Formation of Hydrogen Peroxide and Oxygen Dependence of Photosynthetic CO2 Assimilation by Intact Chloroplasts

1. Hydrogen peroxide excretion by photosynthesizing intact spinachchloroplasts was determined. The rates were dependent on theoxygen concentration and on the ATP/NADPH requirement of thefinal electron acceptor. Upon CO₂ assimilation a maximum rateof 0.9 µmol H₂O₂/mg chlorophyll/hr and half saturationat 7.5 x 10^–5 M O₂ were found. Excretion of H₂O₂ was considerablyreduced upon photosynthetic reduction of glycerate 3-phosphateor oxaloacetate.
2. Light- and HCO₃-saturated CO₂ assimilationwas inhibited bymore than 50% by anaerobic conditions, whereuponquantum efficiencywas also drastically decreased. However,no anoxic influencewas detected with glycerate 3-phosphateas the terminal electronacceptor and the quantum requirementwith this acceptor wasnot increased by anaerobiosis. Thus theenhancing effect ofoxygen on CO₂ assimilation was ascribedto an improvement ofphotosynthetic ATP supply.
3. Since thestimulation of anaerobic photosynthetic CO₂ assimilationbyoxygen was markedly greater than the concomitant increaseinH₂O₂ evolution, photosynthetic oxygen reduction alone isnotsufficient to produce the required additional ATP for theobservedenhanced CO₂ assimilation. But it provides a meansto avoidthe over-reduction of photosynthetic electron carriersand thusenables aerobic cyclic photophosphorylation. This supportsthehypothesis that cyclic photophosphorylation is not an alternativeto ATP formation by "pseudocyclic" electron transport, but ratherthat it depends on the latter.

(Received January 5, 1981; Accepted March 9, 1981)