Inhibition of Ion Transport in Excised Barley Roots by Abscisic Acid; Relation to Water Permeability of the Roots

Previous papers have shown that abscisic acid can inhibit transport of ions across the root to the xylem vessels, resulting in reduced exudation from excised roots or inhibiting guttation from intact plants. However, it has not been established whether the inhibition was due to a reduction in salt transport (Js) or in permeability of the roots to water (Lp). This paper investigates the effect of ABA on Lp and Js separately. It is shown that Lp increased in ABA and then fell, but was about the same as in control roots when transport was inhibited. The effect of ABA on exudation therefore appeared to be mainly due to reduction in Js. Inhibition of Js was also present in intact, transpiring plants and so was not due to reduced water flow. The inhibition of ion release to the xylem affected Na+, Mg2+, Ca2+, and phosphate as well as the major ion in the exudate, K+. It is concluded that ABA inhibits salt transport to the shoot by acting on ion transport into the xylem, and not by reducing water flow coupled with salt transport.