[Electron paramagnetic resonance study of electron transport in photosynthetic systems. VIII. The interplay between two photosystems and kinetics of P700 redox transients under various conditions of flash excitation].

The kinetics P700 redox transients in bean chloroplasts induced by short (t 1/2 = 7 microsecond) flashes of white light has been studied. The flashes were fired in the dark or simultaneously with the continuous background far red light (707 nm). After oxidation of electron transport chain by 707 nm light only 50% of P700+ centres were reduced by photosystem 2 in response to the first flash of saturating intensity. The total reduction of P700+ centres was observed only after two or more saturating flashes. The similar data were obtained for various native photosynthetic systems (bean leaves and leaves of other higher plants). It has been shown that the quantitative difference in P700 behaviour after the first and following flashes may be interpreted in terms of a mechanism of the plastoquinol oxidation by photosystem 1. Twofold oxidation of one molecule of plastoquinol was assumed as its successive oxidation to plastosemiquinone and plastoquinone by different acceptors localized nea the same P700 centre (cytochrome f and plastocyanine). It has been shown that one molecule of plastoquinol was arised due to the interaction of two plastosemiquinones reduced by different P680 centre.