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.