ENHANCEMENT OF THE LIGHT-TRIGGERED ELECTRICAL RESPONSE IN PLANT-CELLS FOLLOWING THEIR DEENERGIZATION WITH UNCOUPLERS

Light-triggered membrane potential changes in cells of a liverwort Anthoceros are greatly enhanced by the ionophorous uncouplers nigericin and monensin. Stimulation of the light-triggered electrical response (LTER) by nigericin occurred concomitantly with inhibition of a slow decline in the chlorophyll fluorescence, which suggests that the transmembrane pH gradient in thylakoids is not essential for generation of LTER at the plasma membrane. The extent of monensin-stimulated LTER remained high under a diminished driving force for the ionophore-induced proton-cation exchange across the plasma membrane (elevation of the external Nai concentration from 1 to 50 mM), which indicates that energy uncoupling in chloroplasts is more related to the electric response enhancement than the induction of the H+/K+(Na+) exchange at the plasma membrane. Enhancement of LTER by ionophores occurs in parallel with stimulation of light-triggered pH changes (alkalinization) in the vicinity of the cell surface, which suggests an association of trans-membrane HC fluxes with LTER. The results are consistent with the hypothesis that illumination produces a temporary inhibition of the plasma membrane H+ pump with a subsequent activation of Sated channels and transient rapid depolarization of the cell.