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Oxygen consumption in Mn-depleted photosystem II (PSII) preparations under continuous and pulsed illumination is investigated. It is shown that removal of manganese from the water-oxidizing complex (WOC) by high pH treatment leads to a 6-fold increase in the rate of O(2) photoconsumption. The use of exogenous electron acceptors and donors to PSII shows that in Mn-depleted PSII preparations along with the well-known effect of O(2) photoreduction on the acceptor side of PSII, there is light-induced O(2) consumption on the donor side of PSII (nearly 30% and 70%, respectively). It is suggested that the light-induced O(2) uptake on the donor side of PSII is related to interaction of O(2) with radicals produced by photooxidation of organic molecules. The study of flash-induced O(2) uptake finds that removal of Mn from the WOC leads to O(2) photoconsumption with maximum in the first flash, and its yield is comparable with the yield of O(2) evolution on the third flash measured in the PSII samples before Mn removal. The flash-induced O(2) uptake is drastically (by a factor of 1.8) activated by catalytic concentration (5-10microM, corresponding to 2-4 Mn per RC) of Mn(2+), while at higher concentrations (>100microM) Mn(2+) inhibits the O(2) photoconsumption (like other electron donors: ferrocyanide and diphenylcarbazide). Inhibitory pre-illumination of the Mn-depleted PSII preparations (resulting in the loss of electron donation from Mn(2+)) leads to both suppression of flash-induced O(2) uptake and disappearance of the Mn-induced activation of the O(2) photoconsumption. We assume that the light-induced O(2) uptake in Mn-depleted PSII preparations may reflect not only the negative processes leading to photoinhibition but also possible participation of O(2) or its reactive forms in the formation of the inorganic core of the WOC.

Hydrogen peroxide production in isolated pea thylakoids was studied in the presence of cytochrome c to prevent disproportionation of superoxide radicals outside of the thylakoid membranes. The comparison of cytochrome c reduction with accompanying oxygen uptake revealed that hydrogen peroxide was produced within the thylakoid. The proportion of electrons from water oxidation participating in this hydrogen peroxide production increased with increasing light intensity, and at a light intensity of 630 micromol quanta m(-2) s(-1) it reached 60% of all electrons entering the electron transport chain. Neither the presence of a superoxide dismutase inhibitor, potassium cyanide or sodium azide, in the thylakoid suspension, nor unstacking of the thylakoids appreciably affected the partitioning of electrons to hydrogen peroxide production. Also, osmolarity-induced changes in the thylakoid lumen volume, as well as variation of the lumen pH induced by the presence of Gramicidin D, had negligible effects on such partitioning. The flow of electrons participating in lumen hydrogen peroxide production was found to be near 10% of the total electron flow from water. It is concluded that a considerable amount of hydrogen peroxide is generated inside thylakoid membranes, and a possible mechanism, as well as the significance, of this process are discussed.

It was found that the contribution of segments of photosynthetic electron transport chain (PETC) besides Photosystem I (PSI) to oxygen reduction increased with increase in light intensity, and at high intensities achieved 50% at pH 5.0, and was higher than 60% at pH 6.5 and pH 7.8. The data are explained as the result of O2 reduction in plastoquinone (PQ) pool as well as in PSI followed by reduction of superoxide radicals generated in both processes by plastohydroquinone., (Copyright 2004 Elsevier B.V.)