THE ROLE OF CALCIUM IN THE PH-DEPENDENT CONTROL OF PHOTOSYSTEM-II

pH-dependent inactivation of Photosystem (PS) II and related quenching of chlorophyll-a-fluorescence have been investigated in isolated thylakoids and PS II-particles and related to calcium release at the donor side of PS II. The capacity of oxygen evolution (measured under light saturation) decreases when the DELTApH is high and the pH in the thylakoid lumen decreases below 5.5. Oxygen evolution recovers upon uncoupling. The pH-response of inactivation can be described by a 1 H+-transition with an apparent pK-value of about 4.7. The yield of variable fluorescence decreases in parallel to the inactivation of oxygen evolution. pH-dependent quenching requires light and can be inhibited by DCMU. In PS II-particles, inactivation is accompanied by a reversible release of Ca2+-ions (one Ca2+ released per 200 Chl). In isolated thylakoids, where a DELTApH was created by ATP-hydrolysis, both inactivation of oxygen evolution (and related fluorescence quenching) by internal acidification and the recovery of that inactivation can be suppressed by calcium-channel blockers. In the presence of the Ca2+ -ionophore A23187, recovery of Chl-fluorescence (after relaxation of the DELTApH) is stimulated by external Ca2+ and retarded by EGTA. As shown previously (Krieger and Weis 1993), inactivation of oxygen evolution at low pH is accompanied by an upward shift of the midpoint redox-potential, E(m), of Q(A). Here, we show that in isolated PS II particles the pH-dependent redox-shift (about 160 mV, as measured from redox titration of Chl-fluorescence) is suppressed by Ca2+-channel blockers and DCMU. When a redox potential of - 80 to - 120 mV was established in a suspension of isolated thylakoids, the primary quinone acceptor, Q(A), was largely reduced in presence of a DELTApH (created by ATP-hydrolysis) but oxidized in presence of an uncoupler. Ca2+-binding at the lumen side seems to control redox processes at the lumen- and stroma-side of PS II. We discuss Ca2+ -release to be involved in the physiological process of 'high energy quenching'.