Rapid effect of light on the K+ channel in the plasmalemma ofNitella

Summary Chlorophyll fluorescence, plasmalemma potential and resistance were measured simultaneously and subjected to a kinetic analysis. It was found that the light-induced changes of all three signals have two time constants in common. The faster one (₄=ca. 20 sec) was assigned to the action of light-induced proton uptake across the thylakoid membrane on the plasmalemma H⁺ pump. The slower one (_5a=40 sec) is related to the light action of an unknown photosynthetic process on the potassium channel. The action on the K⁺ channel was revealed from the reversal potential of the related effect on membrane potential. The comparison of the data with findings of other authors led to the hypothesis that the unknown photosynthetic mechanism is the depletion of NADP⁺, which stimulates the uptake of Ca²⁺ from the cytosol, which is required for the NAD-kinase. The resulting change in cytosolic Ca²⁺ modulates the number of open K⁺ channels.