Non-photochemical quenching of chlorophyll a fluorescence in isolated chloroplasts under conditions of stressed photosynthesis

Non-photochemical quenching of chlorophyll a fluorescence after short-time light, heat and osmotic stress was investigated with intact chloroplasts from Spinacia oleracea L. The proportions of non-photochemical fluorescence quenching (q_N) which are related (q_E) and unrelated (q_I) to the transthylakoid proton gradient (pH) were determined. Light stress resulted in an increasing contribution of q_Ito total q_N.The linear dependence of q. _Eand pH, as seen in controls, was maintained. The mechanisms underlying this type of quenching are obviously unaffected by photoin-hibition. In constrast, q_Ewas severely affected by heat and osmotic stress. In low light, the response of q_Eto changes in pH was enhanced, whereas it was reduced in high light. The data are discussed with reference to the hypothesis that q_Eis related to thermal dissipation of excitation energy from photosystem II. It is shown that q_Eis not only controlled by pH, but also by external factors.Abbreviations and symbols 9-AA 9-aminoacridine - F_o basic chlorophyll fluorescence - F_o variable chlorophyll fluorescence - L₂ saturating light pulse - PS photosystem - q_E pH-dependent, non-photochemical quenching of fluorescence - q_I pH-independent, non-photochemical quenching - q_N entire non-photochemical quenching - q_Q photochemical quenching