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Metabolism and Enzymology

Comparison between NO_x Evolution Mechanisms of Wild-Type and nr₁ Mutant Soybean Leaves ¹

Department of Agronomy, University of Nebraska, Lincoln, Nebraska 68583-0817

The nr₁ soybean (Glycine max [L.] Merr.) mutant does not contain the two constitutive nitrate reductases, one of which is responsible for enzymic conversion of nitrite to NO_x (NO + NO₂). It was tested for possible nonenzymic NO_x formation and evolution because of known chemical reactions between NO₂^– and plant metabolites and the instability of nitrous acid. It did not evolve NO_x during the in vivo NR assay, but intact leaves did evolve small amounts of NO_x under dark, anaerobic conditions. Experiments were conducted to compare NO₃^– reduction, NO₂^– accumulation, and the NO_x evolution processes of the wild type (cv Williams) and the nr₁ mutant. In vivo NR assays showed that wild-type leaves had three times more NO₃^– reducing capacity than the nr₁ mutant. NO_x evolution from intact, anerobic nr₁ leaves was approximately 10 to 20% that from wild-type leaves. Nitrite content of the nr₁ mutant leaves was usually higher than wild type due to low NO_x evolution. Lag times and threshold NO₂^– concentrations for NO_x evolution were similar for the two genotypes. While only 1 to 2% of NO_x from wild type is NO₂, the nr₁ mutant evolved 15 to 30% NO₂. The kinetic patterns of NO_x evolution with time weré completely different for the mutant and wild type. Comparisons of light and heat treatments also gave very different results. It is generally accepted that the NO_x evolution by wild type is primarily an enzymic conversion of NO₂^– to NO. However, this report concludes that NO_x evolution by the nr₁ mutant was due to nonenzymic, chemical reactions between plant metabolites and accumulated NO₂^– and/or decomposition of nitrous acid. Nonenzymic NO_x evolution probably also occurs in wild type to a degree but could be easily masked by high rates of the enzymic process.

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