15 soil samples from each 5 upland fields in the three villages and 102 soil samples from different spots of the flood plain in the three oshanas for comparison. The results indicated that the mean soil pH (H2O) in ondombe was 6.3, the means of organic C and total N were 6.28 and 0.41 g kg-1; respectively, the mean of available P was 4.81 mg P kg-1. The means of exchangeable Ca, Mg, K, and Na in ondombe were 2.31, 1.44, 0.21, and 0.61 cmolc kg-1, respectively. Most soil nutrients were higher in lower ondombe positions than on upper and middle positions. Organic C, exchangeable Mg, and clay at the ondombe soils were significantly higher than those at the croplands. The means of electrical conductivity of saturation extract (ECe) and sodium adsorption ratio (SAR) in ondombe soils were 0.62 ds m-1 and 7.32, respectively; even though most of the ondombe soils did not exhibit salinity and sodicity problems. Hence, one can conclude that an ondombe soil has an appropriate condition for agriculture, and may only be prone to sodicity whenever the sodium content is high, as sometimes observed.

We investigated the origins of rain- and subsurface waters of north-central Namibia's seasonal wetlands, which are critical to the regions water and food security. The region includes the southern part of the Cuvelai system seasonal wetlands (CSSWs) of the Cuvelai Basin, a transboundary river basin covering southern Angola and northern Namibia. We analysed stable water isotopes (SWIs) of hydrogen (HDO) and oxygen (H2(18)O) in rainwater, surface water and shallow groundwater. Rainwater samples were collected during every rainfall event of the rainy season from October 2013 to April 2014. The isotopic ratios of HDO (delta D) and oxygen H218O (delta O-18) were analysed in each rainwater sample and then used to derive the annual mean value of (delta D, delta O-18) in precipitation weighted by each rainfall volume. Using delta diagrams (plotting delta D vs. delta O-18), we showed that the annual mean value was a good indicator for determining the origins of subsurface waters in the CSSWs. To confirm the origins of rainwater and to explain the variations in isotopic ratios, we conducted atmospheric water budget analysis using Tropical Rainfall Measuring Mission (TRMM) multi-satellite precipitation analysis (TMPA) data and ERA-Interim atmospheric reanalysis data. The results showed that around three-fourths of rainwater was derived from recycled water at local-regional scales. Satellite-observed outgoing longwave radiation (OLR) and complementary satellite data from MODerate-resolution Imaging Spectroradiometer (MODIS) and Advanced Microwave Scanning Radiometer (AMSR) series implied that the isotopic ratios in rainwater were affected by evaporation of raindrops falling from convective clouds. Consequently, integrated SWI analysis of rain-, surface and subsurface waters, together with the atmospheric water budget analysis, revealed that shallow groundwater of small wetlands in this region was very likely to be recharged from surface waters originating from local rainfall, which was temporarily pooled in small wetlands. This was also supported by tritium (H-3) counting of the current rain- and subsurface waters in the region. We highly recommend that shallow groundwater not be pumped intensively to conserve surface and subsurface waters, both of which are important water resources in the region.

Mixed cropping is a cultivation method widely practiced in tropical regions. The newly developed close mixed planting technique mitigates the flood stress of drought-adapted upland cereal species by co-growing rice (Oryza sativa) plants under field flood conditions. We tested the hypothesis that O-2 was transferred from rice to upland crops using the model system of hydroponic culture. To confirm the hypothesis, the phenomena of O-2 absorption and release by plants were evaluated in a water culture condition without soil. Experiments were conducted in a climate chamber to estimate the amount of O-2 released from the roots of rice and pearl millet (Pennisetum glaucum) under both O-2 rich (20.0 +/- .0% conc. in phase I) and O-2-free dark (.8 +/- .0% conc. in phase II) conditions. The total O-2 change (between the two phases) in a single planting of rice and pearl millet was significantly higher than that of the mixed planting of rice and pearl millet, which indicated that O-2 was transferred from rice to pearl millet under a water culture condition. The result indicated that approximately 7 +/-mu M O-2 g fresh root weight(-1) h(-1) was transferred between the two plant species. O 2 transfer was confirmed between the two plant species in a mix cultured in water, implying its contribution to the phenomenon that improved the physiological status of drought-adapted upland crops under field flood conditions.

The purpose of this study was to examine an inhibitory effect of mango leaf extracts on advanced glycation end products (AGEs) formation and to identify these active ingredients, and also to investigate a relationship between leaves maturation and the inhibitory activity. A methanolic extract of old dark green mango leaf extract (OML-ext) exhibited an inhibitory activity of AGEs formation in nonenzymatic glycation of albumin. The inhibitory activity of OML-ext was attributable to 3-C-β-D-glucosyl-2,4,4’,6-tetrahydroxybenzophenone (1), mangiferin (2) and chlorophyll. Inhibitory effect of young dark reddish brown mango leaf extract (YDL-ext) on AGEs formation was similar to that of OML-ext. The inhibitory activity of YDL-ext was attributable to 1 and 2, in addition, a part of the the activity of YDL-ext due to anthocyanins whose content is highest in young dark reddish brown mango leaves. Considering the amounts of leaves obtained from pruning, old dark green leaves may be a reasonable natural resource for the preparation of ingredients with inhibitory activity of AGEs formation.

We investigated the alleviative effects of mixed cropping using ice plant, which is one of the saltaccumulating halophytes, on the damage and growth inhibition of cowpea, which is not tolerant to high salinity. Three cropping patterns (mono cropping of cowpea and ice plant and their combination) were tested. The plants were treated with 0, 100, 200 and 300 mM NaCl for 14 days (consecutive NaCl). The plants were also treated with NaCl for 3 days, followed by 2 weeks (shortterm recovery) and 1 month (long-term recovery) recovery. Salinity levels for short-term recovery were similar to those of the consecutive experiment, while the concentration of long-term recovery was 250 mM. The alleviative effects of mixed cropping in the consecutive NaCl experiment were observed at 200 and 300 mM NaCl. Mixed cropping significantly reduced the Na content in the cowpea leaves at 200 and 300 mM NaCl compared with mono cropping. In addition, the Na content in the soil of mix-cropped cowpea at 200 and 300 mM NaCl was statistically lower than that of the mono cropping. Mixed cropping was effective to recover from high concentration of NaCl in the experiments of short-and long-term recovery. These results indicate that mixed cropping with a halophyte could be effective in mitigating the damage and growth inhibition of a glycophyte not only under salinity but also under recovery periods.

The purpose of this study was to search edible ripe Citrus fruits which are applicable for functional food materials as juice, tea and/or jam with sweet taste and rich aroma. A fifty percent ethanolic extract (CMR-ext) obtained from the edible ripe fruit of Citrus madurensis exhibited an inhibitory activity of antigen-induced degranulation in anti-dinitrophenyl (DNP) IgE antibody sensitized rat basophilic leukemia (RBL) -2H3 cells. The inhibitory effect of the CMR-ext on degranulation in RBL-2H3 cells was attributable to 3’,5’-di-C-β-glucopyranosylphloretin (1) which is a constituent of C. madurensis. The effect of 1 on Akt and mitogen-activated protein kinases (MAPK) phosphorylation was examined in RBL-2H3 cells. Western blot analysis revealed that 1 (50 μM) inhibited the degranulation by suppression of Akt and p38 phosphorylation.

Flash floods, erratically striking semi-arid regions, often cause field flooding and soil anoxia, resulting in crop losses on food staples, typically pearl millet (Pennisetum glaucum L.) and sorghum (Sorghum bicolor (L) Moench). Recent glasshouse studies have indicated that rice (Oryza spp.) can enhance flood stress tolerance of co-growing dryland cereals by modifying their rhizosphere microenvironments via the oxygen released from its roots into the aqueous rhizosphere. We tested whether this phenomenon would be expressed under field flood conditions. The effects of mix-planting of pearl millet and sorghum with rice on their survival, growth and grain yields were evaluated under controlled field flooding in semi-arid Namibia during 2014/2015-2015/2016. Single-stand and mixed plant treatments were subjected to 11-22 day flood stress at the vegetative growth stage. Mixed planting increased plant survival rates in both pearl millet and sorghum. Grain yields of pearl millet and sorghum were reduced by flooding, in both the single-stand and mixed plant treatments, relative to the non-flooded upland yields, but the reduction was lower in the mixed plant treatments. In contrast, flooding increased rice yields. Both pearl millet rice and sorghum rice mixtures demonstrated higher land equivalent ratios, indicating a mixed planting advantage under flood conditions. These results indicate that mix-planting pearl millet and sorghum with rice could alleviate flood stress on dryland cereals. The results also suggest that with this cropping technique, rice could compensate for the dryland cereal yield losses due to field flooding. (C) 2016 The Authors. Published by Elsevier B.V.

Recently, the occurrences of extreme flooding and drought, often in the same areas, have increased due to climate change. Wetland plant species are known to oxygenate their rhizospheres by releasing oxygen (O-2) from their roots. We tested the hypothesis that wetland species could help upland species under flood conditions; that is, O-2 released from the wetland crop roots would ameliorate rhizosphere O-2-deficient stress and hence facilitate upland crop root function. Flooding tolerance of upland-adapted staple crops-pearl millet (Pennisetum glaucum) and sorghum (Sorghum bicolor) mix-cropped with rice (Oryza spp.) was investigated in glasshouse and laboratory. We found a phenomenon that strengthens the flood tolerance of upland crops when two species-one wetland and one drought tolerant-were grown using the mixed cropping technique that results in close tangling of their root systems. This technique improved the photosynthetic and transpiration rates of upland crops subjected to flood stress (O-2-deficient nutrient culture). Shoot relative growth rates during the flooding period (24 days) tended to be higher under mixed cropping compared with single cropping. Radial oxygen loss from the wetland crop roots might be contributed to the phenomenon observed. Mixed cropping of wet and dryland crops is a new concept that has the potential to overcome flood stress under variable environmental conditions. (C) 2016 The Authors. Published by Elsevier GmbH. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

We investigated the soybean productivity of double cropping and relay intercropping in farmer's fields for three years using two black soybean cultivars of Kurozukin and Tanbaguro, which are used for the first and second crop, respectively. Kurozukin is the early maturing cultivar for vegetable soybean harvest and Tanbaguro is the late-maturing cultivar for the harvest of vegetable soybean and seeds. The yield of the first crop (Kurozukin) was similar to the mono cropping regardless of cropping patterns. However, the yield of the second crop (Tanbaguro) was affected by cropping patterns. The yield of Tanbaguro in double cropping was prone to decrease by late sowing. The late sowing was induced by the late sowing and late harvesting of Kurozukin because of the low temperature in April and the large amount of precipitation in rainy season, respectively. In relay intercropping that Tanbaguro was sown between the rows of Kurozukin at about one month before the harvest of Kurozukin, the yield of Tanbaguro was similar to the mono cropping and the competition with Kurozukin was not observed. Thus, the land equivalent ratio value of double cropping was lower than that of relay intercropping. These results suggest that relay intercropping is more useful cultivation system than double cropping to increase the annual soybean production.

Waterlogging is the constraint for soybean growth and yield, because soybean is often cultivated in upland fields converted from paddy in Japan. However, efficient cultivation techniques for alleviating the adverse effects have not been developed. We have proposed the new soybean cultivation technique named crack fertilization which enables yield increase due to enhancing new root growth and N acquisition by increasing nodulation. Waterlogging induces N deficiency due to the suppression of nutrient uptake by the inhibition of root growth and nodule activity. Thus, it is hypothesized that crack fertilization would be effective to alleviate the inhibition of soybean growth and yield. The soybean cultivar of Sachiyutaka was planted in 1/5000 a Wagner pots and root boxes. Two separate waterlogging treatments were imposed to soybean plants at different growth stages, V1 and R4, and crack fertilization was done at V3. After these treatments, soybean plants were sampled at R5 in 2012 and 2013 experiments, respectively. Waterlogging at V1 and R4 inhibited the growth and yield of soybean and nodule growth, and the decreases in physiological parameters of soybean such as photosynthesis, chlorophyll content, and xylem sap exudation rate were observed. The adverse effects of waterlogging at V1 were alleviated by crack fertilization at V3, whereas crack fertilization could not alleviate the adverse effects of waterlogging at R4. Thus, crack fertilization after waterlogging at early vegetative stage would be the cultivation technique that enables to alleviate the adverse effects.

This paper examined the route of water supply into soybean nodules through the new visualization technique of time of flight secondary ion mass spectrometry (Tof-SIMS) cryo system, and obtained circumstantial evidence for the water inflow route. The maximum resolution of the Tof-SIMS imaging used by this study was 1.8 mu m (defined as the three pixel step length), which allowed us to detect water movement at the cellular level. Deuterium-labeled water was supplied to soybean plants for 4 h and the presence of deuterium in soybean nodules was analyzed by the Tof-SIMS cryo system. Deuterium ions were found only in the endodermis tissue surrounding the central cylinder in soybean nodules. Neither xylem vessels nor phloem complex itself did not indicate any deuterium accumulation. Deuterium-ion counts in the endodermis tissue were not changed by girdling treatment, which restricted water movement through the phloem complex. The results strongly indicated that nodule tissues did not receive water directly from the phloem complex, but received water through root cortex apoplastic pathway from the root axis. (C) 2015 Elsevier GmbH. All rights reserved.

1. はじめに
アジアやアフリカの農村地域では、農家が自給用作物を栽培する際、複数の作物を同一の畑で栽培する混作や間作、単作など、多様な作付様式がみられる。作物種の組み合わせは農家ごとに異なる場合が多く、畑地空間の環境認識や生計戦略など、作付様式には各農家の主体的な営為が反映されている。しかし、調査者がインタビューなどで把握する農家の認識と実践との間にはズレがある場合が多く、作付様式に反映された農家の意図を調査者が把握するためには、農家の認識と実践の両面に注目した多面的なアプローチが必要となる。本稿では、アフリカのナミビア共和国北部地域を対象とし、農家の作付様式から彼らの創意工夫を理解するための調査手法について、メンタルマップによる農家の認識把握や自律型飛行体(UAV)による航空写真の活用というアプローチから検討した。

2. 方法
2012年11月～2014年9月にかけて、ナミビア北中部に位置するO村において現地調査を実施した。予備調査により、32世帯を対象に、村の農業体系について把握した。その情報を基に8世帯を抽出し、作付様式に関する調査を実施した。調査では、その年に栽培した作物種を聞き、その後、画用紙に畑のスケッチを描いてもらった。その後、そのスケッチをもとに各作物種の栽培場所の選択理由、作物種の組み合わせの理由をインタビューし、彼らの認識を把握した。インタビュー調査が終わった後に畑へ移動し、実際の耕作範囲をGPS受信機を用いて記録した。また、スケッチに描かれた内容を基に、実際の作付様式について説明をしてもらった。最後に、UAVによって畑の航空写真を撮影し、実際の作付様式について把握した。

3. 結果と考察
(1) 作付様式に対する農家の認識：調査世帯が作付を行っている作物種は12種であった。メンタルマップおよびインタビューにより、これらの作物種には単作としてのみ栽培される作物と混作によって栽培される作物があることが明らかとなり、混作の組み合わせ方は世帯によって異なる傾向がみられた。例えば、マメ科のササゲはトウジンビエと混作する世帯とモロコシと混作する場合がみられ、組み合わせを実施する背景には様々な意図がみられた。

(2) 認識と実践のずれ：メンタルマップによって把握した農家の認識と実際の作付様式の間には、様々な要因によるずれがみられた。インタビューによる調査では、一般的な傾向として、作物種間の混作様式が説明される傾向がみられるが、実際に農家が行っている作付様式と比較すると、農家が小規模に試みに実施している作付方法などが浮き彫りとなり、農家の主体的な取り組みが抽出された。

(3) 作付様式を採用する多様な意図：年間降水量が少なく、降雨の経年変動が大きい本地域では、混作は不確実な降雨に対するリスク軽減の意味合いがみられた。例えば、乾燥に強いトウジンビエと湿害に強いモロコシの種子を混ぜ、同じ場所に混播する方法を行っている農家では、洪水や干ばつなどの気象災害発生時でも、どちらか一方の作物が生存することを意図して実施すると説明した。また、砂質土壌に覆われる本地域では土壌の肥沃度も微地形によって異なると説明され、各世帯の畑の環境条件に適応した作付方法として、多様な混作が実践されているとみられる。

Crack fertilization is a soybean cultivation technique for nodulation control in which midterm subsoiling is used to supply fertilizing materials to deep soil just before the flowering stage. This study examined the effects of fertilizing materials and the continuous application of nodulation control, on soybean yield enhancement in two field experiments. The survival of nodule bacteria in deep soil was also evaluated by a bioassay of nodule bacteria in a root box. When the nodule bacteria on biochar were continuously applied without any other chemical fertilizers for three successive years, seed weight was significantly heavier being up to 1.34 times that of the control. The application of nodulation control in the previous year but not in the experimental year did not have residual effects on seed weight. The enhancement of seed weight in a farm field converted from a paddy was much lower. This may be partly attributed to the midterm tillage practice, which destroys the crack structure after the nodulation control, together with soil water status and cultivar differences. Nodule growth and nitrogen fixation activities significantly increased in the soybean plants grown on the soil collected from the subsoil to which nodule bacteria on biochar had been applied the previous year. This suggests that nodule bacteria in the subsoil survived in the biochar habitat for at least a year after application. These results indicate that nodulation control by the crack fertilization technique leads to yield enhancement when nodule bacteria on biochar are continuously applied.

