Photoregulation of Proton Extrusion by Cucumber Hypocotyls and the Role of ATPase Activity

The photoregulation of H⁺ extrusion by hypocotyl segments of Cucumis sativus L. was studied. Irradiation with red light (to give Pfr) enhanced H⁺ efflux in the presence of Mg²⁺ and K⁺. This effect of red light was decreased by far red irradiation (to give Pr). The acidification was inhibited by sodium orthovanadate. These results imply that H⁺ extrusion is coupled to plasma membrane ATPase activity. In vivo pretreatment with filipin, which interferes with phytochrome association with membranes, reduced both photoregulation of ATPase activity and H⁺ extrusion. A similar effect was found after preirradiation with red light before a prolonged dark period at 4 °C, a treatment which is assumed to lower phytochrome levels.