Inhibitors of electron transport associated with photosystem II in chloroplasts

The modes of actions of six inhibitors on the electron transport system in the vicinity of system II in chloroplasts were studied. The first group, including piericidin A, ioxynil and broxynil, showed relatively simple modes of action on the Hill reaction, fluorescence of chlorophyll α and the photobleaching of photosynthetic pigments, which are similar to the action of DCMU. As compared with inhibitors of the first group, the inhibitory actions of salicylaldoxime, antimycin A and azide on the Hill reaction were more complicated in that they were influenced more strongly by reaction conditions, i.e. duration of incubation, pH of the reaction mixture and illumination of chloroplasts. Inhibitors of the second group suppressed the rise in fluorescence in the induction period. However, this effect was not observed in the presence of DCMU or dithionite. Salicylaldoxime and azide were effective in inducing photobleaching of photosynthetic pigments, whereas antimycin A inhibited the photobleaching induced by ferricyanide or CCCP. Inhibition sites of the inhibitors in the first group are assumed to be similar to that of DCMU, whereas the inhibitors in the second group are effective in blocking electron transport on the oxidizing side of system II between the primary electron donor of system II and an intermediary electron carrier which receives electrons from artificial electron donors for system II.