AbstractThe effects of extracellular K+concentration ([K+]o) on the pH of cell sap, “bulk cytoplasm” and vacuole have been investigated inElodea densaleaves under conditions of either low or high activity of the plasmalemma electrogenic H+pump. Cell sap pH was evaluated directly in the cell sap expressed after freezing and thawing. Cytoplasmic and vacuolar pH were calculated by the weak base and weak acid distribution method, DMO and benzylamine appearing to be a suitable acid and base, respectively, for this purpose in this material.When added to the basal medium (no rapidly permeating ions present), 5 mM K+induced an increase in intracellular pH, larger for the cell sap and the vacuole (about 0.2 units), and smaller but still significant for the cytoplasm (0.07 units). This alkalinizing effect of K+was thus associated with a significant decrease in the pH difference across the tonoplast. The alkalinizing effect of K+was markedly and synergistically enhanced by the presence of fusicoccin, a condition inducing a marked activation of H+extrusion and of K+uptake.The correlation between these effects of [K+]oon intracellular pH and those on H+extrusion indicates that changes in extracellular K+concentration, and thus in K+influx, can influence cytoplasmic and vacuolar pH by modulating the rate of H+extrusion by the plasmalemma H+p