Influence of antimycin a and uncouplers on anaerobic photosynthesis in isolated chloroplasts.

Anaerobiosis depresses the light- and bicarbonate-saturated rates of O(2) 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 mum antimycin A or by low concentrations of the uncouplers 0.3 mm NH(4)Cl or 0.25 mum 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 HCO(3) (-), glycerate 3-phosphate, and oxaloacetate. The acceleration of bicarbonate-supported O(2) 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.