Charge accumulation and recombination in Photosystem II studied by thermoluminescence. I. Participation of the primary acceptor Q and secondary acceptor B in the generation of thermoluminescence of chloroplasts

Abstract

In the glow curves of chloroplasts excited by a series of flashes at +1°C the intensity of the main thermoluminescence band appearing at +30°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 S 3 state of the water-splitting system in the radiative charge recombination reaction. After long-term dark adaptation of 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°C as well as by a single flash in which case the water-splitting system undergoes only one transition (S 1 → S 2). The experimental observations and computer simulation of the oscillatory patterns suggest that the B band originates from charge recombination of the S 2B − and S 3B − 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 second band (Q band; Q, primary acceptor of Photosystem II) appears in the glow curve at +10°C which overlaps with the B band. In chloroplasts excited by flashes prior to DCMU 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 (S 1 → S 2). 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 S 2Q − or S 3Q − states depending on whether the water-splitting system is in the S 2 or the S 3 state. The half-time of the back reaction of Q − with the donor side of PS II (S 2 or S 3 states) is 3 s. It was also observed that 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 S 2 state to the S 3 state. This result implies that the midpoint potential of the water-splitting system remains unmodified during the S 2 → S 3 transition.