Thermoluminescence study of charge recombination in Photosystem II at low temperatures. I. Characterization of the Zv and A thermoluminescence bands

The characteristics of the Z_v and A thermoluminescence bands appearing in the glow curve at about -75 and -30°C, respectively, were investigated in spinach chloroplasts. Inhibitory concentrations of DCMU decreased the amplitude of the Z_v band by half and completely abolished the A band. On the other hand, after two preflashes at +2°C before freezing, the A band could be charged by low-temperature illumination even when the electron transport was interrupted between Q_A and Q_B by DCMU addition after the preflashes. Two-flash preillumination greatly enhanced the amplitude of the A band, but diminished that of the Z_v band. Tris washing and NH₂OH treatment, which inactivated the oxygen-evolving system, almost completely abolished the Z_v band, but did not affect the A band. Severe trypsin treatment, which also impaired the oxygen-evolving system, resulted in a very large intensification of the Z_v band. The half-times of the A and Z_v bands, determined by theoretical analysis of the thermoluminescence data, proved to be about 4 ms and 200–500 μs, respectively. These results, taken together with EPR data from the literature, suggest that the A band arises from charge recombination between a negatively charged acceptor located before the DCMU block (most probably Q⁻_A) and the oxidized donor Z⁺ (which accounts for the EPR Signal II_vf and Signal II_f). The electron carrier responsible for the Z_v band is also a component located prior to the inhibitory site of DCMU (Q⁻_A); its interacting counterpart is an unidentified donor which is involved in charge exchange with the S states.