The back reaction in the primary electron transfer couple of photosystem II of photosynthesis

Changes of C-550, cytochrome b₅₅₉ and fluorescence yield induced in chloroplasts by single saturating flashes were studied at low temperature. A single saturating flash at −196°C was quite ineffective in reducing C-550, oxidizing cytochrome b₅₅₉ or increasing the fluorescence yield, presumably because most of the charge separation induced by the flash was dissipated by a direct back reaction in the primary electron transfer couple. The back reaction, which competes with the dark reduction of the oxidized primary electron donor by a secondary electron donor, becomes increasingly important as the temperature is lowered because of the temperature coefficient of the reaction with the secondary donor. The effect of the back reaction is to lower the quantum yield for the production of stable photochemical products by steady irradiation. Assuming a quantum yield of unity for the photoreduction of C-550 at room temperature, the quantum yield for the reaction is about 0.40 at −100°C and 0.27 at −196°C.