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Steiger, HM; Beck, E: Formation of hydrogen-peroxide and oxygen dependence of photosynthetic CO2 assimilation by intact chloroplasts, Plant and Cell Physiology, 22, 561-576 (1981), doi:10.1093/oxfordjournals.pcp.a076199
Abstract:
Hydrogen peroxide excretion by photosynthesizing intact spinach chloroplasts was determined. The rates were dependent on the oxygen concentration and on the ATP/NADPH requirement of the final electron acceptor. Upon CO2 assimilation a maximum rate of 0.9 μmol H2O2/mg chlorophyll/hr and half saturation at 7.5 × 10−5M O2 were found. Excretion of H2O2 was considerably reduced upon photosynthetic reduction of glycerate 3-phosphate or oxaloacetate. Light- and HCO3-saturated CO2 assimilation was inhibited by more than 50% by anaerobic conditions, whereupon quantum efficiency was also drastically decreased. However, no anoxic influence was detected with glycerate 3-phosphate as the terminal electron acceptor and the quantum requirement with this acceptor was not increased by anaerobiosis. Thus the enhancing effect of oxygen on CO2 assimilation was ascribed to an improvement of photosynthetic ATP supply. Since the stimulation of anaerobic photosynthetic CO2 assimilation by oxygen was markedly greater than the concomitant increase in H2O2 evolution, photosynthetic oxygen reduction alone is not sufficient to produce the required additional ATP for the observed enhanced CO2 assimilation. But it provides a means to avoid the over-reduction of photosynthetic electron carriers and thus enables aerobic cyclic photophosphorylation. This supports the hypothesis that cyclic photophosphorylation is not an alternative to ATP formation by “pseudocyclic” electron transport, but rather that it depends on the latter.

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