Effects of pH and Oxygen on Photosynthetic Reactions of Intact Chloroplasts

Oxygen inhibition of photosynthesis was studied with intact spinach (Spinacia oleracea L.) chloroplasts which exhibited very high rates of photosynthetic CO(2) reduction and were insensitive to additions of photosynthetic intermediates when CO(2) was available at saturating concentrations. Photosynthetic rates were measured polarographically as O(2) evolution, and the extent of the reduction of substrate was estimated from the amount of O(2) evolved. With CO(2) as substrate, inhibition of photosynthesis by O(2) was dependent on pH. At pH values above 8, rates of O(2) evolution were strongly inhibited by O(2) and only a fraction of the added bicarbonate was reduced before O(2) evolution ceased. The extent of O(2) evolution declined with increasing O(2) concentration and decreasing initial bicarbonate concentration. At pH 7.2, the initial photosynthetic rate was inhibited about 30% at high O(2) levels, but the extent of O(2) evolution was unaffected and most of the added bicarbonate was reduced. Photosynthetic O(2) evolution with 3-phosphoglycerate as substrate was similarly dependent on pH and O(2) concentration. In contrast, there was little effect of O(2) and pH on oxaloacetate-dependent oxygen evolution. Acid-base shift experiments with osmotically shocked chloroplasts showed that ATP formation was not affected by O(2). The results are discussed in terms of a balance between photosynthetic O(2) evolution and O(2) consumption by the ribulose diphosphate oxygenase reaction.