Energy-dependent quenching of chlorophyll a fluorescence: effect of pH on stationary fluorescence and picosecond-relaxation kinetics in thylakoid membranes and Photosystem II preparations

Quenching of chlorophyll a fluorescence by low pH ('high-energy quenching') has been characterized by stationary fluorescence and time-resolved fluorescence decay in thylakoid membranes and photosystem PS II preparations. Fluorescence is quenched when the pH on the lumen-side of PS II decreases below 5.5. It is likely to involve an one-proton transition (apparent pK = 4.5-4.8) and quenching is accompanied by an inactivation of the photosynthetic oxygen evolution. The overall quenching is due to a decline in the two nanosecond fluorescence decay components (τ = 1.6 ns and 5 ns at pH 6.5), which mainly contribute to the stationary fluorescence in the presence of the reduced acceptor Q_A. Two picosecond components (τ = 30 ps and 500 ps at pH 6.5) are little affected by low pH and the stationary F_u^--fluorescence (with oxidized Q_A) remains constant. When artificial e- donors were added at low pH, the fluorescence recovered, suggesting that quenching is caused by inhibition of e donation from the water-splitting side. There is no indication that the transfer of excitation energy from the antennae to centers or the trapping process itself is disturbed by internal acidification. We propose that, at centers with an inactive donor side, excitation-energy is 'quenched' by charge recombination between Q_A and P-680⁺. Under physiological conditions quenching induced by a high ΔpH may serve to dissipate excess excitation-energy and to keep Q_A oxidized, even at high irradiance.