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 (ΔpH) were determined. Light stress resulted in an increasing contribution of q Ito total q N.The linear dependence of q. Eand ΔpH, 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 ΔpH 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 ΔpH, but also by external factors.