Enhancement of the ΔpH-dependent dissipation of excitation energy in spinach chloroplasts by light-activation: correlation with the synthesis of zeaxanthin

The extent of energy-dependent quenching of chlorophyll fluorescence in broken spinach chloroplasts has been quantitatively related to the size of the thylakoid proton gradient as measured by the quenching of 9-aminoacridine fluorescence by titration at constant irradiance with the uncoupler nigericin or by change in irradiance. It was found that chloroplasts prepared from leaves that had been pre-illuminated with strong light for 30 min showed energy-dependent quenching at a lower proton gradient than chloroplasts prepared from dark-adapted leaves. Measurement of the carotenoid composition of the thylakoids showed that light treatment raised the ratio of zeaxanthin: violaxanthin. The possible dependence of energy-dependent quenching on xanthophyll composition and the physiological implications of this light-activation process to the regulation of photosynthetic electron transport are discussed.