| Recent studies on soil moisture cycling and balance and conversion of five kinds of water (atmospheric water, plant water, surface water, soil moisture and underground water) are based on a Soil-Plant-Atmosphere Continuum (SPAC). The soil-plant system is an important constituent of SPAC. Due to limitations of research apparatus and techniques, studies on soil-plant water relations are relatively difficult. Hence, the studies on the topic have very important value for perfecting water transport theories of SPAC and defining related water-controlling mechanisms and techniques. Some important water problems in the soil-plant system were studied through PVC columns and field plots on maize and wheat, including (1) Axial changes of root xylem water potential, radial and axial hydraulic conductivity of maize under simulated soil conditions, (2) The critical value of relative water content of maize nodal root destroyed by drought, (3) Effect of drying-rewetting on root growth and water uptake rate of spring wheat, (4) Validation of Feddes and van Genuchten root-water-uptake models in the bess area, (5) Evaluation of soil moisture availability to maize with leaf photosynthetic rate, (6) Effect of N deficiency on root radial and axial hydraulic conductivity of maize, (7) Some physio-ecobogical responses of spring wheat root to water and nitrogen, (8) Relationship between water use efficiency (WUE) and nitrogen use efficiency (NUE) across different wheat evolution materials. The major conclusions are as the follows: 1. Multi-potometer and anatomical methods were used separately to study axial changes of root xylem water potential, radial and axial hydraulic conductivity of maize under quasi-soil condition, respectively. The results indicated that root xylem water potential decreased continuously across 9cm from root apex. Soil drought not only decreased root xylem water potential but also increased the drop degree of root xylem water potential along the 7cm from the root apex. The apical zone was the most sensitive one to water stress. Root radial hydraulic conductivity gradually increased in the apical 3cm, plateaued to a maximum value over 3-40cm, decreased at 10cm from the root apex. Soil drought decreased radial hydraulic conductivity earlier and the gradient of its axial changes was smaller. Root axial hydraulic conductivity in the apical 2cm increased quickly but it was not sensitive to soil drought. Water stress decreased axial hydmulic conductivity significantly after 4cm from the root apex. 2. The critical value of relative water content of maize nodal roots destroyed by drought was probed through the shape changes of root dehydration curves. Root dehydration curves could be described by the Weibull function. During two successive dehydration processes, the difference of shape parameters between the 1st and 2nd dehydration processes was very little while the scale parameter difference had an obvious upturn, so the scale parameter was closely related with the drying degree. The critical value of the relative water content for Shandan 9 with strong drought-resistance was 0.365, which was lower than that of Gedabai (0.428) with relatively weak drought- resistance. 3. Effects of drying-rewetting at different growth stages on root growth and water- uptake rate of spring wheat were studied. The results showed that both drought at the seedling and shooting-flowering stage decreased root length, changed the time cou...
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