Triggered Permeation of Methyl Viologen into In Situ Chloroplasts upon Electrical Excitation of a Plant Cell

Action potential (AP) generation at the plasmalemma of Chara corallina has a strong influence on photoprocesses in chloroplasts. Under physiological conditions the AP generation transiently suppresses photosynthetic electron transport and reversibly increases thermal losses of chlorophyll excitations. However, these changes provoked by membrane excitation became irreversible in the presence of artificial electron acceptor methyl viologen (MV) in the external medium. Incubation of Chara cells under resting conditions in the presence of this herbicidal agent had no effect on kinetics of chlorophyll P700 photooxidation, indicating that permeation of MV divalent cations across the cell and chloroplast membranes is a limiting factor and makes MV inaccessible at the sites of its interactions with photosystem I. On the other hand, the AP generation in the presence of MV irreversibly modified the P700 photooxidation signals, as measured from the difference of absorbance changes at 810 and 870 nm ( A810). The results suggest that permeation of MV into in situ chloroplasts is triggered during or after AP generation. Photoinduced changes of cell membrane potential, similarly to A810 signals, were insensitive to the presence of MV in the external medium until the application of the first excitatory stimulus. However, the AP generation in the presence of MV irreversibly modified cell electrical photoresponses, indicating that photosynthetic electron flow was redirected to MV reduction. In the herbicide-treated plants, the effect of AP on photosynthesis seems to be complex and includes permeability changes to MV in the system of membrane barriers comprising the plasmalemma and the chloroplast envelope. _____________________________________________________________________________________________________________