Land use changes in wetland areas can alter evapotranspiration, a major component of the water balance, which eventually affects the water cycle and ecosystem. This study assessed the effect of introduced rice-cropping on evapotranspiration in seasonal wetlands of northern Namibia. By using the Bowen ratio-energy balance method, measurements of evapotranspiration were performed over a period of 2.5 years at two wetland sites-a rice field (RF) and a natural vegetation field (NVF)-and at one upland field (UF) devoid of surface water. The mean evapotranspiration rates of RF (1.9 mm daytime(-1)) and NVF (1.8 mm daytime(-1)) were greater than that in UF (1.0 mm daytime(-1)). RF and NVF showed a slight difference in seasonal variations in evapotranspiration rates. During the dry season, RF evapotranspiration was less than the NVF evapotranspiration. The net radiation in RF was less in this period because of the higher albedo of the non-vegetated surface after rice harvesting. In the early growth period of rice during the wet season, evapotranspiration in RF was higher than that in NVF, which was attributed to a difference in the evaporation efficiency and the transfer coefficient for latent heat that were both affected by leaf area index (LAI). Evapotranspiration sharply negatively responded to an increase in LAI when surface water is present according to sensitivity analysis, probably because a higher LAI over a surface suppresses evaporation. The control of LAI is therefore a key for reducing evaporation and conserving water. Copyright (C) 2013 John Wiley & Sons, Ltd.

The use of coffee grounds in crop fields were evaluated in terms of crop growth enhancement, soil improvement, and weed control during four successive cropping seasons for two years. Six summer and three winter green manure crops were grown from June 2009 to May 2011. In the first cropping season, the growth of all green manure crops was significantly inhibited by the application of 10 kg m(-2) of coffee grounds. However, the inhibitory effects spontaneously diminished after the second cropping season (about 12 months later), and the growth of guinea grass, sorghum and sunflower was about 2-fold higher than that of the control. The application of horse manure at 10 kg m(-2) effectively alleviated the inhibitory effects, even though the high concentration of coffee grounds was included. Although top dressing application of coffee grounds at 16 kg m(-2) permitted weed control, the impact maintained enough only for half a year. Coffee grounds application effectively increased both carbon and nitrogen contents of soil and reduced CN ratio. The soil amendment effects were significantly higher in terms of nitrogen enrichment and CN ratio improvement as compared with the horse manure application. These results indicated that coffee grounds are useful to enhance long term crop growth, short duration weed control, and soil improvement in agricultural fields by considering the inhibitory effects on the plant growth for half year after the application. Agricultural use of coffee grounds was also discussed in term of fallow periods in crop rotation.

Agricultural use of wetlands is important for food security in various regions. However, land-use changes in wetland areas could alter the water cycle and the ecosystem. To conserve the water environments of wetlands, care is needed when introducing new cropping systems. This study is the first attempt to evaluate the water dynamics in the case of the introduction of rice-millet mixed-cropping systems to the Cuvelai system seasonal wetlands (CSSWs) in north-central Namibia. We first investigated seasonal changes in surface water coverage by using satellite remote sensing data. We also assessed the effect of the introduction of rice-millet mixed-cropping systems on evapotranspiration in the CSSWs region. For the former investigation, we used MODIS and AMSR-E satellite remote sensing data. These data showed that at the beginning of the wet season, surface water appears from the southern (lower) part and then expands to the northern (higher) part of the CSSWs. For the latter investigation, we used data obtained by the classical Bowen ratio-energy balance (BREB) method at an experimental field site established in September 2012 on the Ogongo campus, University of Namibia. This analysis showed the importance of water and vegetation conditions when introducing mixed-cropping to the region.

An appropriate combination of rice cultivar and cropping system that maximizes water use efficiency (WUE) may improve yield of rainfed lowland rice. In the paddy field, a large amount of water is consumed by evaporation during the early growth period, and it can be reduced by canopy coverage especially in semi-arid regions. Therefore, we evaluated the role of canopy coverage in WUE of rice in the early growth period in semi-arid region. A pot experiment was conducted in Namibia to investigate the genotypic and species difference in WUE, and another pot and a field experiment were conducted to investigate the effects of planting density on WUE. Although no significant difference was observed among species, the mean WUE was in the decreasing order of Oryza sativa, and Oryza glaberrima followed by the interspecific progenies including NERICA. In contrast, there was a significant difference in WUE at the genotypic level. Highly tillering genotypes such as WAB1159-2-12-11-5-1 and WITA 2 showed a high WUE. Furthermore, WUE was significantly correlated with the number of tillers (R-2 = 0.453), and higher planting density resulted in a higher WUE. In contrast, stomatal conductance had no significant correlation with WUE (R-2 = 0.081). Thus, the physical conditions affected by number of tillers and planting density had greater impacts on WUE than physiological characteristics such as stomatal conductance. The suppression of surface water evaporation by coverage was significant, probably contributing to WUE improvement. To increase WUE in semi-arid regions, we recommend the increase of canopy coverage and higher planting density.

This paper presents evidence for the existence of both a circadian and an ultradian rhythm in the elongation growth of rice roots. Root elongation of rice (Oryza sativa) was recorded under dim green light by using a CCD camera connected to a computer. Four treatment conditions were set-up to investigate the existence of endogenous rhythms: 28 degrees C constant temperature and continuous dark (28 DO); 28 degrees C constant temperature and alternating light and dark (28 LD); 33 degrees C constant temperature and continuous dark (33 DD); and diurnal temperature change and alternating light and dark (DT-LD). The resulting spectral densities suggested the existence of periodicities of 20.4-25.2 h (circadian cycles) and 2.0-6.0 h (ultradian cycles) in each of the 4 treatments. The shorter ultradian cycles can be attributed to circumnutational growth of roots and/or to mucilage exudation. The average values across all the replicate data showed that the highest power spectral densities (PSDs) corresponded to root growth rhythms with periods of 22.9, 23.7, and 2.1 h for the 28 DD, 28 LD, and 33 DD treatments, respectively. Accumulation of PSD for each data set indicated that the periodicity was similar in both the 28 DD and 33 DD treatments. We conclude that a 23-h circadian and a 2-h ultradian rhythmicity exist in rice root elongation. Moreover, root elongation rates during the day were 1.08 and 1.44 times faster than those during the night for the 28 LD and DT-LD treatments, respectively. (C) 2011 Elsevier GmbH. All rights reserved.

Enhancement of nitrogen fixation activities by a cultivation technique is one of the potential targets to improve soybean yield in Japan. A cultivation technique named "crack fertilization" is described and the nodulation control by this technique is analyzed by experiments in two fields with different soil and environmental conditions, and a root box experiment. Crack fertilization is a technique that utilizes the irregularly shaped soil cracks formed by subsoiling just before the flowering stage of soybean, and introduces root nodule bacteria, fertilizers, and so on to the deep subsoil layer. Production of ureide-form nitrogen at six weeks after the crack fertilization (only nodule bacteria application to the subsoil layer) was 1.4 times higher than the control in the Hikone field. Acetylene reduction activity at four or eight weeks after the crack fertilization (application of both nodule bacteria and low level of fertilizer to the subsoil layer) tended to be higher than in the control in both fields. In the root box experiment, nodule number was four times higher than that in the control at the lower portion of the root system, where modified crack fertilization treatment was conducted, and the acetylene reduction activity was increased significantly by the treatment. These results indicated that the soybean nodulation control, i.e., timing and position of nodulation, as well as the enhancement of nitrogen fixation activity could be achieved by this crack fertilization technique.

Water movement between cells in a plant body is the basic phenomenon of plant solute transport; however, it has not been well documented due to limitations in observational techniques. This paper reports a visualization technique to observe water movement among plant cells in different tissues using a time of flight-secondary ion mass spectrometry (Tof-SIMS) cryo-system. The specific purpose of this study is to examine the route of water supply from xylem to stem tissues. The maximum resolution of Tof-SIMS imaging was 1.8 mu m (defined as the three pixel step length), which allowed detection of water movement at the cellular level. Deuterium-labelled water was found in xylem vessels in the stem 2.5 min after the uptake of labelled water by soybean plants. The water moved from the xylem to the phloem, cambium, and cortex tissues within 30-60 min after water absorption. Deuterium ion counts in the phloem complex were slightly higher than those in the cortex and cambium tissue seen in enlarged images of stem cell tissue during high transpiration. However, deuterium ion counts in the phloem were lower than those in the cambium at night with no evaporative demand. These results indicate that the stem tissues do not receive water directly from the xylem, but rather from the phloem, during high evaporative demand. In contrast, xylem water would be directly supplied to the growing sink during the night without evaporative demand.

Salinity tolerance has been extensively studied in Oryxa sativa, but little is known about the salt tolerance levels in Oryxa glaberrima and the interspecific progenies including New Rice for Africa (NERICA). In this study, the salinity tolerance of the three cultivated rice species, O. glaberrima (54 genotypes), the interspecific progenies (21) including NERICA (7) and O. sativa (41) mainly grown in West Africa were examined comparatively. At 10 days after sowing (DAS) 80 mM NaCl was added to the culture solution, and the plants were grown for 10 more d. The ratio of shoot biomass in the 80 mM NaCl solution to that in the control was significantly higher in the interspecific progenies than in the other two species, and the relative root biomass was significantly lower in O. glaberrima than in the others. The vegetative growth of six genotypes including the salt tolerant Pokkali, and NERICA4 and its parents were evaluated further in pot experiments irrigated with 80 mM NaCl solution from 22 to 52 d after sowing. At 30 d of the salt stress, CG14 and Mala noir IV (O. glaberrima) were killed by salt, while WAB56-104 and NERICA4 survived; Pokkali maintained the highest relative shoot biomass growth at all sampling times of 10 d intervals. These results indicate that O. glaberrima is relatively weaker to NaCl salinity, while the interspecific progenies are fairly tolerant during the seedling stage, and that the relatively high salt stress tolerance of NERICA4 is derived from the O. sativa parent, WAB56-104.

We investigated the effects of combined soil physical stresses of compaction and drought. oil the production of fully hydrated mucilage (mucilage) and root. border cells (RBCs) in maize. The exudation of carbon and water were also estimated using stable isotopes of (13)C and deuterated water (D(2)O) under same soil conditions. As plant age progressed during seedling stage, mucilage production increased, however, RBCs release did not. Soil compaction increased the release of D(2)O, RBCs, and production of mucilage which implies the function of roots to reduce mechanical impedance during root penetration. Drying stress increased only carbon release, but reduced the others. This indicates that RBCs adhere more strongly to the root cap due to drying of mucilage, and water release may be reduced to save the water loss. The highest rhizodeposition of mucilage, RBCs and D(2)O were occurred under wet compact soil condition, however, that of carbon Occurred under dry compact soil condition.

We introduced subsoiling to a field of wheat-soybean rotation where no-tillage practice had been conducted for five years and whose yield tended to decrease or stagnate. By subsoiling a half of each plot just: before wheat: sowing, treatments of tillage/no-tillage x subsoiling/no-subsoiling were established. Root distribution, shoot growth, water uptake and yield of both crops were examined to elucidate whether the subsoiling improves tale productivity such as shoot biomass and yield through the modification of root system development, and how differ the effects of subsoiling between tilled and non-tilled fields. In wheat, roots were less concentrated in surface (0-5 cm) layer in no-tillage, and distributed more in deep (20-25 cm) layer of the soil. Deuterium labeled heavy water analysis revealed that: the subsoiling enhanced water uptake from the deep soil layer in the no-tillage field. Both the no-tillage and subsoiling showed positive and significant effect on total biomass and yield. The effect. of subsoiling must be related to water supply by deep roots in spring. In soybean no-tillage significantly increased the productivity, but subsoiling did not though distribution of the roots was modified by both practices. Soybean in non-tilled accumulated roots in the surface soil layer, but subsoiling did not significantly modify the root distribution especially in the deep soil layer. Water uptake trend and yield was thus not changed significantly by subsoiling. Subsoiling in the non-tilled field increased rooting depth and showed the possibility of braking yield stagnation in long-term no-tillage cultivation in wheat, but: not in soybean.

Plant roots release mucilage and root border cells (RBCs) into rhizosphere, which function as it complex at. the root-soil interface. The dynamics of RBCs in rhizosphere soil, however, remains Unknown. In this study the ratio of crushed root cap cells during root penetration into Soil and Survival of the RBCs after the release from the root cap wan estimated in maize seminal root. In addition, the effects of long term Soil compaction on RBCs release were investigated. During the root penetration into rhizosphere soil, 78, 56, and 45% of sloughed root cap cells were estimated to be cashed at the first, second, and third day after planting, respectively. The number of surviving RBCs decreased with time, but 6% of the RBCs in the rhizosphere still retained their cell walls at one month after planting. These cell were estimated to remain in the soil for at least 10 d after the release from lateral roots. Furthermore, RBCs release from newly emerged nodal root increased with aging of plants, and the cell release ins significantly, increased by soil compaction only at the seedling stage. In conclusion,, significant number of RBCs wan crushed during root. penetration into Soil, however many RBCs remained in We rhizosphere sod for a relatively longer period. Soil compaction significantly increased cell release on, at We seedling stage.

