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Formation of Hydrogen Peroxide and Oxygen Dependence of Photosynthetic CO2 Assimilation by Intact Chloroplasts
Authors:Steiger  Hans-Martin; Beck  Erwin
Institution:Lehrstuhl für Pflanzenphysiologie, Universität Bayreuth, Universitätsstraße 30 D 8580 Bayreuth, W-Germany
Abstract:
  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)
Keywords:
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