Morio Iijima

Effects of soil compaction on the development of rice and maize root systems

Abstract 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 ☐es (24 cm long × 2 cm wide × 40 cm deep) with soil bulk densities of 1.33 g/cm 3 (control) and 1.50 g/cm 3 (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.

Deep Root Water Uptake Ability and Water Use Efficiency of Pearl Millet in Comparison to Other Millet Species

Abstract 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.

Structure and function of the root cap.

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.

Water Competition of Intercropped Pearl Millet with Cowpea under Drought and Soil Compaction Stresses

Abstract 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 under wet, 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 any significant changes under these stress conditions.

Which roots penetrate the deepest in rice and maize root systems

Abstract 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.

Physiol-morphological analysis on axile root growth in upland rice

Abstract 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 13G. 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 13C abundance in the axile root tip zone was found at 21 hrs after feeding 13CO2. The length of the apical unbranched zone behind the axile root tip positively correlated with the 13C 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.

Combined soil physical stress of soil drying, anaerobiosis and mechanical impedance to seedling root growth of four crop species.

Abstract 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 Ψw) enhanced root elongation by 17-18%, but severe drying stress (−900 kPa Ψw) reduced it by 17-21% as compared with the control environment (−10 kPa Ψw). 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.

Crop Production in Successive Wheat-Soybean Rotation with No-Tillage Practice in Relation to the Root System Development

Abstract 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.

Hydrogen Stable Isotope Analysis of Water Acquisition Ability of Deep Roots and Hydraulic Lift in Sixteen Food Crop Species

Abstract 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%; ψ = –5 kPa), while soil moisture in the upper (topsoil) ones was adjusted to 25% (ψ = –7 kPa) and 12% (ψ = –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.

No-Tillage Enhanced the Dependence on Surface Irrigation Water in Wheat and Soybean

Abstract 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.