In rhizosphere soil, mucilage and root border cells (RBCs) form a functional entity, the mucilage border-cell complex (MB complex). Carbohydrates of the MB complex are utilized by rhizosphere bacteria, which are under strong grazing pressure of the soil food web, in particular protozoa. We investigated the role of the MB complex for protozoan effects on plant growth. First, the MB complex formed by 16 rice cultivars belonging to different ecotypes and subspecies were quantified. These cultivars were subsequently used to investigate protozoan effects on plant growth. The differences between the highest and lowest MB complex producers were 3.1 and 5.3 times for fully hydrated mucilage and RBCs, respectively. Mucilage production and RBCs showed a significant positive regression (R-2 = 0.92) in Japonica. Presence of protozoa generally enhanced shoot biomass, lateral root growth and plant nitrogen uptake. Further, upland cultivars showed significantly higher growth enhancement than lowland cultivars in presence of protozoa. A significant positive regression between MB complex and increased lateral, root growth by amoeba revealed that the MB complex facilitated protozoan effects on plant growth, which is the first evidence for a new functional role of the MB complex.

The root cap (RC) is a multilayered dome of spindle-shaped parenchyma cells that overlies the growing root tip. It is present in the roots of almost all crop species. This paper briefly reviews some topics on the structure and function of the RC in the major crop species such as maize and rice. Special attention is placed on its contribution to the root system formation, that is, the elongation and growth direction of axile roots. The cells produced in the RC meristem are pushed forward as new cells form beneath them, and eventually the cells on the periphery of the RC fall off. The life cycle of RC cells of maize has been studied extensively and ranges from one to seven days. Approximately 4,000 to 21,000 cells are present in a complete maize RC, and 1,400 to 3,200 sloughed cells can be found in the rhizosphere soil per day per root. These cells, called root border cells (RBCs), mix with RC mucilage and play important roles for the root growth in soil. The RBC-mucilage complex effectively reduces the resistance roots experience during penetration into field soil, about 30-40% of the resistance being reduced by the presence of RC alone. The RC is also a tissue integral to gravitropism, and is known to determine the direction of root growth. The size of amyloplasts and coumellae in RCs has a strong influence on determining the growth angle of axile roots. The function of the individual regions of the RC and how the RC tissues and cells are formed should be studied further to advance our understanding regarding the critical roles of the RC in crop root growth.

Soil compaction often creates combined physical stresses of drought, anaerobiosis, and mechanical impedance in field soil. This paper aims to analyze the effect of combined and independent soil physical stresses on crop root growth to find out the species-specific response to the physical stresses, which has not been reported before. Drying stress without the increase of mechanical impedance was evaluated in a very loose pot soil environment. This drying stress did not modify the root elongation rates of rice and pea by the 48 h exposure to the stress environment. For maize and cotton, however, mild drying stress (-80 kPa Psi w) enhanced root elongation by 17-18%, but severe drying stress (-900 kPa Psi w) reduced it by 17-21% as compared with the control environment (-10 kPa Psi w). The combined stress of drying and mechanical impedance nearly stopped the root elongation in all the species, while that of anaerobiosis and mechanical impedance did not stop the elongation of rice and. cotton; cotton elongated about 32% of control environment. In maize, root diameter was reduced by the severe drying stress due to the reduction in the number of cortical cell layer and diameters of both central cylinder and xylem vessel. In contrast, cotton showed a significant increment of cortex diameter, although overall diameter was not statistically increased by the severe drying stress. The ability of cotton to continue elongation under anaerobiosis and mechanical stress implied the higher penetration ability to the hard pan layer under the anaerobic condition just after the heavy rainfall.

No-tillage often affects crop root development due to the higher mechanical impedance to root elongation, resulting in yield reduction under an unfavorable rainfall pattern, such as drought. In this study, we analyzed the changes in water source of wheat and soybean under drought stress in a continuous no-tillage field. Deuterium-labeled irrigation water was applied at different growth stages of crops to analyze their water uptake pattern. Mechanical impedance of the surface soil was 3.5 and 4.4 times higher in the no-tillage than in the conventional tillage under wet and drought conditions, respectively. Root length density and root branching index (the length of lateral roots per unit axile root length) of soybean in the surface soil layer were higher in the no-tillage field. This indicates that the increased branching by the higher mechanical impedance of undisturbed surface soil causes roots to accumulate in the surface soil layer. The deuterium concentration in the xylem sap of both crops was significantly higher in the no-tillage than in the tillage under a drought condition. This indicates that the crops in the no-tillage field depend highly on the newly supplied easily accessible water (irrigation water and/or rainfall) as compared with those in the conventional tillage field under a limited water supply. In conclusion, enhanced surface root growth in the no-tillage condition would result in higher dependence on surface supplied irrigation water than in the conventional tillage under drought.

Seasonal wetlands, locally called oshanas, are characteristic of the densely populated northern Namibia, a desert country in southwest Africa. The formation of seasonal wetlands, which will sustain the water balance of a semiarid environment, was quite unstable depending entirely on the variable rainfall in the upper catchments of Angola. The objective of the present study was to evaluate the use of seasonal wetland water by pearl millet, the local staple food crop intercropped with cowpea, to discuss the water competition pattern of intercropped species. Root system development of the intercropped species was also evaluated together with the water source analysis. For this purpose, field experiments using pearl millet intercropped with cowpea in the seasonal wetland in Namibia University (Exp. 1) and monocropped pearl millet in the local farmers field (Exp. 2) were conducted in northern Namibia. Both pearl millet and cowpea developed deeper root systems as the distance from the seasonal wetland water increased. At flowering time, the 6D value of intercropped cowpea was similar to that of wetland water, while that in pearl millet was much lower than those of both the wetland water and groundwater. This indicated that intercropped pearl millet did not have full access to the wetland water when there was competition with cowpea for water derived from various water sources. Under such circumstances, intercropped pearl millet probably relies more on the rainfall water, which is just sufficient to sustain its growth in a semiarid environment. By contrast, intercropped cowpea wins in the competition with pearl millet and can acquire water from the existing stored wetland water.

The practice of intercropping pearl millet with cowpea is widespread among subsistence farmers in northern Namibia. In this region, the scarce and erratic rainfall may enhance competition for the limited soil water between intercropped plants. Trials were conducted on a field of the University of Namibia (on-station) and on a farmer's field (off-station) to determine the effects of competition between pearl millet and cowpea on the water sources and plant growth of each crop. The deuterium analysis showed that pearl millet, intercropped with cowpea, significantly increased its dependence on the recently supplied labeled irrigation water. Intercropped cowpea also showed an increased trend of the dependence but it was not statistically significant. At the university field, intercropped pearl millet showed higher dependence on the irrigation water than monocropped pearl millet. At the farmer's field, the dependence of intercropped pearl millet on the irrigation water was low in the pearl millet-dominant zone. In contrast, the dependence on the irrigation water was high in the cowpea-dominant zone, indicating that the dependence on the irrigation water changes according to the size of the pearl millet canopy. The water sources of cowpea did not show a significant difference at either pearl millet-dominant or cowpea-dominant zone, indicating a stable water uptake trend under competitive conditions. Competition with cowpea significantly increased the root-weight density of intercropped pearl millet in the deep soil layers, but decreased that in the shallow layers. The root-weight density of intercropped cowpea, however, was reduced in most of the soil layers. In conclusion, cowpea has a higher ability to acquire existing soil water, forcing pearl millet to develop deep roots and shift to the surface irrigation water.

We performed a controlled experiment with rice seedlings (Oryza sativa L.) growing in Petri dishes on homogeneous nutrient agar containing a simple rhizosphere food web consisting of a diverse bacterial community and a common soil protozoa, Acanthamoeba castellanii, as bacterial grazer. Presence of amoebae increased bacterial activity and significantly changed the community composition and spatial distribution of bacteria in the rhizosphere. In particular, Betaproteobacteria did benefit from protozoan grazing. We hypothesize that the changes in bacterial community composition affected the root architecture of rice plants. These effects on root architecture affect a fundamental aspect of plant productivity. Root systems in presence of protozoa were characterized by high numbers of elongated (L-type) laterals, those laterals that are a prerequisite for the construction of branched root systems. This was in sharp contrast to root system development in absence of protozoa, where high numbers of lateral root primordia and short (S-type) laterals occurred which did not grow out of the rhizosphere region of the axile root. As a consequence of-nutrient release from grazed bacteria and changes in root architecture, the nitrogen content of rice shoots increased by 45% in presence of protozoa. Our study illustrates that interactions over three trophic levels, i.e. between plants, bacteria and protozoa significantly modify root architecture and nutrient uptake by plants. (c) 2006 Elsevier Ltd. All rights reserved.

Intercropping pearl millet with cowpea is a common practice in semiarid areas. Under limited water environments, competition for soil water between intercropped plants may be strong. Furthermore, the increasing soil compaction problems, due to the use of heavy machinery, may intensify competition for limited resources, particularly in the topsoil. Two field trials were Conducted to evaluate the water competition ability of intercropped pearl millet when subjected to drought and soil compaction during the 2004 Japanese summer. For this purpose plant water Sources were determined by the hydrogen stable isotope (deuterium) technique. Plant water relations and biomass production were also evaluated. According to the deuterium concentration values in xylem sap, pearl millet water sources were changed by the Competition With cowpea. Pearl millet was forced to rely more on recently supplied (irrigation/rainfall) water. In contrast, the water sources of cowpea were unchanged by plant Competition. When plants were subjected to drought, the transpiration rate of pearl millet was reduced by 40 % of its monocropped potential by Competition, but that of cowpea was not. Moreover, intercropped pearl Millet, under drought and soil compaction, showed lower leaf water potential and biomass than their respective monocropped Counterparts. Cowpea had a higher competitive ratio tinder vet, dry, and compaction treatments, while pearl millet was more competitive under loose conditions. In conclusion, under drought and soil compaction, water competition restricted the water use of intercropped pearl millet, forcing pearl millet to shift to the recently supplied water. In contrast, cowpea did not show air), significant changes under these stress conditions.

In semi-arid areas, pearl millet is an important staple food crop that is traditionally intercropped with cowpea. This study evaluated the water competition between pearl millet and cowpea using deuterated water. At vegetative stage, pearl millet biomass production was lower in the pearl millet-cowpea (PM-CP) combination than in the pearl millet-pigeon pea (PM-PP) and pearl millet-bambara nut (PM-BN) combinations. PM-CP used more water than PM-PP and PM-BN under well-watered conditions; however, all combinations used similar amounts of water under dry conditions. The biomass production, photosynthetic rates, transpiration rates, and midday leaf water potential of pearl millet at early flowering stage were not significantly reduced by mixed planting with cowpea sown two weeks later as compared with single planted pearl millet. When pearl millet and cowpea were sown at the same time, mix planting significantly increased the recovery rates of recently irrigated heavy water in pearl millet, but not in cowpea in both vegetative and early flowering stages. Midday leaf water potential and transpiration rates in pearl millet were lowered by mixed planting but those in cowpea were not. These indicate that the water source of pearl millet is shifted to the recently irrigated and easily accessible water. By contrast, when cowpea was sown two weeks later than pearl millet, this trend was not observed. These results provide new evidence on water competition in the PM-CP intercropping system; cowpea has higher ability to acquire existing soil water than pearl millet when both crops are sown at the same time.

Pearl millet is better adapted to hot and semi-arid conditions than most other major cereals. The objective of this study was to compare the deep water uptake ability and water use efficiency (WUE) of pearl millet among millet species. First, the WUE of six millet species was evaluated in pots under waterlogging, well-watered (control), and drought conditions. Secondly, the water uptake from deep soil layers by pearl millet and barnyard millet, which showed the highest drought and waterlogging tolerance, respectively, was compared in long tubes which consisted of three parts (two loose soil layers separated by a hardpan and a Vaseline layer). Soil moisture was adjusted to well-watered and drought conditions in the upper (topsoil) layer, while the lower (deep) layer was always kept wet. WUE was significantly reduced in all millet species by waterlogging but not by drought. The ratio of WUE to the control condition indicated that pearl millet had the highest and lowest resistances to drought and waterlogging conditions, respectively, while barnyard millet was the most stable under both conditions. The deuterium concentration in xylem sap water, relative water uptake from deep soil layers, and water uptake efficiency of deep roots were significantly increased in barnyard millet but not in pearl millet by drought in topsoil layers. In conclusion, the drought resistance of pearl millet is explained by higher WUE but not by increased water uptake efficiency in deep soil layers as compared to barnyard millet, another drought-resistant millet species.

In Japan, wheat-rice crop rotation with the practice of rice transplanting has been quite popular in the past. Mechanized direct-planted wheat-rice sequential cropping was developed at the Aichi Prefecture Agricultural Research Center by intercropping them for two months in spring. An objective of this study was to evaluate the introduction of continuous no-tillage to the cropping system with emphasis on water stress. The water source of intercropped wheat was also elucidated using deuterated heavy water to analyze water competition between crops. Continuous no-tillage of wheat-rice direct planting was performed for six seasons (three years) in an experimental small paddy field. No-tillage resulted in a doubled soil penetration resistance in the surface layer of soil, indicating the risk of suppressing root development. The higher yield of wheat in the dry plot suggested that excess-moisture stress occurs in the field. In the no-tillage plot, light transmission to intercropped rice seedlings increased significantly due to the reduced wheat biomass production. Wheat and rice yields were not statistically lowered by the no-tillage practice. This indicated that it is possible to introduce continuous no-tillage to the cropping system. The no-tillage significantly increased the deuterium concentrations in the xylem sap in wheat after the application of simulated rainfall with deuterated water. This indicated that the water uptake dependency of wheat shifted from stored soil water to recently applied water, which suggested the higher competition between the crops may occur under no-tillage conditions.

Water extraction from subsoil in upland rice (Oryza sativa L.) was examined as related to topsoil desiccation and subsoil compaction. The water extraction was observed by measurements of heavy water concentrations in transpiring plants. The plants were grown in pots that were filled with sandy soil and vertically compartmented into two columns. Heavy water was applied to the subsoil. Plants exposed to mild topsoil desiccation (-120 kPa in water potential) eventually increased water extraction from the subsoil and maintained photosynthetic rate and stomatal conductance at the wet condition level. The rates of the plants subjected to severely droughted topsoil (-190 kPa) were significantly lowered due to less water uptake from the subsoil. Subsoil compaction at bulk densities of 1.45 and 1.50 Mg m(-3) inhibited increase of root length densities. Limited water extraction from the subsoil was insufficient to maintain plant productivity under drought conditions. Daily water uptake per unit of root length in the lower tube did not apparently increase even if water demand on the unit root length increased. When water to topsoil was completely withheld, water extraction from the subsoil gradually increased as the topsoil dried out. Plants that were watered and rewatered took up very little water from the subsoil. The extraction from the subsoil occurred only when water potential of the topsoil was below about -190 kPa.

Deep root penetration, which allows access to deep soil water and hydraulic lift, may help plants to overcome drought stress. The aim of this study was to evaluate the ability of sixteen food crop species to take up water from deep soil layers and the extent of hydraulic lift by the use of deuterated water. Plants were grown in pots consisting of two loose soil layers separated by a hardpan and a Vaseline layer. The lower (deep) layers were always kept wet (32%; psi = -5 kPa), while soil moisture in the upper (topsoil) ones was adjusted to 25% (psi = -7 kPa) and 12% (psi = -120 kPa) in the well-watered and drought treatments, respectively. The deuterium labeling of the deep soil water provided evidence that wheat, Job's tears, finger millet, soybean, barnyard millet, rice, and rye (in decreasing order of D2O increments) extracted more water from the deep layers under drought than well-watered in topsoil. These species showed significantly greater hydraulic lift under drought, except for soybean. Most of these species also showed increased root length density in deep soil layers and sustained high photosynthetic rates under drought. In contrast, pigeon pea, cowpea, common millet, pearl millet, foxtail millet, maize, barley, and oat did not show a significant increment in either deep-water uptake or hydraulic lift under drought. In summary, increased extraction of deep soil water under drought was closely related with the magnitude of hydraulic lift.

