Effect of light upon membrane potential, conductance, and ion fluxes inRiccia fluitans

Summary

The membrane potentialE _m , slope conductanceg _m , and fluxes of⁸⁶Rb⁺/K⁺ and³⁶Cl⁻ have been measured on cells of the aquatic liverwortRiccia fluitans in artificial pond water of pH 4 to 8 and 0.1 to 10mm K ⁺₀ , in the dark and 1 mW/cm² of white light. In the darkE _m reflects a passive diffusion potential according to the Goldman equation with relative ionic permeabilities,P _H/P _K/P _Na=10∶1∶0.02. In the light,E _m of −200 to −240 mV exceeds the most negative ion diffusion potential, i.e.E _K, by more than 100mV, is more sensitive to H ⁺₀ than K ⁺₀ , and is reduced to the dark level by uncouplers of phosphorylation. At 1mm K ⁺₀ ,g _m is 33 and 50 μS/cm² in the light and dark, respectively.g _m is sensitive to H ⁺₀ only in the light, and more sensitive toK ⁺₀ in the dark. Current-voltage relationships are given. Light increases the influx at the plasmalemma of⁸⁶Rb⁺/K⁺ and³⁶Cl⁻ less than would be expected from the increase ofE _m . It is concluded that the electrogenic pump operates in the dark as a constant current source which is shunted by the diffusive channels whereas in the lightE _m approaches the H⁺-dependent electromotive force of the electrogenic pump.