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A new hypothesis is presented to explain the major molecular process that regulates the efficiency of light harvesting by chloroplast membranes. It is proposed that in excess light the decrease in the thylakoid lumen pH causes an increase in aggregation of the light harvesting complexes of photosystem II resulting in formation of an efficient pathway for non-radiative dissipation of excitation energy. The aggregation is potentiated by the conversion of violaxanthin to zeaxanthin. This hypothesis is based upon (i) similarity between the spectroscopic changes associated with energy dissipation and those observed upon aggregation of isolated light harvesting complex; and (ii) the link between changes in light scattering and increased energy dissipation.

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.