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

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(+) ((22)Na(+)) 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(+).