Abscisic acid effects on intracellular pH and ion fluxes in barley leaf segments

Changes in intracellular pH and in H⁺, K⁺ and Cl^? fluxes were evaluated in different experimental conditions in leaf segments of barley (Hordeum vulgare cv. Georgie) incubated in the dark, at pH 5.5, in the presence or absence of abscisic acid (ABA), and a comparison was made between the effects of ABA and those of erythrosin B (EB), a plasmalemma H⁺-pump inhibitor. In all conditions tested, ABA induced a cell sap acidification, an alkalinization of the external medium, a decrease in K⁺ intracellular contents, and an increase in the contents of Cl^?. The ABA-induced decrease in K⁺ content was chiefly due to the inhibition of K⁺ influx. On the contrary, ABA did not influence the uptake of Cl^?, but inhibited Cl^? efflux, the inhibition satisfactorily accounting for the larger Cl^? content observed in the presence of the hormone. The intracellular acidification and the decrease in apparent outward net transport of H⁺ observed with ABA were seemingly not associated with the activity of the proton pump, the transmembrane electrical potential difference, or K⁺ transport. On the contrary, a correlation was evident with the changes in Cl^? content. These results and, in particular, the similarity between the effects of ABA and those induced by 4,4 -diisothiocyano-2,2-disulfonic acid stilbene (DIDS), a Cl^? channel-blocking agent, suggest that the ABA-induced changes in intracellular pH and in H⁺ transport might depend on the capability of ABA to inhibit Cl^? efflux, more than on a primary inhibition of the H⁺ pump, and propose an important role for ABA in regulating the Cl^? channels.