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Metabolism and Enzymology

Further Studies on O₂-Resistant Photosynthesis and Photorespiration in a Tobacco Mutant with Enhanced Catalase Activity

Department of Biochemistry and Genetics, The Connecticut Agricultural Experiment Station, P. O. Box 1106, New Haven, Connecticut 06504

The increase in net photosynthesis in M₄ progeny of an O₂-resistant tobacco (Nicotiana tabacum) mutant relative to wild-type plants at 21 and 42% O₂ has been confirmed and further investigated. Self-pollination of an M₃ mutant produced M₄ progeny segregating high catalase phenotypes (average 40% greater than wild type) at a frequency of about 60%. The high catalase phenotype cosegregated precisely with O₂-resistant photosynthesis. About 25% of the F₁ progeny of reciprocal crosses between the same M₃ mutant and wild type had high catalase activity, whether the mutant was used as the maternal or paternal parent, indicating nuclear inheritance. In high-catalase mutants the activity of NADH-hydroxypyruvate reductase, another peroxisomal enzyme, was the same as wild type. The mutants released 15% less photorespiratory CO₂ as a percent of net photosynthesis in CO₂-free 21% O₂ and 36% less in CO₂-free 42% O₂ compared with wild type. The mutant leaf tissue also released less ¹⁴CO₂ per [1-¹⁴C]glycolate metabolized than wild type in normal air, consistent with less photorespiration in the mutant. The O₂-resistant photosynthesis appears to be caused by a decrease in photorespiration especially under conditions of high O₂ where the stoichiometry of CO₂ release per glycolate metabolized is expected to be enhanced. The higher catalase activity in the mutant may decrease the nonenzymatic peroxidation of keto-acids such as hydroxypyruvate and glyoxylate by photorespiratory H₂O₂.

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