9 Background and Aims The impedance to root growth imposed by soil can be decreased by both mucilage secretion and the sloughing of border cells from the root cap. The aim of this study is to quantify the contribution of these two factors for maize root growth in compact soil. Methods These effects were evaluated by assessing growth after removing both mucilage (treatment I - intact) and the root cap (treatment D - decapped) from the root tip, and then by adding back 2 muL of mucilage to both intact (treatment IM - intact plus mucilage) and decapped (treatment DM - decapped plus mucilage) roots. Roots were grown in either loose (0.9 Mg m(-3)) or compact (1.5 Mg m(-3)) loamy sand soils. Also examined were the effects of decapping on root penetration resistance at three soil bulk densities (1.3, 1.4 and 1.5 Mg m(-3)). Key Results In treatment 1, mucilage was visible 12 h after transplanting to the compact soil. The decapping and mucilage treatments affected neither the. root elongation nor the root widening rates when the plants were grown in loose soil for 12 h. Root growth pressures of seminal axes in D, DM, I and IM treatments were 0.328, 0.288, 0.272 and 0.222 MPa, respectively, when the roots were grown in compact soil (1.5 Mg m(-3) density; 1.59 MPa penetrometer resistance). Conclusions The contributions of mucilage and presence of the intact root cap without mucilage to the lubricating effect of root cap (percentage decrease in root penetration resistance caused by decapping) were 43% and 58%, respectively. The lubricating effect of the root cap was about 30% and unaffected by the degree of soil compaction (for penetrometer resistances of 0.52, 1.20 and 1.59 MPa). (C) 2004 Annals of Botany Company.

To elucidate the effect of no-tillage practice on the root system development and productivity in a wheat-soybean rotation system in Japan, we continuously cultivated these crops under tilled and non-tilled field conditions and compared the growth and yield for three years. Effect of presence or absence of tillage on the root growth was evaluated by the quantitative analysis for the root systems obtained by the core sampling method. The total shoot biomass and yield of wheat were significantly higher in the tilled field than in the non-tilled field in the first and second seasons, whereas, they were significantly higher in the non-tilled field in the third season. On the other hand, no significant difference between the tilled and non-tilled field was found in the soybean yield for the three seasons. Root length per unit area had a significant positive correlation with both the total shoot biomass and yield in wheat but not in soybean. The continuous no-tillage practice improved the soil condition for root development and resulted in an enhancement of the shoot growth and yield of wheat. In soybean, on the other hand, the root system development greatly fluctuated from season to season, especially, in the non-tilled field, but the productivity in the non-tilled field was relatively stable equivalent to that in the tilled field. Thus, stable production equivalent to that obtained by conventional tillage can be achieved by the no-tillage practice in a typical Japanese climate regardless of the fluctuation in root system development.

At mid-elevation terraces in the southern part of Sumatra Island, Indonesia, cassava is widely cultivated as one of the most important cash crops for farmers. However, the prominent cassava cultivation system includes the intercropping of cereal crops and rapidly depletes soil fertility. Hence establishment of a sustainable foodcrop production system without cassava cultivation is required. A three-way experiment with maize-soybean-cowpea sequential cropping was designed to investigate the following main effects: tillage or no-tillage, mulching or no-mulching and government recommendation or farmers' traditional fertilization. Crop productivity and soil erosion were used as indicators of sustainability. At the end of the experiment, root system development of soybean was assessed to elucidate the cumulative effects of treatments on the soil environment for root growth. Fertilization treatments increased yields in all years, whereas mulching significantly increased only from the third year. The no-tillage treatment tended to reduce yields. Soybean root growth at surface level was markedly reduced by no-tillage and slightly improved by mulching. Erosion was reduced by mulching, no-tillage, and fertilization by the government recommendation. Because of its cumulative effect on increasing soil fertility and reducing erosion, the practice of mulching was highly recommended. A sufficient amount of fertilization (government level) was also recommended to maintain the fertility and to support sufficient plant growth to minimize erosion. The no-tillage practice was not recommended because it reduced crop yields, although the treatment efficiently controlled soil erosion.

Root border cells are cells that detach from the growing root cap, and serve both physical and biological roles in the rhizosphere. Most work on border cells has been confined to agar, or hydroponic culture, because of the difficulty in separating them from soil particles. We present a new method to separate the root border cells from soil, and quantify border cell numbers in non-sterile sandy loam soil at contrasting matric potentials (-20 and -300 kPa). Recovery rates of 90 +/- 1% were achieved using a combination of surfactants, sonication, and centrifugation. Root border cell numbers in the dry soil (1.4 x 10(3) after 24 h) were significantly decreased as compared with those in the wetter soil (1.7 x 10(3) after 24 h). Possible reasons for the decreased release of border cells are discussed. (C) 2004 Elsevier Ltd. All rights reserved.

In Gunung Batin, the southern end of Sumatra Island, Indonesia, cassava is widely cultivated on gently sloping areas for starch materials. The monoculture system and/or the intercropping system without legume plants commonly adopted in this region may tend to accelerate soil degradation. The objective of this study is to compare the productivity among several cassava cropping patterns to propose the most beneficial one in this region. A field experiment of five cropping patterns {cassava (Manihot esculenta Crantz) single-cropping, three cassava-based intercropping patterns, and a crop rotation} was conducted for three years. The cropping pattern that recorded the highest net income varied with the year. In 1997, the driest year of the past several decades, cassava single-cropping was the highest in income. The proposed intercropping system {cassava/(maize soybean --> cowpea)} was the highest in 1998, a year with moderate rainfall. In 1999, when severe insect damage occurred to legume crops, the farmers' conventional intercropping was the highest. In an average of the three years, the proposed intercropping pattern was same as cassava single-cropping, although cowpea cultivation as the dry season cropping was not possible in this region. The amount of soil erosion was relatively high in cassava monoculture in comparison with the other intercropping and crop-rotation systems. Cassava roots penetrated to only 0.5 m deep and extended 1 to 2 m in a horizontal direction depending on the planting density. These results lead to the conclusion that the proposed cassava cropping system would be the most beneficial in terms of economy and control of soil erosion.

In the middle terrace area of south Sumatra, Indonesia, where red acid soils poor in crop productivity are widely distributed, the effects of cropping pattern and cultivation techniques on physico-chemical properties of soil were investigated. Five patterns for cassava cropping, including monoculture, a rotation with annual food crops, and three intercroppings with differences in the combination with annual crops and in the planting density, were evaluated in Experiment I. In Experiment II, eight plots composed of the combinations of two tillage methods (no-tillage or conventional tillage), the presence or absence of surface mulch from crop residues, and two rates of chemical fertilizers were established for a maize-soybean-cowpea sequential cropping pattern. At the end of 3 years, there was no difference in total C and total N concentrations among the plots in Experiment I irrespective of the mulch treatment using crop residues. Soil organic matter (SOM) concentration was not affected even in the no-tillage plot where the maximum crop residues (20 t ha(-1)) was given as surface mulch with the increased root residues due to higher rates of fertilizers (Experiment II). In Experiment I, available P concentration was highest in an intercropping with higher fertilizer rates and lowest cassava planting density. In Experiment II, an increase in available P was attained by mulching and the higher rate of fertilizers, and a minor positive effect of fertilizer was also observed in exchangeable Mg and K concentrations. Surface mulch resulted in less clay fraction compared with the non-mulch plots in both the experiments, suggesting its effect on the maintenance of soil particle distribution. An additional finding suggested no prominent influence of cassava monoculture on the level of SOM in this area based on the comparison with other major land uses, including secondary forest, rubber plantation, and mixed cultivation of fruits with crops. Nevertheless, the introduction of crop residue mulch and higher rates of fertilizers are recommended for sustaining soil quality and achieving higher crop yields. (C) 2003 Elsevier B.V. All rights reserved.

To assess the influence of mechanical impedance on cell fluxes in the root cap, maize (Zea mays) seedlings were grown in either loose or compacted sand with penetration resistances of 0.2 MPa and 3.8 MPa, respectively. Numbers of cap cells were estimated using image analysis, and cell doubling times using the colchicine technique. There were 5930 cells in the caps in the compact and 6900 cells in the loose control after 24 h growth in sand. Cell production rates were 2010 cells d(-1) in compact and 1570 cells d-1 in loose sand. These numbers represent accumulations of 4960 and 3540 detached cells d-1 around the cap periphery following the two types of treatment. The total number of detached cells was estimated as sufficient to completely cover the whole root cap in the compact sand, but only 11% of the root cap in the loose sand. In conclusion, mechanical impedance slightly enhanced meristematic activities in the lateral region of the root cap. The release of extra border cells would aid root penetration into the compact sand.

The root cap assists the passage of the root through soil by means of its slimy mucilage secretion and by the sloughing of its outer cells. The root penetration resistance of decapped primary roots of maize (Zea mays L. cv. Mephisto) was compared with that of intact roots in loose (dry bulk density 1.0 g cm(-3); penetration resistance 0.06 MPa) and compact soil (1.4 g cm(-3); penetration resistance 1.0 MPa), to evaluate the contribution of the cap to decreasing the impedance to root growth. Root elongation rate and diameter were the same for decapped and intact roots when the plants were grown in loose soil. In compacted soil, however, the elongation rate of decapped roots was only about half that of intact roots, whilst the diameter was 30% larger. Root penetration resistances of intact and decapped seminal axis were 0.31 and 0.52 MPa, respectively, when the roots were grown in compacted soil. These results indicated that the presence of a root cap alleviates much of the mechanical impedance to root penetration, and enables roots to grow faster in compacted soils.

In a hilly area in Lampung State, the southern end of Sumatra Island in Indonesia, coffee is commonly cultivated on hillsides with steep slopes and soil erosion affects sustainable coffee production. A field experiment on coffee cultivation was conducted for 4 years to evaluate the effects of alley cropping and no-tillage on the seed production of coffee and on erosion control in a steep slope area in this region. The cultivation of intercropped vegetables, red peppers, tomatoes, and long beans was introduced into the coffee fields when the coffee plants were small. No significant differences in the yields of either the coffee plants or the intercropped vegetables were noted among the treatments. Coffee shoot fresh weight, however, was slightly increased by no-tillage treatment. Soil erosion was reduced by 37% as a result of no tillage and by 64% with alley cropping. The amount of soil erosion for local farmers practice, tillage & no-alley, was more than four times that in no-tillage and alley treatment. These results indicate that alley cropping and no-tillage are effective for erosion control on coffee fields on steep slopes and that yield is not affected by these practices. The introduction of intercropped vegetables is beneficial in terms of farm economy, especially when the income from coffee cultivation is limited.

Direct evidence on the functions of root-cap mucilage during plant root growth in soil is limited mainly due to the lack of a method for in situ measurements. In this paper, we offer a method that facilitates the measurement of mucilage exudation when roots are growing in soil. We observed the mucilage exudation directly through a transparent panel located on the side of a root box in which plant roots were growing. We used a CCD camera attached to a microscope to observe and record mucilage exudation. Using image analysis, the activity of mucilage exudation was evaluated based on the area occupied by the mucilage on the root tip. The area of mucilage observed on the root tips after 1-h growth in soil corresponded with the weight of mucilage that was originally observed on the tips before they were transplanted. This relationship suggests that the observed area on root tip relates to total exudation. The area of mucilage exudation on the root tips was high (0.48 mm(2)) at night and low (0.35 mm(2)) at midday, suggesting that the activity of mucilage exudation follows diurnal changes. Furthermore, the mucilage exudation positively correlated with the root elongation rate, implying that fast-growing roots exude more mucilage.

The growth directions and elongation rates of axile roots that compose the framework of an upland rice root system are quite varied. The objective of this study was to elucidate the direction of growth of the axile roots relative to their root diameter and the structural characteristics of their root caps. The relationships of photosynthate translocation to either the growth direction or the elongation rate of the axile roots were also examined using a stable isotope C-13. The growth direction of the axile roots significantly correlated with their diameter. The axile roots with a relatively large diameter tended to elongate vertically in the vegetative stage, though the regression coefficients varied according to phyllochrons. The roots that emerged at the reproductive stage elongated horizontally relative to the large diameter. In the roots that emerged at the same phyllochrons, the prophyll roots elongated more vertically than the proximal roots did. The axile roots that elongated vertically formed wide columellae and large amyloplasts in the cap cells. The highest C-13 abundance in the axile root tip zone was found at 21 hrs after feeding (CO2)-C-13. The length of the apical unbranched zone behind the axile root tip positively correlated with the C-13 abundance in the root apical zones during the first 21 hrs after feeding, indicating that the roots that elongated fast would be superior in photosynthate intake in the apical zone. The axile roots that elongated vertically took in more photosynthate in their apical zones, however, the relationship was not particularly close.

The root system development and its localized distribution were examined using cassava plants grown in narrow root boxes of appropriate sizes which were adjusted to the root growth. Analyses based on fractal geometry were applied in addition to measurements of the weight and length of the root systems to evaluate the root distribution quantitatively. The root system distribution was converted to a digitized image, and the fractal dimension was calculated by the box-counting method (D-box) for the whole root system and different positions in the soil. Furthermore, 1000 random measurements for local fractal dimension by the mass-radius method (D-local) were conducted per root system, and the multifractal spectrum was analyzed. The total root weight increased until 90 days after planting (DAP), whereas the total length ceased to increase at 60 DAP. Neither apparent bulking of storage root nor decrease in root length was observed during the experimental period. The potential rooting habit of cassava was two-dimensionally exhibited under a uniform soil environment in the boxes. The complexity in root distribution as a whole root system shown by the global D (D-box) was almost constant. However, both the trends in positional D-box difference and multifractal analysis showed that the root distribution was localized and heterogeneity in the localization changed with root system development. The root system of cassava grown in the boxes can be characterized as an abundant but uneven root distribution with a highly advanced branching pattern by large global D and kurtosis of the multifractal spectra.

In contrast to cereals or other crops, legumes are known to acidify the rhizosphere even when supplied with nitrates. This phenomenon has been attributed to N-2 fixation allowing excess uptake of cations over anions; however, as we have found previously, the exposure of the shoot to illumination can cause rhizosphere acidification in the absence of N-2 fixation in cowpea (Vigna unguiculata L. Walp). In this study, we examined whether the light-induced acidification can relate to photosynthetic activity and corresponding alterations in cation-anion uptake ratios. The changes of rhizosphere pH along the root axis were visualized using a pH indicator agar gel. The intensity of pH changes (alkalization/acidification) in the rhizosphere was expressed in proton fluxes, which were obtained by processing the images of the pH indicator agar gel. The uptake of cations and anions was measured in nutrient solution. The rhizosphere was alkalinized in the dark but acidified with exposure of the shoots to light. The extent of light-induced acidification was increased with leaf size and intensity of illumination on the shoot, and completely stopped with the application of photosynthesis inhibitor. Although the uptake of cations was significantly lower than that of anions, the rhizosphere was acidified by light exposure. Proton pump inhibitors N,N'-dicyclohexyl carbodimide and vanadate could not stop the light-induced acidification. The results indicate that light-induced acidification in cowpea seedlings is regulated by photosynthetic activity, but is not due to excess uptake of cations. (C) 2002 Annals of Botany Company.

