pH-Dependent Regulation of Electron and Proton Transport in Chloroplasts In Situ and In Silico

Abstract The analysis of electron and proton transport in chloroplasts of higher plants has been carried out on the basis of a mathematical model that takes into account the pH-dependent regulation of electron transport and the operation of the ATP synthase. Numerical experiments aimed at simulation of these processes under pseudocyclic electron transport (water–water cycle) have shown good agreement with experimental data on the kinetics of electron transfer to photosystem 1 (PS1) in class B chloroplasts in metabolic states corresponding to high (state 3) and low (state 4) ATP synthase activity. The simulation of electron transport processes that took into account the Calvin–Benson cycle (CBC), cyclic electron transport around PS1, pH-dependent heat dissipation of energy in photosystem 2 (PS2), and nonphotochemical quenching (NPQ) made it possible to estimate the contribution of these factors to the kinetics of induction phenomena in chloroplasts in situ. It has been shown that the multiphase kinetics of the photooxidation of P700 (a primary electron donor in PS1) reflects the redistribution of electron flows between cyclic and non-cyclic electron transfer pathways, caused by the activation of CBC due to the alkalization of the stroma, as well as the change of the limiting stage in the electron transport chain, induced by a decrease in the intrathylakoid pH (pHin). The electron flux between PS2 and PS1 decelerates with pHin decrease, which may be caused by the reduced rate of plastoquinol oxidation and attenuated activity of PS2 due to NPQ.