ABSTRACT
Oxygen ist reduced by the electron transport chain of chloroplasts during CO2 reduction. The rate of electron flow to oxygen is low. Since antimycin A inhibited CO2-dependent oxygen evolution, it is concluded that cyclic photophosphorylation contributes ATP to photosynthesis in chloroplasts which cannot satisfy the ATP requirement of CO2 reduction by electron flow to NADP and to oxygen. Inhibition of photosynthesis by antimycin A was more significant at high than at low light intensities suggesting that cyclic photophosphorylation contributes to photosynthesis particularly at high intensities. Cyclic electron flow in intact chloroplasts is under the control of electron acceptors. At low light intensities or under far-red illumination it is decreased by substrates which accept electrons from photosystem I such as oxaloacetate, nitrite or oxygen. Obviously, the cyclic electron transport pathway is sensitive to electron drainage. In the absence of electron acceptors, cyclic electron flow is supported by far-red illumination and inhibited by red light. The inhibition by light exciting photosystem II demonstrated that the cyclic electron transport pathway is accessible to electrons from photosystem II. Inhibition can be relieved by oxygen which appears to prevent over-reduction of electron carriers of the cyclic pathway and thus has an important regulatory function. The data show that cyclic electron transport is under delicate redox control. Inhibition is caused both by excessive oxidation and by over-reduction of electron carriers of the pathway.
ABSTRACT
The development of photosystem I activity of plastids isolated from greening barley (Hordeum distichum, L.) leaves was studied. The electron transport activity in photosystem I was measured as anthraquinone-mediated oxygen uptake and as light induced absorbance changes of the reaction centre molecule P700. P700 oxidation was observed after one hour of greening though an electron transport leading to oxygen uptake was observed after 30 minutes. Phenazine methosulphate had no effect on the oxidation of P700 until after four hours of greening. The ratio chlorophyll/P700 decreased from about 2300/l at one hour to 640/l at sixteen hours of greening. The light intensity dependence of the electron transport of photosystem I showed that the photosynthetic units gradually increased in size as the greening proceeded. The increase of the rate of the oxygen uptake, calculated on plastid basis, decreased after eight hours while the P700 content, calculated on plastid basis, increased continuously between three and sixteen hours. Chromatographic separations and fluorimetric analyses of the chlorophyll pigments showed that the reaction centre molecule could not be protochlorophyllide or chlorophyllide.
ABSTRACT
In photosynthetically competent chloroplasts from spinach the quantum requirements for oxygen evolution during CO2 reduction were higher, by a factor often close to 1.5, than for oxygen evolution during reduction of phosphoglycerate. Mass spectrometer experiments performed under rate-limiting light indicated that an oxygen-reducing photoreaction was responsible for the consumption of extra quanta during carbon dioxide assimilation. Uptake of 18O2 during reduction of CO2 was considerably higher than could be accounted for by oxygen consumption during glycolate formation and by the Mehler reaction of broken chloroplasts which were present in the preparations of intact chloroplasts. The oxygen reducing reaction occurring during CO2 assimilation resulted in the formation of H2O2. This was indicated by a large stimulation of CO2 reduction by catalase, but not of phosphoglycerate reduction. Catalase could be replaced as a stimulant of photosynthesis by dithiothreitol or ascorbate, compounds known to react with superoxide radicals. There was no effect of dithiothreitol and ascorbate on phosphoglycerate reduction. A main effect of superoxide radicals and/or H2O2 was shown to be at the level of phosphoglycerate formation. Evidence for electron transport of oxygen was also obtained from 14CO2 experiments. The oxidation of dihydroxyacetonephosphate during a dark period or after addition of carbonyl cyanide p-trifluoromethoxyphenyl-hydrazone in the light was studied. The results indicated a link between the chloroplast pyridine nucleotide system and oxygen. Oxygen reduction during photosynthesis under conditions where light is rate limiting is seen as important in supplying the ATP which is needed for CO2 reduction but is not provided during electron transport to NADP. A mechanism is discussed which would permit proper distribution of electrons between CO2 and oxygen during photosynthesis.
