Inhibition of quantum yield of PS II electron transport in Spirulina platensis by osmotic stress may be explained mainly by an increase in the proportion of the Q(B)-non-reducing PS II reaction centres

Modulated chlorophyll fluorescence and fluorescence induction kinetics were used to evaluate the PS II photochemistry in Spirulina platensis exposed to osmotic stress (0-0.8 M mannitol). Osmotic stress decreased the efficiency of excitation energy capture by open PS II reaction centres F(v)'/F(m)') and more significantly, decreased photochemical quenching (q(p)). Osmotic stress also decreased the maximal efficiency of PS II photochemistry (F(v)/F(m)). There was no significant change in non-photochemical quenching (q(N)), indicating that the decreased F(v)'/F(m)' was not due to an increase in q(N). Analyses of the fast fluorescence induction kinetics indicated that osmotic stress caused a significant increase in the proportion of the QB-non-reducing PS II reaction centres. Based on the results in this study, we suggest that a substantial increase in the proportion of the QB-non-reducing PS II reaction centres may be responsible for the decrease in q(p) and F(v)'/F(m)', of which both resulted in the decrease in the quantum yield of PS II electron transport (Φ(PS II)).