CHARGE ACCUMULATION AND RECOMBINATION IN PHOTOSYSTEM-II STUDIED BY THERMO-LUMINESCENCE .1. PARTICIPATION OF THE PRIMARY ACCEPTOR-Q AND SECONDARY ACCEPTOR-B IN THE GENERATION OF THERMO-LUMINESCENCE OF CHLOROPLASTS

In the glow curves of chloroplasts excited by a series of flashes at +1.degree. C the intensity of the main thermoluminescence band appearing at +30.degree. C (B band; B, secondary acceptor of Photosystem II) exhibits a period-4 oscillation with maxima on the 2nd and 6th flashes indicating the participation of the S3 state of the water-splitting system in the radiative charge recombination reaction. After long-term dark adaptation of [maize] chloroplasts (6 h), when the major part of the secondary acceptor pool (B pool) is oxidized, a period-2 contribution with maxima occurring at uneven flash numbers appears in the oscillation pattern. The B band can even be excited at -160.degree. C as well as by a single flash in which case the water-splitting system undergoes only one transition (S1 .fwdarw. S2). The experimental observations and computer simulation of the oscillatory patterns suggest that the B band originates from charge recombination of the S2B- and S3B- redox states. The half-time of charge recombination responsible for the B band is 48 s. When a major part of the plastoquinone pool is reduced due to prolonged excitation of the chloroplasts by continuous light, a 2nd band (Q band; Q, primary acceptor of Photosystem II) appears in the glow curve at + 10.degree. C which overlaps with the band. In chloroplasts excited by flashes prior to DCMU [3-(3,4-dichlorophenyl)-1,1-dimethylurea] addition only the Q band can be observed showing maxima in the oscillation pattern at flash numbers 2, 6 and 10. The Q band can also be induced by flashes after DCMU addition which allows only one transition of the water-splitting system (S1 .fwdarw. S2). In the presence of DCMU, electrons accumulate on the primary acceptor Q, thus the Q band can be ascribed to the charge recombination of either the S2Q- or S3Q- states depending on whether the water-splitting system is in the S2 or the S3 state. The half-time of the back reaction of Q- with the donor side of PS II (S2 or S3 states) is 3 s. In a sequence of flashes the peak positions of the Q and B bands do not depend on the advancement of the water-splitting system from the S2 state to the S3 state. This result implies that the midpoint potential of the water-splitting system remains unmodified during the S2 .fwdarw. S3 transition.