QUANTUM EFFICIENCY FOR THE PHOTO-ACCUMULATION OF REDUCED PHEOPHYTIN IN PHOTOSYSTEM-II

The quantum efficiency for the photoaccumulation of reduced pheophytin has been estimated using Photosystem II (PS II)-enriched membranes (with Q(A) present) and isolated PS II reaction centres (with Q(A) absent). Sodium dithionite was present in the reaction medium in order to reduce Q(A) in the PS II membranes and also to act as an electron donor to photooxidised P680. The kinetic analyses of the rate of reduction of pheophytin, measured as absorption changes at 685 nm within the two systems, indicated that this reaction was three orders of magnitude more efficient in the isolated PS II reaction centre than in PS II-enriched membranes. When calibrated against the quantum efficiency of Q(A) photoreduction within the PS II-enriched membranes, it was concluded that the absolute quantum yields for the photoaccumulation of pheophytin were 5.2% for the isolated PS II reaction centres and 0.005% for the PS II membranes. Although several factors could account for this very large difference it is suggested that, in part, a possible explanation is that there is a decrease in the quantum efficiency centre when Q(A) is present in its reduced state. Also discussed is the shape of the long wavelength reduced-minus-oxidised pheophytin difference spectrum obtained with the isolated PS II reaction centre in which any effects of the presence of reduced Q(A) are avoided.