In vivo mechanisms of non-photochemical quenching that contribute to energy dissipation in higher plants are still a source of some controversy. In the present study we used an exogenous oxidized quinone, 5-hydroxy-1,4- naphthoquinone to induce quenching of chlorophyll excited states in photosynthetic light-harvesting antenna and to elucidate the mechanism of non-photochemical quenching of chlorophyll fluorescence by this quinone. Excitation dynamics in isolated spinach thylakoids in the presence of an exogenous fluorescence quencher was studied by a combined analysis of data gathered from independent techniques (fluorescence yields, effective absorption cross-sections and picosecond kinetics). The application of a kinetic model for photosystem II to a combined data set of fluorescence decay kinetics and absorbance cross-section measurements was used to quantify antenna quenching by a model antenna quencher, 5-hydroxy-1,4-naphthoquinone. We observed depressions in F(O) and photosystem II absorption cross-sections, paralleled with an increase of the rate constant for excitation decay in antenna. This approach is a first step towards quantifying the amount of antenna quenching contributing to non-photochemical quenching in vivo, evaluation of the contributions of antenna and reaction centre mechanisms to it and localization of the sites of non-photochemical energy dissipation in intact plant systems.
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