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 QB-non-reducing PS II reaction centres

Abstract

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_PSII ).