Influence of Antimycin A and Uncouplers on Anaerobic Photosynthesis in Isolated Chloroplasts 1

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RESUMO

Anaerobiosis depresses the light- and bicarbonate-saturated rates of O2 evolution in intact spinach (Spinacia oleracea) chloroplasts by as much as 3-fold from those observed under aerobic conditions. These lower rates are accelerated 2-fold or more by the addition of 1 μm antimycin A or by low concentrations of the uncouplers 0.3 mm NH4Cl or 0.25 μm carbonyl cyanide m-chlorophenylhydrazone. Oxaloacetate and glycerate 3-phosphate reduction rates are also increased by antimycin A or an uncoupler under anaerobic conditions. At intermediate light intensities, the rate accelerations by either antimycin A or uncoupler are inversely proportional to the adenosine 5′-triphosphate demand of the reduction process for the acceptors HCO3−, glycerate 3-phosphate, and oxaloacetate. The acceleration of bicarbonate-supported O2 evolution may also be produced by adding an adenosine 5′-triphosphate sink (ribose 5-phosphate) to anaerobic chloroplasts. The above results suggest that a proton gradient back pressure resulting from antimycin A-sensitive cyclic electron flow is responsible for the depression of light-saturated photosynthesis under anaerobiosis.

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