ENERGY-DEPENDENT QUENCHING OF CHLOROPHYLL-A-FLUORESCENCE - THE INVOLVEMENT OF PROTON-CALCIUM EXCHANGE AT PHOTOSYSTEM-2

The pH-dependent quenching of chlorophyll (Chl) fluorescence (the high energy quenching) was characterized by stationary Chl-alpha-fluoresce nce in the thylakoid membranes and photosystem (PS) 2 preparations. Th e variable part of fluorescence was quenched, when the pH in the thyla koid lumen decreased below 5.5, i. e., at high DELTApH. This quenching was caused by an inhibition of electron donation from the manganese c luster to the reaction centre of PS 2. The pH response of quenching su ggested that a 1 H+-transition with an apparent pK of 4.7 was involved . Parallel to quenching at low pH, a Ca2+-release was measured (1 Ca2 per 200 Chl). When the DELTApH relaxed and the pH on the lumen side i ncreased again, fluorescence recovered provided Ca2+ was present (K(D) = 100 muM). Both the quenching at low pH and the reactivation at pH > 5.5 are light-dependent processes. In the presence of high concentrat ion of external Ca2+, fluorescence recovered even at low pH. Inhibitio n of the donor-side of PS 2 directly affected the acceptor-side of PS 2, as seen by a shift of the redox potential of Q(A) from -120 mV (pH 7.0) to +40 mV (pH 4.2). We propose that at high DELTApH (1) the water splitting side is inactivated by release of Ca from a high affinity b inding site, and (2) Q(A) is converted to a high-potential form. Excit ation energy is then dissipated at the PS 2 reaction centres by a reco mbination reaction between donor and acceptor side. As a result, Q(A) (and the intersystem electron transport chain) remains oxidized, even in the excessive light.