Modulation of the plasmalemma proton pump activity by intracellular pH in Elodea densa leaves: correlation between acid load and H+ pump activity.

Treatments with 2mM 5,5-dimethyloxazolidine-2,4 dione (DMO) or isobutyric acid (IBA) induced in Elodea densa leaves a significant acidification of both the cytosol and the vacuole as evaluated by the weak acid and weak base distribution method. The decrease in cytosolic pH was equal or even slightly enhanced by the presence in the medium of K+ or, even more, of K+ plus fusicoccin (FC), which in the absence of weak acids increase cytosolic and vacuolar pH. The weak acid-induced decrease in intracellular pH was associated with a significant increase in the K+ -dependent H+ extrusion. This effect on H+ extrusion was somewhat more than additive with the stimulating effect of FC, indicating that FC and the weak acid-induced cytosolic acidification stimulte the H+ pump by distinct modes of action. The dose-response curves obtained with increasing concentrations of IBA, and the results obtained by comparing the effects of equal acid loads imposed on the cytosol by supplying DMO and IBA at different concentrations establish a close relationship between acid load, cytosolic acidification and activation of the plasmalemma H+ pump.