Deep rooting of wheat has been suggested that it influences the tolerance to various environmental stresses. In this study, the nodes from which the deepest penetrated roots had emerged were examined in winter wheat. The wheat was grown in long tubes with or without mechanical stress and in large root boxes. The length and growth angle of each axile root were examined to analyze the difference in the vertical distribution of the roots between the two wheat cultivars, one with a deep and one with a shallow root system. In Shiroganekomugi, a Japanese winter wheat cultivar with a shallow root system, the rooting depths of the seminal and nodal roots decreased as the rooting nodes advanced acropetally. Six out of nine deepest roots were seminal root in the non-mechanical stress conditions. In Mutsubenkei, a Japanese winter wheat cultivar with a deep root system, grown in root boxes, not only the seminal roots but also the coleoptilar and the first nodal roots penetrated to a depth of more than 1.3 m in the root box, and became the deepest roots. In both cultivars, the seminal roots became the deepest roots under the mechanical stress conditions. There were no clear tendencies in the root growth angles among the rooting nodes in the wheat root system. This indicates that the length of the axile roots can explain the differences in the rooting depths among axile roots in a wheat root system. On the other hand, the axile roots of Mutsubenkei elongated significantly more vertically than those of Shiroganekomugi. This suggests that not only seminal but also nodal roots exhibit strong positive gravitropism and penetrate deeply in a cultivar with a deep root system. In wheat cultivars, it is likely that the extent of its Root Depth Index results partly from the gravitropic responses of both seminal and nodal roots.

The cap of the primary root of maize produces several thousand border cells that are shed from the outside of the cap each day. Border cell production is important in the penetration of soil by roots, and in influencing the activity of both beneficial and pathogenic organisms in the rhizosphere. To improve understanding of the dynamics of border cell production, it is desirable to know the number of cells in different parts of the root cap. An image analysis procedure was used to quantify cell dimensions and locations in the median longitudinal section of maize (Zea mays L.) root caps. Calculations based on root symmetry were then used to estimate the number of cells in 3-dimensions. Our estimation procedure was tested initially using regular arrays of identical square and hexagonal shapes to represent cells. It was then tested using two different tissues showing analogous arrays: a transverse section through the maize root cap junction, and a transverse section through a barley root. Good linear correlations were obtained between the number of cells estimated and the number of cells actually counted in the microscope. The numbers of cells in the whole maize root cap (8870 +/- 390) were then estimated from longitudinal sections. These numbers of cap cells agreed with values that had been estimated for maize by other methods. In the first tier of the cap meristem, ten-times more meristematic cells were located in the cap flanks (> 500 cells) than were present in the columella portion. Similarly, only 7% of cells in the outermost layer of the root were associated with the columella region of the cap, a fraction which compared well with previous measurements of sloughed cells extracted from rhizosphere sand. This present technique can be applied to estimate the numbers of cells in any cylindrically symmetrical tissue from two-dimensional sections. (C) 2001 Annals of Botany Company.

Sloughing of root cap cells and exudation of mucilage plays an important role in the penetration of compacted soils by roots. For the first time we have quantified the rate of sloughing of root cap cells in an abrasive growth medium that was compacted to create mechanical impedance to root growth. The number of maize (Zea mays) root cap cells sloughed into sand increased as a result of compaction, from 1930 to 3220 d(-1) per primary root. This represented a 12-fold increase in the number of cells sloughed per mm root extension (from 60 to > 700). We estimated that the whole of the cap surface area mas covered with detached cells in compacted sand, compared with c. 7% of the surface area in loose sand. This lubricating layer of sloughed cells and mucilage probably decreases frictional resistance to soil penetration. The total carbon deposited by the root was estimated at c. 110 mu g g(-1) sand d(-1). Sloughed cells accounted far <10% of the total carbon, the vast majority of carbon being contained in mucilage exudates.

Deep penetration of an axile root is one of the important factors that allow crops to form deep root systems. In this study, the nodes from which the deepest penetrated roots had emerged were examined at the heading stage in upland rice and maize grown in large root boxes and in the field. Both experiments were designed to measure the direction, length, and rooting nodes of each root. In maize, the growth angles of axile roots increased with vertical elongation as rooting nodes acropetally advanced. The roots that emerged from the lower nodes, namely from coleoptilar to the second node, exhibited conspicuously horizontal elongation in the field, reaching 2.3 m in width at the maximum. The roots that emerged from higher than the fifth node were too short to penetrate deeply. Thus, these roots became the deepest root in less or no probability under field conditions. On the other hand, the fourth nodal root, which had an intermediate growth angle and length, had the highest probability. In upland rice, the deepest roots emerged from the nodes lower than the forth node on the main stem in the root boxes. In the field, however, the deepest roots emerged at later stages, that is, the roots from the middle nodes on the main stem and from the low nodes on the primary and secondary tillers were the deepest roots. Five out of nine of the deepest roots were from the prophyll nodes in three field-grown upland rice. The deepest roots from the same plant were estimated to have emerged and grown at approximately the same stage.

The effect of fluctuation in soil moisture on the root development of sweetpotato was studied during its establishment period, i.e., from planting to about one month after planting. The number of leaves, shoot dry weight and vine length were suppressed significantly by deficient moisture but the same were markedly increased by excessive moisture regardless of the time of occurrence relative to the initial development of the plant. In terms of its roots, the number and total length of the root system components were either increased or decreased depending upon the soil moisture regime in which they were subjected immediately prior to the time of sampling. Deficient soil moisture reduced the number and total length of the root system components while excessive moisture caused notable stimulation. Continuous exposure to normal soil moisture caused substantial reduction in the number of adventitious roots and consequent depression in total length but this could be attributed to sink competition among adventitious roots. Under fluctuating soil moisture, normal moisture content appeared to restore the aberrant development of the roots caused by deficient or excessive levels. The suppressed formation and elongation of the root system components under deficient soil moisture was alleviated when the soil moisture condition was changed to normal. On the other hand, the stimulated root formation and elongation in the excessive moisture was abated when the normal soil moisture condition prevailed afterwards. The result was the opposite when the soil moisture condition was reversed, i.e., normal then followed by either deficient or excessive soil moisture.

Root zone temperature (RZT) is an important factor that affects the establishment of crops in the field. This study was conducted to determine the effect of high RZT on the root system development of cassava and sweetpotato and to assess the response of each root system component of cassava and sweetpotato to high RZTs. Cassava and sweetpotato are widely grown crops in soils that are prone to reach a high temperature especially during the dry season. The plant root system was the primary object of investigation in this study. Cassava and sweetpotato cuttings were grown in pots for 20 days in a growth chamber where two different soil temperature regimes were maintained : 40°C (high RZT) and 25°C (normal). High RZT significantly decreased the total length of the adventitious roots (ARs) and the number and total length of the first order lateral roots (LRs) in both cassava and sweetpotato. Reduction in these root growth parameters was greater in the latter than in the former. High RZT caused a marked increase in the number of the second order LRs in sweetpotato and tended to enhance the formation of the third order LRs. Under high RZT, in cassava, a higher proportion of the total number of ARs and the first order LRs, and thus, a greater fraction of the total root length came from the lower nodes of the cuttings that were buried deep in the soil at planting. In sweetpotato, a greater proportion of the total number and total length of the ARs and the first order LRs was observed in the nodes of the cuttings that were buried closer to the soil surface than those buried deeper. An opposite trend was evident in both cassava and sweetpotato grown under a normal RZT. In the screenhouse experiment where plants were grown for 12 days, root growth of cassava was significantly reduced by the high RZT that prevailed when the soil was not covered with mulch materials. The root development in the mulched soil was similar to that under normal RZT in the growth chamber.

In spite of the important role it plays for water and nutrient acquisition, information on the root system development in cassava (Manihot esculenta Crantz) is limited. To examine the root length and branching pattern with reference to shoot growth and tuber bulking, we grew cassava plants in containers under natural climatic conditions in the southern end of Sumatra Island, Indonesia. One 20-cm length cutting of cassava (cv. Ardira IV) was planted in either a plastic bucket or a wooden box. The containers, which were filled with heavy clay soil, had different sizes depending on the growing period. At 30, 60, 100, 140, 180, and 270 days after planting (DAP), both the shoot and roots were sampled for quantitative analysis. The dry weight of both shoot and roots increased rapidly with the leaf area. The axile root number, however, decreased from 60 to 140 DAP as a result of the abscission of roots emerging from the basal part of the cutting during tuber bulking. The total root length reached its maximum at 60 DAP and significantly decreased thereafter because of decay and decomposition during tuber bulking. On the other hand, the root branching either increased the branching order or retained it, as determined from a topological point of view. The root branching during the later growing period compensated for the decrease in total root length and contributed to maintain a sufficient root surface area. The surviving roots with a well-developed branching pattern could absorb water and nutrients essential for tuber bulking.

The soil sheath (SS) distribution and its change with the passage of time in maize root systems were investigated. Maize seeds were planted in deep pots, and the roots were sampled at 2, 3, 4 and 6 weeks after sowing. The distribution of SS on the axile roots and the 1st order lateral roots was examined quantitatively. On axile roots, as classified by the nodes from which they emerged, younger roots were more thickly sheathed than older ones. A similar tendency was observed in the 1st order lateral roots. Significant correlations were observed between the thickness of SS, and the root length or the diameter in one part of the axile roots. L-type 1st order lateral roots (branched 1st order laterals) were sheathed more than S-type lateral roots (non-branched). Quantitative analysis revealed that the amount of SS was closely related to the root length, thickness, branching capacity, and root age.

Plant growth is sometimes restricted with soil compaction under no-tillage conditions, although undisturbed soils are favorable to arbuscular mycorrhizal (AM) fungi of a symbiont. We examined growth responses of the pigeon pea (Cajanus cajan (L.) Millsp.) to soil disturbance and inoculation with an AM fungus (Gigaspora margarita Pecker & Hall) in a pot experiment. The AM fungal inoculum was added to the soil and wheat was grown. After 6 months the shoot of wheat was removed and the soil was either disturbed or remained undisturbed. Subsequently, pigeon pea was grown and harvested after 3 months. The colonization and spore density of Gigaspora were significantly greater in undisturbed soil than in disturbed soil. Undisturbed soil showed higher penetrometer resistance and resulted in impaired shoot growth of the pigeon pea with lower shoot-to-root (SIR) ratio than disturbed soil. However, inoculation with the AM fungus reduced the stress impact of undisturbed soil on the pigeon pea without affecting the soil resistance and SIR ratio. A possible reason for reducing the stress impact was increase in specific root length, rather than P inflow with the AM formation. It is a novel finding that AM formation in undisturbed soil could promote root elongation despite the fact that soil was seriously compacted. (C) 1998 Elsevier Science B.V.

This paper describes a technique to monitor the root elongation rate (RER) per hour for several days, and variation in RER during the day and night. Rice (Oryza sativa L.) and sorghum (Sorghum bicolor Moench) were grown in root boxes placed inside a growth chamber set at 25 degrees C with a 12 h photoperiod. Seminal root axes were sandwiched between a transparent acrylic board and filter paper placed on a loamy sand soil. The roots were photographed under dim green light using a CCD camera connected to a time lapse video recorder. The environment of the root, including temperature, light, nutrient, water and air supply, was controlled precisely and maintained constant. RER fluctuated hourly in sorghum and to a greater extent in rice. Maximum RERs were 1.4 to 4.4 limes faster than minimum rates. RERs during the day and night did not differ statistically when temperatures were the same. (C) 1998 Annals of Botany Company.

Penetration of the roots deep into soil layer (deep roots) may alleviate growth inhibition under various soil stress conditions. In this study, the nodes from which deep roots had emerged were examined at the heading stage in rice and maize grown in a 2 m long tube. The effect of soil mechanical stress on the rooting nodes of deep roots was also examined. The roots that emerged in a relatively early growth stage, that is, the roots from coleoptilar, 1st and 2nd node in rice, and the seminal root and roots from the coleoptilar, 1st and 2nd nodes in maize, penetrated into the deep soil layer. The node which produced the highest number of deep roots was the 1st node in rice and the coleoptilar node in maize. Seminal root of rice and seminal adventitious roots of maize did not penetrate into the deep soil layer although they emerged at an early growth stage. In the rice root system, the nodal roots, emerged from the upper portion of the node, tended to penetrate deeper than the nodal roots emerged from the lower portion of the same node. Soil compaction did not affect these tendencies.

To elucidate genotypic variations in the root system architectures of rice, which are determined by the emergence (branching) and elongation of lateral roots, we compared the development of seminal root systems (seminal root axis and lateral roots) among seven cultivars with different ecotypes using an in vitro culture method which can simplify growth conditions. Four different culture treatments were prepared by combining two factors (nitrogen source in medium and presence/absence of shoot) to evaluate genotypic variations as a whole. After three weeks of culture, sampled seminal root systems were developmentally, topologically and geometrically analyzed. First, we researched the responses of root system development to culture conditions. It was especially notable that the responses of L-type and S-type first order lateral roots were completely different, and that root system size was affected by both the composition of medium and the presence/absence of shoot, while branching pattern was mainly controlled by the former. Though it was difficult to find a general trend in cultivar variations throughout the treatments, we characterized and classified the seven cultivars mainly based on the root system size and responses to culture conditions. Instinctive genotypic variations were clearly recognized under culture conditions as compared to soil conditions. Thus, this study showed the possibility that selection of genotypes focused on architecture could be facilitated using an in vitro culture method.

We analyzed and compared the development of the seminal root systems (seminal root axis and lateral roots) of seven rice cultivars grown under two different soil moisture conditions (upland and paddy treatment). The aim was to elucidate cultivar variation and environmental effects with emphasis on the configuration determined by the emergence and elongation of lateral roots (architecture). After growing in a root box for three weeks, the seminal root systems were sampled. The architectures were evaluated using topological, geometrical and fractal analyses in parallel with developmental analysis. Root system development laid emphasis on branching (emergence) of lateral roots under upland treatment and their elongation under paddy treatment. The cultivar variations in total root length were mainly due to variations in total root number and average external link length in the former and latter treatments, respectively, which are similar to those emphasized in each treatment. We found it possible to determine morphological differences in root systems among cultivars or treatments using mathematical parameters. However, we also found that to evaluate a root system architecture exactly, two features of the ''complexity of branching pattern'' must be understood (i.e., the degree of higher branching order and the frequency of lateral root branching using two parameters, such as topological indices and branching density). Furthermore, the significance of fractal dimension must be clarified according to the strategy of root system development.

Different responses among legume species were observed. but the morphological and physiological differences that confer drought resistance or susceptibility are not well explained. The objective of this study was the determination of variation of morphological characteristics within 7 field bean and 4 field pea cultivars as related to drought tolerance. Also differences in the effect of drought on seed germination and seedling growth in 2 field bean and 2 field pea cultivars of different drought tolerances were investigated. The examined cultivars were characterized by variation of certain morphological characteristics regarded as xeromorphic features associated with the ability of plant to survive under drought. The drought resistant cultivars (field bean Gobo and field pea Solara) in comparison with the sensitive ones (field bean Victor and field pea Bareness) were characterized by more favourable relations between the size of the above-ground parts and the size of root, as well as the frequency and size of stomata. Moreover, in the resistant cultivars there was observed, a smaller influence of simulated drought (Psi = -0.6 MPa) on the increase of dry matter of the above-ground parts and of the roots. Also there was smaller influence on the height of seedlings and on the length of lateral roots. The correlation coefficients between the measured characteristics and the values of the drought susceptibility index (DSI) were in most cases statistically not significant, although, on the whole, they were very high. This may be an indication of a relatively high participation of the measured characteristics in the total variation of the drought tolerance in the cultivars. In cultivars regarded as belonging to the group of sensitive ones, a more disadvantageous effect of simulated drought (Psi = -0.6 MPa) on seed germination was observed, especially in the determination of the promptness index (PI).

