In vivo analyses of the electron and proton circuits in the higher plant photosynthesis

Higher plant chloroplasts have sophisticated regulatory mechanisms to optimize photosynthetic reactions depending upon physiological status and environmental conditions. In vivo analysis of the electron and proton transfer reactions has revealed that the transthylakoid proton gradient or pH in the lumen has a central control for down regulation of photosynthesis. Relative change in lumen pH was estimated from kinetic analysis of membrane potential-dependent electrochromic shift and absolute pH value was estimated by calibrating it using secondary pH-dependent signals, whose pH responses were observed from in vitro experiments. When the light reaction exceeds dark reactions, stromal Pi concentration likely decreases below Km of the CFO-CF1 ATP synthase. Pi depression increase lumen acidity by decreasing proton conductivity at CFO-CF1 ATP synthase. The light capture and electron transfer reactions hierarchically respond to the lumen acidity for optimizing the reduction pressure and preventing photodamage. Activation of energy dependent excitation quenching in antenna complex is more sensitive to lumen pH than down regulation of plastoquinol oxidation at cytochrome b6f complex, so that excess amount of excitation energy is effectively dissipates as a heat.