HEAT-STRESS STIMULATION OF OXYGEN-UPTAKE BY PHOTOSYSTEM-I INVOLVES THE REDUCTION OF SUPEROXIDE RADICALS BY SPECIFIC ELECTRON-DONORS

A Photosystem I submembrane fraction isolated from spinach was used to study the mechanism of heat-stress stimulation of oxygen uptake by the photosystem. Various artificial electron donors were shown to generate electron transport reactions with various degrees of thermally induced stimulation. A strong stimulation was observed with durohydroquinone as electron donor with a maximal effect at 50-degrees-C. The degree of stimulation obtained was independent from the redox potential of the electron donors and from their oxidation site because the enzyme superoxide dismutase fully inhibited the stimulation. Instead, it is proposed that thermal stress causes the release of membrane bound superoxide dismutase from the thylakoids thus allowing the reduced form of electron donors with specific properties to reduce O2- radicals to H2O2 besides the usual disproportionation of O2 into O2 and H2O2.