ΔpH-dependent chlorophyll fluorescence quenching indicating a mechanism of protection against photoinhibition of chloroplasts

Intact isolated spinach chloroplasts were subjected to photoinhibitory conditions (high light and lack of CO₂). Photoinhibition of the electron transport system was considerably diminished when the chloroplasts were in a low-fluorescent state related to a high proton gradient across the thylakoid membranes, as compared to a high-fluorescent state in which ΔpH-dependent fluorescence quenching was abolished by addition of uncouplers. The hypothesis is discussed that in chloroplasts exposed to excess light, photoinhibition is partly prevented by increased thermal dissipation of excitation energy, as expressed by ΔpH-dependent (‘energy-dependent’) chlorophyll a fluorescence quenching.