Action potential opens access for the charged cofactor to the chloroplasts of Chara corallina cells

Effects of inhibitors and cofactors of cyclic and noncyclic electron transport on nonphotochemical quenching of chlorophyll fluorescence. induced by action potential (AP) was investigated in Chara corallina cells. Under control conditions, energy-dependent quenching (qE) develops upon the increase in photosynthetically active radiation (PAR); it also arises and reversibly disappears after AP generation at moderate irradiances. The treatment of cells with diuron (DCMU) completely eliminated qE established at high irradiances and qE induced by AP generation. The activation of cyclic electron transport by DCMU in combination with phenazine methosulfate restored qE at high irradiances but did not restore qE imposed after AP generation. The presence in the medium of a PSI acceptor, methyl viologen at concentrations from 100 mu M to 0.83 mM had no effect on fluorescence and photosynthetic activity of chloroplasts until the application of a single excitatory stimulus. Once a single AP was generated in the presence of methyl viologen, it induced irreversible qE at a wide range of irradiances, which indicates the AP-triggered redirection of a part of electron flow from the main pathway to the artificial acceptor. It is concluded that AP generation opens access for permeation of a divalent cation methyl viologen from the medium to the chioroplast stroma across two membrane barriers, the plasmalemma and the inner membrane of the chloroplast envelope.