Fluorescence analysis during steady-state photosynthesis

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Abstract
Photosynthetic gas exchange of attached leaves has been measured under steady-state conditions at different light intensities and correlated with simultaneous measurements of chlorophyll fluorescence and oxidized P 700 (by absorbance changes at 820 nm). The data suggest that during light-saturated assimilation, photosystem II (PSII) photochemistry is mainly controlled by non-photochemical and non-radiative dissipation of excitation energy, rather than by accumulation of reduced acceptor, Q A , and this could be related to ‘high-energy quenching’ of fluorescence. The occurrence of oxidized P 700 at saturating light and low concentration of CO 2 suggests that in the steady state PSI photochemistry is controlled by a shortage of electron donation from the plastoquinone pool (photosynthetic control), rather than by excess electrons at the acceptor side. The significance of the oxidized form of P 700 as a ‘quencher’ of excitation energy is discussed. This control of photosystems I and II, both related to the proton gradient across the thylakoid membrane, may serve to match the potential rate of net photochemistry to the demand by the biochemical reactions. However, when light-saturated assimilation is not limited by CO 2 , PSI activity is controlled by accumulation of reduced electron acceptors, rather than by photosynthetic mechanisms. Photosynthetic control has been found to determine the redox state of the ferredoxin-thioredoxin system.