Electron spin resonance of electron transport in photosynthetic systems. IX. Temperature dependence of the kinetics of P700 redox transients in bean chloroplasts induced by flashes of different duration

The temperature dependence of the kinetics of P700 redox transients in bean chloroplasts was studied. The flashes of white light with different duration (7 microseconds, 0.5 and 0.75 ms) were fired simultaneously with the background continuous far red light (lambda max = 707 nm). It was shown that the rate of P700+ reduction was temperature dependent and increased with the rise of the concentration of the reductants in the electron transport chain between photosystems. Photosystem 2 donates electrons to P700+ at temperatures from -5 to 45 degrees C under various modes of flash illumination. Experiments with spin labels showed that there were correlation between the physical state of lipids in the chloroplasts membrane and the rates of different steps of electron transport from photosystem 2 to photosystem 1--plastoquinone reduction by photosystem 2 and plastoquinol oxidation by photosystem 1. We assume that the rates of electron transport reaction of the plastoquinone shuttle are controlled by diffusion of plastoquinone and plastoquinole in the hydrophobic part of the thylakoid membrane. Additional evidence in support of that proposal was obtained from the temperature dependence of light induced spin label reduction which occurred due to its interaction with the plastoquinol of plastosemiquinone.