Action potential in a plant cell lowers the light requirement for non-photochemical energy-dependent quenching of chlorophyll fluorescence

This study deals with effects of membrane excitation on photosynthesis and cell protection against excessive light, manifested in non-photochemical quenching (NPQ). In Chara corallina cells, NPQ and pericellular pH displayed coordinated spatial patterns along the length of the cell. The NPQ values were lower in H+-extruding cell regions (external pH ∼ 6.5) than in high pH regions (pH ∼ 9.5). Generation of an action potential by applying a pulse of electric current caused NPQ to increase within 30–60 s. This effect, manifested as a long-lived drop of maximum chlorophyll fluorescence (Fm′), occurred at lower photosynthetic flux densities (PFD) in the alkaline as compared to acidic cell regions. The light response curve of NPQ shifted, after generation of an action potential, towards lower PFD. The release of NPQ by nigericin and the rapid reversal of action potential-triggered NPQ in darkness indicate its relation to thylakoid ΔpH. Generation of an action potential shortly after darkening converted the chloroplasts into a latent state with the Fm identical to that of unexcited cells. This state transformed to the quenched state after turning on weak light that was insufficient for NPQ prior to membrane excitation of the cells. The ionophore, A23187, shifted NPQ plots similarly to the action potential effect, consistent with a likely role of a rise in the cytosolic Ca2+ level in the action potential-induced quenching. The results suggest that a rapid electric signal, across the plasma membrane, might exert long-lived effects on photosynthesis and chlorophyll fluorescence through ion flux-mediated pathways.