Abscisic Acid Effect on Root Exudation Related to Increased Permeability to Water

Abstract

It has been recently reported (7) that application of abscisic acid to the root medium of decapitated tomato plants increases their exudation rate. This effect could be due to an increase in driving force or to increased permeability to water. The distinction between these two possibilities cannot easily be made by determining the effect of ABA on the osmotic concentration of the exudate. The common practice of assessing xylem osmotic concentration by measuring the osmolarity of the collected exudate has recently been subjected to serious criticism. Klepper (5) showed that the measured value is an underestimation of the real solute concentration in the water absorption region, whereas Meiri and Anderson (6) proved that under their experimental conditions the value obtained is an overestimation. It is accepted that the discrepancy between the osmolarity of the exudation fluid collected at the cut surface and that of the xylem sap at the region of osmotic water flux is a result of absorption or secretion of ions by cells surrounding the xylem. Any treatment which affects such ion transport may cause an error in determining the parameter affected. The present paper describes experiments in which bi-directional water fluxes through the root system were measured. Evidence is presented that ABA affects the hydraulic conductivity of the root system.