Relatively little research has been conducted to determine different responses to drought among cultivars of the legume species. The objective of this study was to identify differences in seedlings growth, water relations and leaf conductances resulting from drought imposed on two field bean and two field pea cultivars that had been observed to differ in their drought tolerances, and special emphasis was placed on the root system development. Distinct differences between resistant and sensitive cultivars of field bean and field pea became evident in measurements of the characteristics of the lateral root. The drought treatment induced statistically significant decrease in the number of the developed laterals, their total length and dry matter. In the drought resistant cultivars (field bean Gobo and field pea Solara) this reduction was smaller in comparison with sensitive ones (field bean Victor and field pea Bareness). The effect of drought on growth of tap root in the drought resistant and drought sensitive cultivars was smaller and statistically not significant. The results showed that drought resistant cultivars when compared with drought sensitive one would demonstrate less abundance in the above-ground part and greater dimensions of the root system. The measurements of leaf water potential and stomata diffusive resistance measurements indicate that the physiological reasons for the different reactions to drought between the resistant and the sensitive field bean and field pea cultivars may be due to a more effective protection of the level of tissue hydration and due to increase stomata diffusive resistance in the resistant cultivars. During recovery period it has been also demonstrated that in the drought resistant cultivars a tendency exists for a more complete return to the level of the control plants.

Plant roots produce abscisic acid (ABA) and zeatin riboside (ZR) which are known to counteract the aging of plant organs. Changes in ABA and ZR levels were determined by an enzyme immunoassay method in rice roots, in order to evaluate their roles in root system development, especially of seminal root axis (SRA) and lateral roots (LR). Rice planes were grown for 35 days after sowing (DAS) under submerged soil conditions in root boxes. In the seminal root system, ABA and ZR levels reached the highest peaks at 10 and 21 DAS, respectively. The ABA peak corresponded with the times when the nitrogen concentrations in LR decreased to the lowest level and the ZR peak coincided with the 2nd peak of the nitrogen level, as indicated by our previous finding. A drastic increment in ZR level in the seminal root tip at 6 DAS coincided with rooting of 2nd order LR and closely related to emergence of 4th leaf and Ist node nodal roots, which indicated the significant role of ZR in the early development of rice seedlings. Comparison of LR and SRA revealed that SRA showed a much higher ZR level and much lower content ratio of ABA to ZR than those of LR. Furthermore, the ratio in the seminal root tip was very similar to that of SRA. This indicates that the hormonal characteristics of LR and SRA. would be far different.

For the topological analysis of plant root systems, two sets of topological indices have been proposed; the ratios of measured altitude (a) and total external pathlength (Pe) to the expected value of the respective parameter when lateral root branching is assumed to occur randomly {a/E(a), PelE(Pe)} and to the maximum value of the respective parameter when all lateral roots emerge from the main root axis (herringbone type) {a/Max(a), Pe/Max (Pe)}. We traced the timecourse changes in these indices with root system development of two rice cultivars grown with the excised root tip culture method, and evaluated their adaptability for the actual complexity of the branching pattern. The a/E(a) and Pe/E(Pe) did not adequately express the progress of complication with root growth and the difference between cultivars in branching pattern because root system size substantially affected the values. On the other hand, changes in a/Max(a) and Pe/Max(Pe) showed close correlation with those in several root developmental characters such as the number and mean branching order of lateral roots (MBO). Thus, we conclude that a/Max(a) and Pe/Max(Pe) are better indices than a/E(a) and PelE(Pe) for the topological characterization of morphology in the developmental study of rice root systems.

Aging pattern of rice seminal root system grown under submerged soil conditions was evaluated by the measurement of succinic dehydrogenase (SDH) and peroxidase (POD) activities with special reference to the difference between the basal 11 cm-portion of the seminal root axis (SRA) and the lateral roots (LR) initiated from this portion. SDH activity in both SRA and LR showed high levels just after their initiation, and subsequently these activities decreased sharply with aging. The activity in LR always exceeded that in SRA. The percentage of the LR that positively reacted to the reduction assay of triphenyltetrazolium chloride was 100% in the first 4 days after initiation, followed by a drastic decrease ranging from 14 to 24%. Even at heading stage, however, about 8% of the LR still showed a positive reaction. In contrast, POD activity in both SRA and LR increased with aging. The activity in LR, again, always exceeded that in SRA, and attained the highest level which was 3.6 times higher than that in SRA at heading. This elevation of POD activity with aging was discussed in terms of induced protective reaction which may delay senescence. These results indicated that the senescence of SRA precedes that of LR, and LR plays a major role in the physiological activity of the seminal root system. In addition, the rice seminal root system was found to be alive up to the heading.

The development of cultured seminal root system of rice was analyzed to determine the most appropriate culture period for the characterization of root system morphology. Excised seminal root tips of Taichung Native 1 (TN-1, indica type) and Yukara (japonica type), were cultured for six weeks with the conventional tissue culture method, and their growth was evaluated with developmental, topological and fractal analyses. Developmental root growth parameters, such as the number and length of each root system component, reached almost maximum values at the fourth week in TN-1, and the third week in Yukara. On the other hand, it was possible to characterize the root system morphology by the branching densities of L- and S-type first order lateral roots and the average length of S-type first order lateral roots at the third week. In the timecourse changes of a topological parameter (log a/log mu), which indicates the branching pattern, the difference between cultivars became clear at the third week. The fractal dimension (D), which indicates the complexity of root morphology, was higher in Yukara than in TN-1 throughout the experimental period. This suggested the possibility for characterizing the root system morphology of each cultivar by the D value. Therefore, we concluded that the most appropriate sampling time for the morphological research of cultured seminal root system was three weeks when the conventional excised root culture method is used.

The allocation pattern of carbon (C) and nitrogen (N) with aging among several organs of rice seedling was investigated with particular attention to the difference in seminal root axis (SRA) and concomitant lateral roots (LR). Rice (Oryra sativa L. cv. Aichiasahi) was grown in root boxes under submerged conditions for 35 days after sowing (DAS). Overall C and N concentration of LR were higher than those of SRA, and the difference was more pronounced in N concentration. In the seminal root portion of the same age, C and N content decreased in both SRA and LR with progression of age, however a trend of declining N content in LR was alleviated by the rooting of higher (2nd order) LR. The ratios of root length to C content and dry weight in LR were 27 times that of SRA, and the ratio of root length to N content in LR was 12 times that of SRA after Day 21. These results suggest that the aging patterns of LR and SRA are different, and LR plays a more important role in controlling the metabolic activity of the whole root system than SRA. Moreover, increase in N content in SRA after Day 12 and the increase in the number of LR up to 35 DAS suggested that the seminal root of rice was alive and maintained its metabolic activity for at least 35 DAS.

Tissue culture of excised root is a useful method with which genetic variation of plane root itself can be determined without the influence of shoot. We aimed to determine the effects of age and elongation rates of the seminal root axis of rice on subsequent growth in a culture medium. Taichung Native 1 (indica type) and Yukara (japonica type) were used in the experiments. The seminal root tips (1-cm-length) were sampled every day after bedding on agar and their morphological and physiological characteristics were monitored by recording the persence of lateral roots and primordia, dry weight and C . N content. Root tips with different ages or elongation rates were then cultured, and subsequent growth was observed after three week culture. Root tips which were older and had lower elongation rates showed inferior root growth in both cultivars. When a mote than three-day-old seminal axis on which lateral roots started to emerge was excised, less L-type fuse order lateral roots was produced than that grown on the two-day-old axis, and this affected the total root number and length. The dry weight or C . N content of excised segments decreased as the excision day, and these characteristics showed a close correlation with the subsequent root growth. Thus, we concluded that the excision of root tip segments at an earlier stage ensures excellent development of seminal root system in vitro.

Effects of soil mechanical impedance on the development of Golgi apparatus in the root cap cells of maize were studied under controlled soil-water conditions Heavily compacted soil (bulk density = 1.50 g cm−2) had 3.3 to 3.4 times greater mechanical impedance than control soil (bulk density = 1.33 g cm−3), but their oxygen diffusion rates were not significantly different The number of dictyosomes and the number and area of secretory vesicles per unit area of tangentially sub-peripheral root cap cells in the heavily compacted soil increased compared to those in the control These results suggest that secretory activity of the root cap cells is promoted by soil mechanical impedance

The length of the seminal root (SR) axis and the number and length of lateral roots (LRs) of sorghum (Sorghum bicolor Moench) were markedly inhibited by taro [Colocasia esculenta (L.) Schott] residues incorporated into a sand growing medium. The sand profile was divided equally into zones with and without residues. Production and elongation of the first-order LRs of the SR axis facing the zone containing taro residues were severely suppressed. On the side facing the zone that was free of residues, production and elongation of LRs was not inhibited. SR and LR growth was drastically impaired and many plants were killed when taro residues were incorporated in large amounts into the uppermost 2 cm of the growing medium. The activity of the allelopathic substances in the root zone appeared to be location-specific.

The aim of this study was to determine the effects of soil compaction on the development of root system components of rice (Oryza sativa L.) and maize (Zea mays L.). Plants were grown for 4 weeks in root boxes (24 cm long x 2 cm wide x 40 cm deep) with soil bulk densities of 1.33 g/cm3 (control) and 1.50 g/cm3 (compact). In the compact treatment the main root axes of rice never penetrated beyond the 10-15 cm soil layer even by the fourth week, while seminal and seminal adventitious roots of maize had penetrated 30-35 cm deep by the third week. Generally, growth of higher order (second and third) lateral roots compensated for the restricted growth of the main root axes in both species. The ratio of first order L-type laterals producing higher order laterals on their axes was greater in the compact treatment for rice, while that of maize was not significantly increased. The root growth responses of rice and maize to soil compaction were different in the downward penetration of the main axis and the growth of the higher order laterals.

Comparison of the root system structures of four cereal species as affected by soil compaction was conducted morphometrically. Plants were grown under two different soil bulk densities of 1.33g/cm3 (control)and 1.50g/cm3 (compact) for two weeks in the root box, and measurements were made on the number, length and diameter of all the roots from every component of the root system. Promoted or less restricted growth was recognized in the mean length and number of higher order laterals in every species under compact treatments. The increased occurrence of L-type lateral roots in the compact treatment contributed to the promoted growth of higher order laterals. Compared to upland rice and Job's tears, sorghum and maize showed higher restriction in both shoot and root growth. As for the occurrence ratio of L-type laterals in the compact treatment, the former two species had 1.5 to 3 times that of the latter two species. Regardless of root diameter, the species with longer root length showed higher restriction of total root length in both 1st and 2nd order laterals. It was suggested that the ability to produce L-type laterals and extension rate of each order lateral roots were closely related to root growth responses under mechanically impeded soil conditions.

根箱内 (幅24×厚さ2×深さ40cm) 土壌の硬度の指標となる貫入抵抗値の測定装置を試作し, 容器内土壌における測定結果の有用性の検討を行った.この装置は, 直径2.3mm, 先端頂角30゜, 長さ50cmのピアノ線を探針とし, ひずみゲージ, 変異変換器などを組み合わせたシステムである.容器内土壌の測定に際して問題になるedge effectは, 6段階の異なる直径の探針を用いて検討したところ, 測定上の影響は, 無視できる結果が得られた.貫入抵抗値のポイント測定と連続測定との比較を行ったところ, 探針のロッドと土壌との摩擦抵抗は, 表層下10cm以降では積算されず, ほぼ一定の値を保った.両者の測定値における, 土壌深度別推移のパターンはよく一致していた.また, 土壌のage hardeningを貫入抵抗値の面から測定した.以上の結果から, 根箱内土壌の貫入抵抗値の土壌深度別推移を, 相対値として40cmの深度まで簡易に連続的に測定するうえで, 本機の有用性を認めた.

土壌容積重, 1.33g/cm³ (対照区) および1.50g/cm³ (圧縮区) に調整した根箱 (幅24×厚さ2×深さ40cm) を用い, 土壌の貫入抵抗値の垂直方向 (根箱内土壌深度) の推移と, これらの根箱で生育させ, ピンボード法で採取した2週齢の陸稲根系の肥大生長との相互関係を検討した.土壌の充填方法に起因する層界面の存在による貫入抵抗値のピークが, 根箱の全層にわたって数ヵ所に出現した.この貫入抵抗値の垂直パターンは, 圧縮区においては種子根軸に沿った各根系構成要素 (種子根・1次側根) の根径の変動パターンとよく一致した.このことは, 根箱内土壌の貫入抵抗値の推移のパターンは, 根端がより高い機械的抵抗に遭遇したときの根系の肥大生長の履歴として根軸に記録されたことを示すものである.本実験の結果は, 根系-土壌間の相互作用の研究において, 土壌の機械的抵抗の分布パターンにも留意する必要性を示唆した.

https://doi.org/10.1007/978-981-10-7308-3

（全ページ）p.406 （共著者）日本作物学会編 安藤・---・飯嶋・---（102名との共同執筆のため以下省略） （分 担） pp.136-137. （概 要）日本作物学会の用語委員会がとりまとめた基本用語の解説書。形態部門の「根系」を解説した．

（全ページ）p.208 （共著者）坂上潤一編、朝川・--・飯嶋・（39名との共同執筆のため以下省略） （分 担） pp.242-244 （概 要）第6章 5．湿地帯でのイネ栽培と塩害 湿地における洪水被害と作物栽培技術の活用の事例として、ナミビア国北中部地域に形成される季節性湿地帯への稲作導入課程と塩害を紹介した．

（全ページ）p.208 （共著者）森田茂紀編、阿部・飯嶋・（以下28名との共同執筆のため省略） （分 担） pp.156-161（概 要）植物の根系が積極的に環境を改変する事例として、インドネシアの傾斜地コ－ヒ－園での事例を紹介した。

At present, simultaneous occurrence of floods and droughts in the same place has become common globally. Under such weather conditions, crop cultivation techniques that might accommodate both extremes of water abundance are required to stabilize crop production. During the process of development of a mixed cropping system of flood-tolerant rice and drought-tolerant grain crops, we found a new possibility in mixed cropping—enhancement of the flood tolerance of upland crops. The “close mixed-planting” is the technique growing the two plant species in a same portion where their root systems tightly tangled each other. This technique improved the growth and yield of upland crops subjected to flush flood stresses. Oxygen transfer was confirmed between the two species mix-cultured in water, implying its contribution to the phenomenon that improved the flood tolerance of upland crops in fields. This technique also enhanced the drought tolerance of rice by the hydraulic phenomenon. Water absorbed by the roots of upland crops from deep soil is released into the shallow soil during night. Close mixed-planting enabled rice to capture the released water resulting the improvement of drought resistance of rice. Mixed cropping of wet and dryland crops is a new concept that will help farmers overcome both flood- and drought stresses under variable environmental conditions due to climate change.

本研究では、申請代表らがこれまでに公表した「萌芽的な技術提案」である、「根粒着生制御」と「接触混植」の複合が、ダイズの湿害ストレス緩和に有効であるかどうかを明らかにすることを目的とする。前年度までの研究進展により、奈良県、滋賀県、兵庫県の近畿地区3県にまたがるフィ-ルド試験を実施し、イネとダイズの接触混植と亀裂処理という萌芽的な栽培技術の融合を目指した基礎研究を展開し、研究目的を達成する。まず、奈良県では近畿大学に隣接する農家圃場と、近畿大学実験圃場内の小型のコンクリ-ト制実験圃場において、接触混植試験を実施した。農家圃場では、イネが先行するリレ-播種によってイネ／ダイズの接触混植試験を実施した。現場での作業体系を考えると接触混植苗をセルトレイで栽培する手法は社会実装には程遠いため、接触混植の直播技術としての成立を目指す。小型の実験圃場では、競合と補完関係を検討するため、イネとダイズの栽培距離を数段階替えて実験を実施した。滋賀県立大学においては、7アール規模の実験水田で前年度と同様な接触混植―亀裂処理実験を実施した。亀裂処理は別研究で開発した亀裂処理機を使用して実施した。亀裂処理の実施後に、長期間の干ばつ被害が発生し易いため、転換畑に備えられた灌漑水を用いてフラッシュ灌漑を実施した。兵庫県立農林水産技術総合センターでは、黒ダイズの丹波黒を供試して亀裂処理試験を実施した。黒ダイズにおける生育初期の土壌湛水ストレス耐性は不明であるため、本年度は初めてのフィールド試験となった。これまでの成果をまとめた学会発表を行うとともに投稿した原著論文が採択されオンライン公表された。

新型コロナウイルス（Covid-19）禍により渡航が制限されたため、オンラインによりナミビア、ボツワナ、日本の3ヵ国での試験や研究成果を詳細に打ち合わせ、以下の研究と、研究成果の共同執筆・国際会議での報告などを実施した。① 接触混植試験：ナミビア北部では、ソルガムとトウジンビエの接触混植が農家の自助努力などにより徐々に広がりを見せている。そこで、ナミビア大学オゴンゴ校内の附属農場施設内で、トウジンビエとソルガムの接触混植試験を実施した。農家圃場においても栽培を希望する農家に種子を提供し、農家圃場における栽培試験を実施した。② 亀裂施肥試験：本試験のためにナミビア国で作成した新型の亀裂施肥機をベースにして日本ではコストを考慮し、亀裂処理に特化した新型アタッチメントを作成した。その走行試験を神戸大学等で実施し、今後の普及を検討中である。 ③ ボツワナ大学では、ナミビアから導入したイネ品種群の栽培試験を2019/2020作付け期に開始し、2021/2022まで3か年に渡る栽培トライアルを継続中である。2021/2022からは小規模フィ―ルドでの作付けを開始し、種子の更新と栽培試験を拡大した。 ④ ナミビアとボツワナの研究サイト周辺の農家圃場では季節湿地が形成されるためメタン発生の増大が懸念される。そこで地球温暖化の進行を阻止するためウルトラファインバブル水灌漑によるメタン削減の可能性を議論し、現地への導入を目指したフィールド試験を日本で開始した。 ⑤ 研究成果5報の原著論文がアクセプトされた。 ⑥ 国際学会（アジア作物学会議）で４報の口頭発表と、国内学会（日本作物学会）で２報の口頭発表を実施した。

ウルトラファインバブル水の作物成長促進効果を明らかにすることを目指す。

アフリカの半乾燥地には、洪水や干ばつによって食糧不足になるリスクが高い地域が依然として多く残されている。砂漠国ナミビアでも季節湿地の不安定な水環境が問題となっており、食糧安全保障の観点から現地農業を再構築する必要がある。そこで本研究では、頻発する洪水や干ばつ年でも常に一定以上の穀物生産が維持されるような新しい栽培システムを考案する。新規導入作物のイネと現地主食のトウジンビエを混作し、水収支と経済性を評価する。社会・自然環境インパクトから、現地の自給自足農民の生活向上に資する農法の導入と半乾燥地の水環境保全とを永続的に両立させ、南部アフリカに広がる季節湿地を最大限に活用したモデル農法を提案する。（参照（参照 https://www.jst.go.jp/global/kadai/h2306_namibia.html および https://www.jica.go.jp/oda/project/1100521/index.html））

本研究では、外部から根粒菌を投与し、ダイズに人為的に根粒着生を促すことで増収を可能にする根粒着生制御技術 (亀裂施肥)の有用性を明らかにすることを目的とした。さらに、北海道と本州という異なる気象条件で亀裂施肥を実施し、本技術の普遍性を検討した。寒冷地である北海道では、本技術の十分な増収効果は得られなかった。しかし、本州では、畑地と水田転換畑において増収効果が確認された。畑地では中耕時に条間に亀裂に沿って炭のみを投与する処理、水田転換畑では、播種前の条直下根粒菌を付与した炭を投与する処理が、最も増収効果が高かった。

砂漠国ナミビアには、隣国からの洪水により雨期の間だけ形成される広大な季節湿地がある。本研究では、これまで作物生産に活用されてこなかった水資源に注目し、新たに稲作を導入することにより、水資源開発と環境保全との両立を検討した。その結果、半乾燥地におけるトウジンビエ在来農法と稲作とが融和しうることと、低位収量レベルであれば水環境を改変しない稲作導入が可能であることを示唆した。

ダイズの窒素固定能力を栽培技術により向上させることは,我が国におけるダイズの収量性を改善するための技術戦略の一つである。本研究では、「亀裂施肥」という新しい栽培技術を提案した。この技術は、ダイズの開花期直前に心土破砕を実施し、土壌深層部に根粒菌や肥料等を投入する技術である。亀裂施肥後、3週から6週目にかけて根粒の窒素固定能力が顕著に促進するとともに下層土壌の根粒数が有意に増加した。これらの結果は、亀裂施肥によりダイズの根粒着生を制御することが可能であることを示した。

今年度の研究では,飛行時間型2次イオン質量分析計(Tof-SIMS)を活用し,安定同位体でラベルした重水の吸収過程を視覚化し,養水分の吸収と移動経路を明らかにするための方法論を確立することを目的とした.まず,これまでに実施したTof-SIMSを用いた作物の水吸収に関わる実験系についての問題点を確認した.すなわち「よく晴れた日の蒸散が盛んな条件下では,根から吸収した水は木部ではなく師部を介して茎部の皮層細胞などの柔組織に分配される」という本研究で得られた新たな知見に対して,投稿した国際雑誌の審査委員から,蒸散がはたして盛んに行われているかどうかの判断基準を強く問われた.そこで,日射と,蒸散量,蒸発量との関係を調査したところ,植物体のサイズにより,蒸散速度と培地からの蒸発量との関係が異なることが判明した.今後,実験条件をそろえるための工夫が必要であることがわかり,予備的な検討を始めたところである.上記の論文投稿は,最終的に不採択となったため,指摘された諸点を訂正した後,別の国際誌に現在投稿中である.他の成果として,根粒への水の流入経路は,木部や師部からではなくアポプラスト経由の水の流れが主体をなす,という結論について関連データを補足実験により確認したうえで,学会発表を行った.他研究者と議論したうえで,本成果についても執筆を開始した.さらに,培地からの水吸収と水放出現象についての研究成果についても,その整理を開始したところである.本研究の最終年度であるため,本研究で得られたすべての成果から,今後の研究の方向性についてさまざまな視点から議論した.

アフリカ南西部に位置するナミビア国は、国土の大半が乾燥地に属する砂漠国である。本研究では、北部地域に広がる未利用の季節性湿地帯の基本的な水環境を改変せずに、あるがままの湿地帯に適合する稲品種を導入し、さらに当地の貧農によるトウジンビエ栽培に融和するような稲栽培体系を提案することを目的とした。まず、海外共同研究者、分担者とともに4年間の研究の方向性を議論し、トウジンビエの水利用試験とともに、水稲の品種選抜、ストレス耐性評価、農家圃場での栽培試験などを実施することとした。その結果、トウジンビエは湿地帯近傍の湿性環境に弱く、かならずしも湿地の水を利用し得ないことが明らかとなるとともに、ヒエとカウピー間作栽培では、サヘル地域で実践されているような播種期をずらすことによって雨水の競合が起こりにくくなることを示唆した。西アフリカ地区で栽培されるアジアイネ、アフリカイネ、両種の種間交雑系統(ネリカ品種群)合計130品種・系統を供試し、湿地帯の中心地と東部の多雨地帯で栽培試験を実施した。3ヶ年の繰り返し試験を実施しているため、本プロジェクト完了直後に、両地区で有望な数品種・系統が選抜される予定である。なお湿地帯では乾季に塩が集積する地域が多数みられるため、塩ストレス耐性選抜試験も実施し、その結果インド在来のNona Bokraが有望であることが判明し、現在種子を増殖中である。さらに、トウジンビエとイネの栽培を一連の作業体系の中で融和させることを目指して、イネの収穫後調整農具を導入した。現地の自給農家へ普及させるため、日本古来の農具を作成し、現地に導入し、現地研究協力者と今後も引き続き研究協力を継続させることを確認した。

作物の根は分泌層で覆われており、この層を介して根圏土壌中の物質が吸収される。本研究は、根-土壌界面に存在する分泌層内での物質の動きを明らかにする方法論を検討し、安定同位体でラベルした水や窒素が根圏から分泌層を介して根面へ取り込まれる過程を明らかにすることを目的として、以下の研究を実施した。今年度は、昨年検討したシリコン板法とともに、寒天培地法も含めて検討を継続するとともに、飛行時間型2次イオン質量分析計(Tof-SIMS)による安定同位体の取り込みに関する実験も実施した。まずシリコン板法では直接、粘液を観察することが困難であった。そこで様々な濃度の寒天培地を試験したが、同様に寒天への封入では、真空中での脱水により寒天がくずれてしまいうまく観察ができなかった。そこで、やはりクライオシステムで観察する必要があることを認識し、まずは植物サンプルでの飛行時間型2次イオン質量分析計での観察手法の確立を先決課題として実施した。イネでは真空中での組織の保持がうまくできなかったため、茎や'根の直径の大きなダイズを用いて、真空中での観察を実施した。クライオミクロトームで切断した根と茎切片を観察したところ、すべてのイオンの総和であるトータルイオン観察によって木部導管や師部組織を観察することが可能となった。次に安定同位体として簡易な暴露が可能な重水を用いて、一定時間吸収させた後の重水の分布を観察したところ、木部導管部位では重水の組織への吸収を認めず、この手法での同位体の吸収を観察することが可能であることを確かめた。次にクライオシステムで同様の観察を行ったところ、より鮮明な観察が可能となった。今後はこれまで試したシリコン板法や寒天培地法をクライオシステムで観察する実験系を確立する必要がある。

本研究では、根圏土壌中における境界複合体の生態的な意義を多面的に解明することを目的として、以下の研究を実施した。初年度は、研究を開始するに当たり、研究代表者が海外出張し4ヵ年にわたるプロジェクト全体の計画調整を行うとともに、海外研究協力者のBarlow氏が来日し研究分担者とともに根冠細胞の生産速度に関する実験手法について討論し、次年度には、海外研究協力者のBryan Griffiths博士とMichael Bonkowski博士が来日し、土壌動物と境界複合体の関係について予備的な実験を実施した。以上の研究討論に基づき、まず、境界細胞の根圏土壌中における生存期間と崩壊速度を検討した。その結果、播種直後の1日目では、根冠の表層から崩壊し易い細胞群が根圏土壌中に放出されるが、その後、細胞の生産速度よりも放出量が上回るため、表層下2〜3層目の比較的壊れにくい細胞群が根圏土壌中に放出することを確かめた。すなわち、根冠の内層の比較的崩壊しにくい細胞群が根圏土壌中に放出されるため、それらの境界細胞が根圏土壌中で1ヶ月以上にわたって残存し続けることが明らかとなった。次に、側根が発生しない根軸領域に残存していた境界細胞の生存期間を調査したところ少なくとも10日間以上は残存することを確かめた。最後に、原生動物の存在下でのイネ品種群の成長促進を確認するとともに、この成長促進程度には境界複合体の放出量が関与することを定量的に示した。すなわち、境界複合体は、土壌の原生動物と植物との共生的関係を仲介する働きがあることを初めて実験値とともに提案した。また、共同研究者の森田、Barlow(海外共同研究者)とともに境界複合体ならびに根冠の形態と機能に関する総説を執筆した。

本研究は、サツマイモ、キャッサバにおける耐旱性品種の育成や乾燥ストレスが問題になっている地域における栽培技術の確立のための基礎的な知見を提供することを目的とした。フィリピン共和国のLeyte State University附属Philippine Root Crop Research and Training Centerと共同で、両種の品種・系統の耐旱性評価を通じ、耐旱性に関わる形質を特定しようとした。そのため、同研究所所有のものに加え、新たに広範な地城から遺伝資源を探索、収集し、評価に供した。また、同時に農家調査を実施し、農民の品種や栽堵技術を選択・導入基準を慎重に検討した。その結果、耐早性は、収量や品質と並んで、選択における重要な形質であることが裏付けられたが、市場性等他の様々な要因を総合的に判断していることが明らかとなり、今後の重要課題として残した。 キャッサバを対象にしたポット実験には、計28品種・系統を供試した。乾物生産=水消費量(蒸散量)×水利用効率の考え方を適用して解析を進めた。乾燥ストレス条件下で相対的に乾物生産が大きく、耐旱性の大きいと判定された品種は、乾燥ストレスに対し、水利用効率を上昇させ、気孔を鋭敏に反応させ、また、根-地上部率を増加させた。サツマイモについては18品種を対象にし、耐旱性が大きい品種は、乾燥条件下で、夜明け前、日中に葉身水ポテンシャルを高く維持する能力があり、気孔の反応も鋭敏であった。また、キャッサバとは対照的に、水消費量の差異が、品種・系統間差異をよく説明した。 圃場試験の結果は、ポット試験で評価した耐旱性をほぼ支持した。とくに、これまで広範に農家に受け入れられてきた品種の耐旱性が高いことが示された。 これらから得られた知見を実際の育種や栽培技術の改良に活かすためには、今後は、遺伝子型×環境相互作用の評価が必須である。

植物根の表層部は粘液等の分泌物に覆われていることが知られている。この粘液層は水や無機塩の植物体への取り入れを介在する働きをもつだけでなく、土壌のさまざまな物理・化学的ストレスから根を保護することが示唆されている。粘液層は以上のような重要な働きを担うにもかかわらず、この層がどのような土壌条件においてどの程度根の表面を覆っているのかは全く明らかにされてはいない。本研究では、植物根の根圏土壌中での粘液分泌動態を視覚的に明らかにするとともに、根圏土壌内の安定同位体炭素および水素の濃度勾配から、分泌物の土壌中への拡散範囲を定量的に把握することを目的とした。 3ヵ年の研究で、植物根による根圏土壌中への粘液分泌動態をCCDカメラにより視覚的に捉え、分泌が日周変動することを明らかにした。さらに、トウモロコシを^<13>CO_2とD_2Oで暴露し、根圏土壌中に放出される安定同位体の存在量を分析し、土壌の乾燥条件下では炭素の放出量は増加するが、水の放出量は減少することを明らかにした。しかしながら根圏土壌内の安定同位体の濃度勾配については、根に付着した土壌鞘の外側への同位体の放出が確認できなかった。現行の方法論では、根の周辺部における根圏土壌と非根圏土壌を選別すること、言い換えれば根毛の発達により形成される土壌鞘と非土壌鞘の選別にはある程度時間がかかる。そのため、解剖顕微鏡下での土壌の選別作業中に土壌水が蒸発し、その結果同位体分別が起こっていることが推定される。そこで、飛行時間型2時イオン質量分析計により土壌表面における同位体水素の検出を試みることを新たな目標として設定した。そのための予備的な試みとして、トウモロコシをD_2Oで暴露し、根軸内での安定同位体の分布を調査しているところである。現状では、同位体が根全体に均一に分布する像しか得られていない。この理由は現状では不明であり、今後、異なる条件下で測定を繰り返すことにより、技術的な問題点を解明していきたい。

