Effects of abscisic acid on sequestration and exchange of Na+ by barley roots

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Abstract
Excised Na+-starved barley roots were suspended in solutions of Na+ in combination with NO 3 - , Cl-, and SO 4 2- , and effects of the added phytohormone, abscisic acid (ABA), to the medium were determined. Abscisic acid increased the rate of Na+ (22Na+) accumulation and the amount of Na+ deposited in the vacuoles. These stimulating effects of ABA were modified by anions following the sequence NO 3 - >Cl->SO 4 2- . Testing whether the magnitude of the pH gradient across the plasmalemma of the cells of the root cortex affects rates of Na+ accumulation and their dependence upon ABA, we observed that, in the pH range from 4 to 8, the ABA-induced stimulation was strongest at pH 5.8, and least at pH 4. Changes in pH during the experiment caused changes in the rates of Na+ accumulation in agreement with experiments performed at constant pH values. Simultaneously with ABA-enhanced accumulation, loss of Na+ occurred. Loss of Na+ was strongest at pH 4 and was affected by anions, being greatest with SO 4 2- and following the sequence SO 4 2- >Cl->NO 3 - . On the basis of the finding that initial acceleration of uptake as well as loss of Na+ depended on the pH of the medium we suggest that, in barley roots, ABA stimulates an exchange of Na+ for H+ at the plasmalemma of the cortical cells. The results indicate that ABA-stimulated expulsion of Na+, in combination with ABA-stimulated sequestration in the vacuoles, constitutes one of the mechanisms which enable barley plants to tolerate higher than normal levels of Na+.