Abscisic Acid raises the permeability of plant cells to water.

A number of recent observations have indicated a role for abscisic acid in maintaining water balance. Treatment with ABA reduces the transpiration rate (3, 5), probably by bringing about stomatal closure (2, 4). Furthermore, water stress leads to a significant increase in ABA concentration in leaves (5, 7). On the other hand, ABA increases the rate of root exudation (6). The present communication reports a marked effect ofABA on the permeability of plant tissues to water. The effect is observable both on Lp, the coefficient of hydraulic conductivity, and on Pd, the coefficient of diffusional permeability to water. The effect on hydraulic conductivity was detected in experiments in which osmotic water flux between cells and medium was followed with time. The equations for net osmotic flux into or out of cells can be written as follows: