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Quenching analysis of chlorophyll fluorescence by the saturation pulse method is used to investigate the pH-dependency of O 2 -dependent electron flow in intact spinach chloroplasts with high ascorbate peroxidase activity. When carboxylase/oxygenase activity is blocked, pho­tochemical and non-photochemical quenching are initially low and increase with illumination time. Quenching shows a pH-optimum around pH 6.5, but only when ΔpH-formation is al­ lowed. It is suggested that overall O 2 -dependent electron flow involves two major components, namely O 2 -reduction (Mehlerreaction) and reduction of the H 2 O 2  formed in the Mehlerreaction, involving enzymic activity of ascorbate peroxidase and monodehydroascorbate reductase. The separated pH-dependencies of light driven O 2 -reduction (presence of KCN) and of H 2 O 2 -reduction (anaerobic conditions) reveal contrasting pH-optima around pH 5 and 8.5, respectively. Energy-dependent, dark relaxable non-photochemical quenching is not observed with O 2 -reduction but with H 2 O 2 -reduction, and only at pH-values above 6.5. The relevance of these findings with respect to regulation of photosynthetic electron flow is discussed. It is suggested that upon limitation of assimilatory electron flow O 2 -dependent non-assimilatory flow is responsible for ΔpH-formation, by which it is autocatalytically stimulated. It is proposed that this autocatalytical reaction sequence is the basis of the so-called “Kautsky effect” of chlorophyll fluorescence induction.

Contrasting pH-Optima of Light-Driven O 2 -and H 2 O 2 -Reduction in Spinach Chloroplasts as Measured via Chlorophyll Fluorescence Quenching