Light-induced activation of electrogenic H+ extrusion and K+ uptake in Elodea densa depends on photosynthesis and is mediated by the plasma membrane H+ ATPase

In Elodea densa leaves light strongly stimulates electrogenic, K +-dependent, vanadate- and erythrosin B-sensitive H+ extrusion and hyperpolarizes the transmembrane electrical potential. These effects of light are suppressed by treatment with DCMU, an inhibitor of photosynthesis, which has no effect on H+ extrusion in the dark. Light-induced H+ extrusion requires the presence of K+ in the medium and is associated with increased K+ uptake and alkalinization of the cell sap. Light-induced H+ extrusion increases with increased CO2 concentration. At constant CO2 concentration (104 parts 10-6) the rate of H+ extrusion is strongly enhanced by an increased light intensity up to 30 W m-2. Different wavelengths, between 400 and 730 nm, induce a significant stimulation of both proton secretion and transmembrane potential hyperpolarization.The stimulating effects of light on H+ extrusion, K+ uptake and cell sap pH are very similar to those induced in the dark by fusicoccin, a toxin known to stimulate strongly ATP-driven, vanadate- and erythrosin B-sensitive H+ transport. In the light, the effects of fusicoccin are only partially additive to those of light, thus suggesting that the two factors influence the same system. The identification of this system with the plasma membrane H+-ATPase is indicated by the observed inhibition of the effects of either light or fusicoccin by the H+-ATPase inhibitors vanadate and erythrosin B.These data indicate that the activation of electrogenic H+ extrusion and of K+ uptake by light is mediated by some products of photosynthesis. The mechanism and the possible physiological implications of this phenomenon are discussed.