ON RESISTANCE TO WATER TRANSPORT IN CROP PLANTS FOR ESTIMATING WATER-UPTAKE ABILITY UNDER INTENSE TRANSPIRATION

Root water uptake ability is supposed to greatly affect the degree of midday depression in the photosynthetic rate of crop plants. However, there is no useful indicator for estimating it quantitatively despite necessity. This study investigated whether resistance to water transport calculated according to Ohm's law can be adopted in estimating water uptake ability by using rice, corn and soybean plants. The resistance to water transport from soil through root to leaf decreased markedly as the transpiration rate increased, especially at low transpiration rates, but became constant at high transpiration rates where there were close linear correlations between transpiration rate and leaf xylem water potential and the linear extrapolation of the regression lines down to zero transpiration rate gave the water potentials quite near to the origin. By measuring transpiration rate and leaf xylem water potential for the leaves on the intact stem and the excised stem with its cut end kept under water, it was supposed that root have a relatively high resistance in the water pathway of plants and this resistance could be attributed to the variable resistance of whole plants. Water uptake decreased and the resistance to water transport increased significantly when the root system was partly excised and when treated with NaN3. On the other hand, the resistance was not affected by lowering the water potential of the culture solution, although water uptake of the plants decreased markedly. Our conclusions are that resistance to water transport in crop plants under intense transpiration could be adopted in estimating water uptake and transport ability of plants and that the differences in root water uptake abilities among plants could be compared by calculating the resistance, since the root water uptake ability very much affects whole plant resistance.