Photoreduction of Molecular Oxygen in Preparations of Photosystem II under Photoinhibitory Conditions

Preparations of photosystem II (PSII) from pea (Pisum sativum L.) leaves were used to study the evolution and reduction of molecular oxygen under photoinhibitory conditions. Under these conditions, the photoinduced oxygen uptake did not exceed 10% of the total oxygen-evolving activity in PSII preparations. Both the Hill and the Mehler reactions were found to occur simultaneously under long-term illumination of PSII preparations with high-intensity light in the presence of potassium ferricyanide. During this light treatment in the presence of potassium ferricyanide, the rate of oxygen uptake increased gradually reaching 30% of the oxygen-evolving activity. The photogeneration of superoxide anion radical at increasing light intensities followed a typical light-response curve with a light saturation at 800 W/m². The results provide evidence that the Mehler reaction is the major source for superoxide and hydrogen peroxide in PSII preparations under photoinhibitory conditions and that the Mehler reaction in PSII proceeds more effectively at high light intensities. The relatively low and sustained rate of oxygen photoreduction in PSII preparations under photoinhibitory conditions substantiates the hypothesis on the involvement of Mehler reaction in cell signaling and regulation.