ナミビア大学農学部の教員陣の能力強化のためナミビア大学講師を受け入れ、論文博士取得に向けた共同研究を実施した。

本研究では、根冠脱落細胞(Border cell;境界細胞)が土壌と根の界面に存在しクッションのような緩衝帯様の働きをするという仮説に着目した。すなわち、Border cellが担う生態的役割の一つとして、土壌と根の界面に生ずる摩擦抵抗は、根冠由来のBorder cellによって減少することを多面的に実証することを目的とした。 研究初年度には、日本側研究者3名が連合王国に出張し、海外研究協力者とともに実験を行った。2年目には、海外共同研究者1名が来日し、日本側研究者とともに研究に関する打ち合わせと研究討論を行った。さらに最終年度には、研究代表者がスコットランド作物研究所を短期間訪問し、海外共同研究者とともに研究打ち合わせを行い、これまでの成果の取りまとめと将来の課題について議論した。 研究成果の要約は以下の通りである。1)トウモロコシ種子根の根冠は土壌の抵抗を減少させる働きを持つことを初めて直接的に証明した。実験に用いた土壌条件下では、抵抗の約40%を減少させる働きを持っていた。これは根冠から分泌される粘液とBorder cellの両者の働きによるものと推定できる。2)根冠の存在によって減少した土壌の抵抗のうち、粘液により46%、境界細胞により54%の抵抗が減少することを明らかにした。3)境界細胞はトウモロコシでは単一の細胞ごとにイネでは層状に剥離脱落することを確かめた。このことは根圏土壌中での、境界細胞の存在形態が種間で異なることを示唆した。4)境界細胞を、粘土粒子を含む壌質砂土から分離する手法を考案した。壌質砂土で成育したトウモロコシでは、乾燥条件下において放出される境界細胞数は減少することを明らかにした。以上の結果は、境界細胞の根圏土壌中における生態的役割を明らかにしていくうえでのパイオニア的研究の一つと考える。

不耕起栽培は、土壌侵食防止や生産コスト低減において利点を有し、また化石エネルギーの消費節減やそれに伴う二酸化炭素排出の抑制など地球環境保全面からも注目されている。しかし、耕起作業の省略によって土壌硬度が増加するために作物根系の発育が阻害されやすく、養分吸収能の底下や倒伏性増大による収量不安定化が不耕起栽培の障壁となっている。多くの作物種の根には、糸状菌の一種VA菌根菌が感染して菌根を形成し、リンをはじめとする各種無機養分の吸収を促進させることが知られている。本研究は、この菌根共生系を活用することで、阻害された作物根系の発育を回復できることを示した。すなわち、菌根菌は植物の根内部に侵入する一方で土壌中に菌糸を伸長させ、菌糸のネットワーク構造を構築する。この構造は各種無機養分を土壌から宿主植物へ供給する役割を担うとともに、次の宿主植物を探索する機能も有している。通常の耕起土壌においては、このネットワーク構造が物理的に破壊されるが、攪乱の少ない不耕起土壌では構造が保持され、後作物の菌根形成を促す。この結果、土壌硬度が増大した不耕起土壌条件下で作物根系の発育阻害を引き起こす圧縮土壌ストレスを大きく緩和させることを明らかにした。また、菌糸ネットワーク構造の破壊に伴う菌根菌感染能の低下は菌種によって異なり、他の菌種の存在にも影響されることが明らかとなった。さらに、根-菌糸(菌根)ネットワークの機能として、作物根の分泌物によって難溶性リン酸から溶出するリン酸イオンを、根圏に存在する他の微生物との競合を乗り越えて宿主作物まで輸送する実態を明らかにし、根と菌との相互作用が重要な役割を果たすことを実証した。

植物の根冠からは粘液が分泌され、根冠周辺部において粘液層が形成される。粘液層は植物根の生長や機能に対して様々な役割を担っていることが指摘されている。しかし、土壌中での粘液分泌の実態が不明であるため、とくに土壌と根の相互作用に関連した役割については、推測の域を出ないものが多い。そこで、本研究では土壌中を伸長している根から分泌される根冠粘液を直接観察する方法を検討し、それが土壌中における粘液の分泌動態を把握する方法論となりうるかどうかを判定することを目的とした。実体顕微鏡を取り付けたモノクロチルドCCDカメラで土壌中を伸長しているトウモロコシ種子根の根端を、透明なアクリル板越しに撮影した。この方法によって、種子根の根端部とアクリル板との間に粘液状の物質が観察された。この物質が根冠粘液であるかどうかを、パネル上の粘液の残存性とその染色性、さらに、移植固体の根端部と針金の先端部の観察の比較等によって総合的に評価した。その結果、この物質が根冠粘液であると結論した。本法によって、粘液の分泌動態を把握しうるかどうかを確かめるため、画像として取り込んだ粘液の分布範囲を面積として定量化することによって、分泌の日周変動を調査した。その結果、粘液面積は、夜間の22時から4時にかけて大きく、昼間の10時から12時に最低値を示した。また、最大値を記録した24時の面積は、最低値であった10時よりも約40%大きな値を示した。以上の結果から、土壌中で分泌された根冠粘液の増減には日周変動があることを明らかにし、本法によって土壌中における根冠粘液の分泌動態が把握しうると結論した。さらに、根冠粘液の分泌と根の伸長生長との関係についての基礎的知見を得るために、分泌量と根の伸長速度との関係についても検討した結果、粘液の分泌量は種子根の伸長生長と関連性があることを定量的に認めた。

一個体の作物の根系を構成する根は、発育学・解剖学的差異を有することが明らかにされつつある。本研究はイネ科・マメ科作物を対象に、それらの根の間の機能的差異を解明しようとした。 まずイネ根系において、根系構造を解析するためには、従来から用いられてきた発育学的解析法に加え、根系構造の複雑さを定量化するフラクタル解析や、分枝構造を定量化するトポロジー解析が有効であることを示した。次に、根箱における土壌水分条件や、根端培養法における培養条件を変化させた場合、とくに側根の発育が鋭敏に反応することを示した。また炭素・窒素の動態や、コハク酸脱水素酵素・パーオキシダーゼの加齢に伴う変動を調べ、根軸と側根とではエイジングのパターンが異なることを明らかにした。さらには内生アブシジン酸やゼアチンリボシドの濃度比や動態の特徴から、側根と根軸は器官としての特徴を異にすることも明らかにした。また数種の主要なマメ科食用作物種の根系構造には、とくに側根の発達程度に起因する明瞭な遺伝的差異が存在し、さらに主根の伸長生長は、異なる環境条件に対して可塑性を発揮することを認めた。またラッカセイ根系において、菌根形成はその大部分が側根上で認められること、各根軸上では形成頻度は向頂的に増加し、その傾向は相対的に齢の進んだ1次側根でより顕著に認められることを明らかにした。また菌根菌接種に対して1次側根と2次側根は異なる発育反応を示し、その結果根系形態が大きく変化することを見出した。 以上のように本研究は、一個体の根系を構成する根の性格をそれぞれの齢に注目しつつ調べることにより、それらは環境要因に対する発育反応、物質代謝・生理活性、菌根菌との親和性などにおいて明瞭に異なり、機能を分担しつつ根系全体の機能の発揮に貢献している実態を明らかにした。

土壌の物理性が不良な耕地条件下では、作物根の伸長生長は抑制されることが知られている。この伸長生長の低下率は、ある一定期間における根の伸長速度として表示されるが、これまでの研究では1日単位の伸長量よりもさらに微細な伸長速度にふれた報告はほとんど見あたらない。そこで、本研究では土壌のストレスに対する根の伸長抑制を評価する上で、これまでの研究で欠けていた、時・分単位の伸長生長の変動を明らかにすることを目的とした。具体的には、根の伸長生長における日周性リズムが存在するか否か、さらにリズムが存在する場合にはそれが内因性のリズムであるか否か、を明らかにすることを本研究の到達点として設定した。この目的を達成するため、根圏の水・養分・温度・光などの環境因子が一定の条件下で根の伸長生長を観測しうるシステムを作成した。低照度域においても感度良好なCCDビデオカメラで撮影した根端部の位置をタイムラプスビデオに記録し、コンピュータ上で画像処理した。観察対象とする根は、イネ、白花ル-ピン、モロコシの種子根とし、播種後約3週間程度に渡って種子根の伸長リズムを調査・解析した。数種の作物根の時・分単位の伸長速度には明らかな変動が認められた。数日間の調査期間中における1時間当たりの伸長速度の最大値は、最小値の1.4から6.7倍であった。昼夜温度が変化する場合には伸長速度は昼夜間で明らかに異なっており、伸長速度は温度に依存する傾向が認められたが、恒温条件下では昼夜間の伸長速度には明確な差異は認められなかった。このことは、伸長速度の変動は日周の温度変動に依存する変動パターンを示しうることを暗示した。今後、明暗が交代する通常の条件と、連続明条件の二つの条件で観察を行ない、その結果の違いによって、伸長パターンが外因性か内因性かを判定する必要を認めた。

作物根の根冠ムシゲルは、土壌中で伸長するより根端と土壌粒子との間の摩擦抵抗を緩和する潤滑油のような働きをしていることが従来より推測されてきたが、土壌の特定部位における粘性の高いムシゲルの動態を捉えることは困難であるため、その実態の直接的な証明は現在までなされていない。本研究では実際に土壌中で生育している作物根の根端部位を解剖顕微鏡レベルでタイムラピスビデオシステムを利用して数日間連続的に観察することを主体として、この仮説の真偽を直接的に証明することを目的とした。透明なアクリル板製の根箱を作成し鉛直下方に対し15度傾斜した状態で固定し、上下方向に根箱が移動できるようにした。ズ-ム式実体顕微鏡に接続したCCDビデオカメラを根箱に垂直になるように固定し、アクリル板上に展開したトウモロコシ種子根根端部とその周辺部の土壌を数十倍の倍率でビデオカメラに撮影した。入力信号は、タイムラプスビデオレコーダーに記録させた。播種方向、播種後の種子根の齢、土壌の充填状態、含水比などの検討を行ない、最も観察に適した条件下において、約2日間のビデオ観察を繰り返した。その結果、種子根根軸とアクリル板の間にムシゲル様物質の存在を確認した。このムシゲル様物質の量は増減しているように観察できたが、今後、根軸とアクリル板との接触状態が一定になるようにして検討を行なう必要がある。現段階では、土壌粒子と根冠との間にあるムシゲルは完全には撮影が行なえていない。ある時間帯において部分的に根冠ムシゲルが観察される場合もあるが、観察結果はまだ安定はしていない。ムシゲルが土壌粒子間に拡散してしまい観察が容易ではないと考えられるので、今後とくに観察視野のコントラストと解像度を高めることと、土壌の物理的条件のさらなる改良を平行して観察を進める予定である。

高根域温度条件がサツマイモとキャッサバの根系および地上部の発育に及ぼす影響の評価 人工気象室内において気温25℃、根域温度25℃、40℃を設定し、両種の地上部の成育反応、根系については、構成要素レベルでの発生・伸長反応を精査した。実験と採取した試料の解析・分析の一部は昨年度中に行ない、その残りを本年度行なった。両種とも、地上部で認められた高根域温度に対する反応は促進的であり、これまで報告した、イネ科作物の場合とは対照的であった。根系反応では、根系全体でみた場合、サツマイモでは促進的、キャッサバでは抑制的であったが、根系要素別にみると、両種ともとくに高次側根の発育が促進されることを見いだした。 なお、この概要は第2回アジア作物学会議(1995年8月21日〜25日)において発表し(様式5、10研究発表参照)、現在投稿論文を取りまとめ中である。 2.キャッサバの活着期の根系発達に及ぼす土壌乾燥の影響の評価 活着期、すなわち植え付けから約3ケ月後までのキャッサバに対する、乾燥の時期および期間の及ぼす影響を、とくに根系発達に注目して検討した。植え付け後9〜44日に乾燥処理を与えた場合(前期乾燥処理)、44〜82日に同処理を与えた場合(後期乾燥処理)、いずれも土壌を湿潤に保った対照区と比べて葉数、地上部乾物重が有意に減少した。また、植え付け後9〜82日目まで乾燥処理を与えた区(全期乾燥処理)では、後期乾燥処理区と類似の地上部生長の減少傾向が認められた。根系発達についてみると、前期乾燥処理区では、不定根数・長、第1次・2次・3次側根数、乾物重が減少した。しかし再灌水後、これらの値は増加する傾向を示した。一方、後期乾燥処理区では、同様の不定根数・長、第1次、第2次側根数は乾燥によって減少したが、第3次側根数は促進される傾向を認めた。また、不定根が太くなることによって、根系乾物重も増加した。全体として、これらの結果はキャッサバは活着期においては乾燥に対して感受性が高いことを示している。 なお、この結果は原著論文として日本作物学会紀事に公表した(様式5、10研究発表参照)。 つづいて、この成果をさらに発展させるために、サツマイモを加え、対象品種、乾燥処理時間・方法を増やした、いくつかのポット実験を昨年度から開始している。現在、順次、採取した試料の解析、分析を進めている。 また、この成果が出た段階で、これらの種・品種の水分生理を含めた耐旱性と根系発達・機能を、圃場試験によって評価することを計画している。現在実験圃場にそのための雨よけシェルターを作成中である。 3.農家圃場におけるサツマイモ根系発達の調査 フィリピン国レイテ州の、4農家のサツマイモ圃場(Abuyog,Salvacion,Calipayan,San Austin)において、土壌の性質、およびサツマイモ根系の土壌深度別分布を調査した。その結果、土性はすべてシルト質壌土であった。また、4調査地間では、表層5cmまでの土壌水分は、13〜21%、有機物含有量は1.76〜1.86%、リン含有量は30.2〜43.4ppm、カリウム含有量は13.7〜33.8%、pHは、5.03〜5.19の範囲にあった。また、根長密度・根重密度および土壌貫入抵抗値の土層別分布についての結果を現在解析中である。 今後もこの調査を継続し、ポット・圃場試験の結果と比較する予定である。 以上のように、本研究は、これまでほとんど未解明であったこれらの作物の根系発達と、その土壌温度・水分に対する反応性を明らかにしつつある。得られた成果の一部は、すでに公表済みか、あるいはその直前の段階に至っているが、他の多くの部分は実験・調査が継続中であり、それらはすべて、現段階ではフィリピン側の研究所において行なわれている。したがって、今後は、実験が終了したものから共同でデータを取りまとめ、これらの作物の根系発達・機能が、それらの耐旱性において果たす役割の実態を明らかにし、論文として公表していく予定である。また、新たな研究のための予算を申請する計画を立案中である。