Physiological Investigations of a Tobacco Mutant with O2-Resistant Photosynthesis and Enhanced Catalase Activity

Experiments are described further indicating that O(2)-resistant photosynthesis observed in a tobacco (Nicotiana tabacum) mutant with enhanced catalase activity is associated with decreased photorespiration under conditions of high photorespiration relative to net photosynthesis. The effects on net photosynthesis of (a) increasing O(2) concentrations from 1% to 42% at low CO(2) (250 microliters CO(2) per liter), and (b) of increasing O(2) concentrations from 21% to 42% at high CO(2) (500 microliters CO(2) per liter) were investigated in M(6) progeny of mutant and wild-type leaf discs. The mutant displayed a progressive increase in net photosynthesis relative to wild type with increasing O(2) and the faster rate at 42% O(2) was completely reversed on returning to 21% O(2). The photosynthetic rate by the mutant was similar to wild type in 21% and 42% O(2) at 500 microliters CO(2) per liter, and a faster rate by the mutant was restored on returning to 250 microliters CO(2) per liter. The results are consistent with a lowered release of photorespiratory CO(2) by the mutant because greater catalase activity inhibits the chemical decarboxylation of alpha-keto acids by peroxisomal H(2)O(2). Higher catalase activity was observed in the tip and middle regions of expanding leaves than in the basal area. On successive selfing of mutant plants with enhanced catalase activity, the percent of plants with this phenotype increased from 60% in M(4) progeny to 85% in M(6) progeny. An increase was also observed in the percent of plants with especially high catalase activity (averaging 1.54 times wild type) on successive selfings suggesting that homozygosity for enhanced catalase activity was being approached.