LOW-PH-INDUCED CA2+ ION RELEASE IN THE WATER-SPLITTING SYSTEM IS ACCOMPANIED BY A SHIFT IN THE MIDPOINT REDOX POTENTIAL OF THE PRIMARY QUINONE ACCEPTOR-Q(A)

pH-dependent regulation of Photosystem (PS) II (observed as 'high-energy quenching') has been characterized by chlorophyll fluorescence and thermoluminescence measurements in PS II particles, thylakoid membranes, alga cells and leaf tissue. Steady state redox titration of fluorescence yield performed at pH 6.5 revealed that the midpoint redox potential of the primary quinone acceptor, Q(A), is shifted towards positive direction from E(m) = -80 mV to E(m) = +40 mV (the absolute values for E(m) were varying by about 40 mV between different preparations) after incubation of PS II particles at pH 4.2 for 15 min in the presence of the Ca2+ chelator, PP(i). The original midpoint potential was restored after the addition of 300 muM CaCl2. Low-pH treatment (pH 4.6) of PS II particles also resulted in a decrease of the Q band of thermoluminescence (appearing between 10-14-degrees-C after DCMU addition) with a concomitant appearance or intensification of a high temperature band between 42-50-degrees-C (C band). In accordance with the results of the redox titration of fluorescence yield the C band is attributed to a low-pH-induced high potential form of Q(A). The interconversion of Q band into the C band was more pronounced in the presence of the Ca2+ chelator, EGTA. Addition of CaCl2 to the low-pH-treated particles diminished the C band and restored the Q band. Light-induced acidification of the thylakoid lumen (DELTApH formation under illumination conditions of 'high-energy quenching') was also accompanied by a transformation of the Q band to the C band in isolated thylakoids, in the green alga, Chlorella vulgaris and in pea leaves. The phenomenon was completely reversed by abolishing the pH gradient with 10 mM NH4Cl. Addition of the Ca2+-channel inhibitor verapamil to the thylakoid suspension before the formation of a DELTApH suppressed the transformation of Q band into the C band. In contrast, when a DELTApH was first established and then verapamil was added, the DELTApH-induced change in the glow curve was irreversible and conversion of C band back to the Q band was prevented. It is suggested that the appearance of the C band is associated with Ca2+-dependent reversible inactivation of the water-splitting system and with a shift in the redox potential of Q(A). We propose that pH-dependent Ca2+-release is a physiological process which controls the electron transport of PS II in vivo.