Quenching of chloroplast fluorescence by photosynthetic phosphorylation and electron transfer.

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Abstract
Of special interest is the striking quenching of chloroplast fluorescence by the cofactors of cyclic phosphorylation. Cyclic electron flow may be an important mechanism for degrading the energy of excited chlorophyll in vivo when the normal path of complete photosynthesis is blocked. Arnon et al. suggested that during the diurnal closure of stomata in leaves, when gas exchange and hence normal photosynthesis is impeded or altogether stopped, cyclic photo-phosphorylation could occur and form extra ATP for metabolic purposes, for example, for the activation of amino acids for protein synthesis. The quenching of chloroplast fluorescence provides a direct and sensitive technique for relating, in a photosynthetically active system, the excitation of chlorophyll by light with the primary conversion reactions.