Thermoluminescence study of charge recombination in Photosystem II at low temperatures. II. Oscillatory properties of the Z v and A thermoluminescence bands in chloroplasts dark-adapted for various time periods

Abstract

The oscillations of the Z V and A thermoluminescence bands were investigated in spinach chloroplasts which had been dark-adapted for various time periods and subjected to a series of flashes at +2°C before continuous illumination at various low temperatures. When excited with continuous light below −65°C, the Z V band exhibited period-4 oscillation, with maxima on preflashes 0, 4 and 8. Above −65°C, the oscillation pattern depended greatly on the dark-adaptation period of the chloroplasts. In preilluminated samples (15 s light followed by 3 min dark), when the Q B pool is half oxidized, the oscillation of the thermoluminescence intensity measured at −50°C was similar to that observed below −65°C. However, after the thorough dark-adaptation of the chloroplasts (6 h), when the major fraction of the Q B pool is assumed to be oxidized, a binary oscillation appeared in the oscillation pattern, with maxima at odd flash numbers. Below −65°C, period-2 oscillation of the Z V band could not be induced by the dark-adaptation of the chloroplasts, suggesting an inhibition of electron exchange between Q A and Q B. Upon excitation of the chloroplasts with continuous light at −30°C, the A band oscillated with a periodicity of 4 with maxima at preflash numbers 2 and 6. At pH 7.5, the period-4 oscillation was converted into a period-2 oscillation by thorough dark-adaptation of the chloroplasts (24 h). Model calculations of the oscillatory patterns suggest that the period-4 oscillations of the Z V and A bands are determined by the concentrations [S 0] + [S 1] and [S 2] + [S 3], respectively, which are present after the preflashes prior to the low-temperature continuous illumination. The period-2 oscillations in the amplitudes of the Z V and A bands reflect the changes occurring in the redox state of the Q B pool in a sequence of flashes. The possible relationship between the characteristics of the Z V and A bands and the temperature-dependence of the S state transitions was investigated. Comparison of the amplitudal changes of the B (S 2Q − B and S 3Q − B recombination) and Q (S 2Q − A recombination) thermoluminescence bands as a function of the excitation temperature suggests that the S 2 → S 3 and S 3 → S 4 transitions are blocked at about −65 and −40°C, respectively. It is also concluded that the thermoluminescence intensity emitted by the reaction center is about twice as high in the S 3 state as in the S 2 state.