Resistant Photosynthesis and Enhanced Catalase Activity

Experiments are described further indicating that 02-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 O2 concentrations from 1% to 42% at low CO2 (250 microliters CO2 per liter), and (b) of increasing O2 concentrations from 21% to 42% at high CO2 (500 microliters CO2 per liter) were investigated in M6 progeny of mutant and wild-type leaf discs. The mutant displayed a progressive increase in net photosynthesis relative to wild type with increasing O2 and the faster rate at 42% 02 was completely reversed on retuming to 21% 02. The photosynthetic rate by the mutant was similar to wild type in 21% and 42% 02 at 500 microliters CO2 per liter, and a faster rate by the mutant was restored on returning to 250 microliters CO2 per liter. The results are consistent with a lowered release of photorespiratory CO2 by the mutant because greater catalase activity inhibits the chemical decarboxylation of a-keto acids by peroxisomal H202. 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 M4 progeny to 85% in Ms 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. The suggestion that the photorespiratory pathway is capable of "leaking" additional CO2 under conditions of high photorespiration relative to net photosynthesis, as occurs at high 02 and high temperature, came from observations on the ease with which H202 decarboxylates keto-acids of the pathway such as hydroxypyruvate and glyoxylate (10). This could lead to carbon ratio stoichiometries ofCO2 release, C02/glycolate, in excess of the 25% that would result if the CO2 were produced only from the conversion of 2 glycines to form seine and CO2 in the pathway (1-3). The possibility was thus raised that increasing catalase activity might decrease the concentration ofperoxisomal H202 generated by the glycolate oxidase reaction and thereby decrease the peroxidation of keto acids and increase net photosynthesis. I have previously (8) described the selection of a tobacco mutant that exhibited 02-resistant photosynthesis in intact seedlings as well as in leaf discs (Tables III, IV, and V in ref. 8. Evidence was presented that the mutant has a higher net photosynthetic rate in normal air and a lower rate of photorespiration than wild type especially at lower CO2 levels and higher 02 concentrations or higher temperatures (8, 9). The resistance trait cosegregated precisely with enhanced catalase activity in M4 progeny of the mutant in which about 60% of the siblings showed enhanced catalase activity (9), while other peroxisomal activities such as glycolate oxidase (8) and hydroxypyruvate reductase (9) were the same as wild type. The ratio of photorespiration/net photosynthesis in C3 plants is regulated reversibly by three factors: the concentration of C02, amount of 02, and temperature. The greater net CO2 assimilation by the mutant relative to the wild type at 38°C versus 30°C (8) was consistent with the hypothesis that the mutant has a decreased photorespiration. The progressive increase in the net photosynthesis ratio of mutant/wild type of 10% or more on increasing the 02 level from 1% to 21% to 42% in intact seedlings and leaf discs (8) was used to define 02-resistant photosynthesis and is also consistent with a decreased photorespiration in the mutant. Net photosynthesis by the mutant and wild type has now been investigated to determine whether any aberrations occur during the stepwise increase in 02 levels. Experiments have also been conducted to test whether 02-resistance by the mutant is abolished by high CO2 in a reversible manner, as would be expected if02-resistance is associated with decreased photorespiration. Observations on the catalase activity in different locations of the lamina during development are described. The frequency of enhanced-catalase progeny (>1 to 2 SDs above the mean of wild type) in successive selfings in the M4, M5, and M6 generations has been determined as well as the proportion with unusually high catalase activity (>3 SDS above the mean of wild type), and it was found that the percent of both categories increased with each selfing suggesting the population is becoming more nearly homozygous for this phenotype. MATERIALS AND METHODS Tobacco (Nicotiana tabacum, L. cv Havana Seed) was grown in the greenhouse as previously described (8, 9). Plants from an environmentally controlled chamber were used in one experiment (normal air; 12 h light, 350 ,umol photons m-2 s-I, 24°C; 12 h dark, 1 8°C). Experiments were conducted on M4, M5, and M6 progeny of plants with high catalase activity (and 02-resistant photosynthesis) that were self-pollinated. All of the mutant plants used were initially derived