THE ORIGIN OF PHOTOSYSTEM-II VARIABLE CHLOROPHYLL FLUORESCENCE

The temperature dependence of fluorescence in photosystem (PS) II subchloroplast particles has been studied at various redox states of Q(A). If the reaction centres (RC) were mainly in the state [PPhQ(A)-.], fluorescence did not decrease with temperature and the application of an external magnetic field did not lead to the increase in chlorophyll fluorescence. Transformation of RC into the state [PPhQ(A) =] by illuminating PS II particles in the presence of dithionite or under anaerobic conditions and dark adaptation of the sample resulted in a monotonous decrease of fluorescence with temperature and the occurrence of the magnetic effect. The analysis and mathematical modelling of these results suggest that the increase in the antenna fluorescence yield due to the transition of RC into the state [PPhQ(A)-.] is due to the decrease in the rate constant of charge separation. If RC are in the state [PPhQ(A) =], part of the emission is a recombination luminescence with the lifetime of congruent-to 2.5 ns and enthalpy of activation of 0.13-0.15 eV.