Effects of Shoot Environment on Apparent Root Resistance to Water Flow in Whole Soybean and Cotton Plants

The response of leaf water potential to change in transpiration rate was examined in young soybean and cotton plants. Leaf water potential measured 1 h after transpiration became constant following a change in humidity and was constant over a wide range of transpiration rates in both species. However, leaf water potential was not in equilibrium with flow until 3 h after transpiration became constant. At equilibrium an increase in transpiration always resulted in a decrease in leaf water potential. It was also found that different responses of equilibrium leaf water potential to transpiration rate occurred depending on whether transpiration was altered by changing humidity, light intensity, or leaf area. Low light and decreased leaf area caused lower leaf water potentials for a given transpiration rate. These increases in root resistance correlated with lower rates of root elongation. The data indicate that shoot-root interactions are occurring which affect apparent root resistance to water flow, and complicate interpretation of whole plant data on leaf water potential and transpiration in terms of the flow dependence of root hydraulic characteristics.