Electron paramagnetic resonance of electron transport in photosynthetic systems. XI. Effects of photosynthetic control: dependence of the rate of electron transport on the energization of bean chloroplast thylakoid membrane

The kinetics of light-induced P700 redox transients in bean chloroplast was studied. It has been shown that the rate of electron transport decreased during few seconds of illumination of coupled chloroplasts without addition of ADP and inorganic phosphate. The evidence were obtained that there is a feedback inhibition of electron transport governed by the internal pH of thylakoid. This results in the overshoot in the kinetics of P700 redox transients induced by continuous actinic light. Under the phosphorylation condition (addition of Mg-ADP and inorganic phosphate) the effect of decreasing of the rate of electron transport between two photosystems was not observed. Addition of uncouplers (FCCP or gramicidine) also increased the steady-state rate of noncyclic electron transport. After adding only Mg-ADP (without phosphate) or Mg-ATP to coupled chloroplasts the effect of the light-driven inhibition of electron transport was observed as in the case of chloroplasts without any additions. We showed that the regulation for the electron transport rate was realized at the step of the plastoquinol oxidation by photosystem 1. Light-driven energization of the thylakoid membrane also leads to the the slowing of the reduction of spin label TEMPO. Evidences were obtained that TEMPO interacts with the semiquinone localized in the acceptor side of photosystem 2. From the comparative study of P700+ and TEMPO reduction by photosystem 2 we have concluded that there are two points of inhibitory action of DCMU localized at the acceptor and donor sides of photosystem 2. The mechanisms of photosynthetic control and the role of transmembrane proton gradient for energy transmission in chloroplasts are discussed.