Dependence of chlorophyll P700 redox transients during the induction period on the transmembrane distribution of protons in chloroplasts of pea leaves

Differential absorbance measurements and fluorometry were applied to examine the impact of dicyclohexylcarbodiimide (DCCD, an inhibitor of H⁺ conductance in thylakoid membranes) and nigericin (a K⁺/H⁺ antiporter) on photoinduced redox state transients of chlorophyll P700 and the induction curves of chlorophyll fluorescence in pea (Pisum sativum L., cv. Premium) leaves. The treatment of leaves with DCCD strongly modified the kinetics of P700⁺ absorbance changes (ΔA ₈₁₀) by promoting rapid photooxidation of P700. These characteristic changes in ΔA ₈₁₀ induction kinetics and P700⁺ accumulation did not appear when the leaves were treated with DCCD in the presence of nigericin. In addition to opposite modifications of ΔA ₈₁₀ kinetics evoked by permeability-modifying agents, the fluorescence induction curves differed conspicuously depending on leaf incubation in DCCD solutions with or without nigericin. The observed modifications of fluorescence induction curves and ΔA ₈₁₀ indicate that DCCD suppresses electron transport from photosystem II (PSII) to P700, whereas this inhibition is removed by nigericin. The results suggest that slowing down of the electron transport rate in the presence of DCCD was caused by elevation of ΔpH in thylakoids. The prevention of pH gradient formation in the presence of protonophore lowered also the steady-state P700⁺ level in far-red irradiated leaves and accelerated the subsequent dark reduction of P700. These findings indicate that PSI-driven cyclic electron flow is accelerated after the removal of the pH gradient.