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

The nr(1) 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 + NO(2)). It was tested for possible nonenzymic NO(x) formation and evolution because of known chemical reactions between NO(2) (-) 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 NO(3) (-) reduction, NO(2) (-) accumulation, and the NO(x) evolution processes of the wild type (cv Williams) and the nr(1) mutant. In vivo NR assays showed that wild-type leaves had three times more NO(3) (-) reducing capacity than the nr(1) mutant. NO(x) evolution from intact, anerobic nr(1) leaves was approximately 10 to 20% that from wild-type leaves. Nitrite content of the nr(1) mutant leaves was usually higher than wild type due to low NO(x) evolution. Lag times and threshold NO(2) (-) concentrations for NO(x) evolution were similar for the two genotypes. While only 1 to 2% of NO(x) from wild type is NO(2), the nr(1) mutant evolved 15 to 30% NO(2). 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 NO(2) (-) to NO. However, this report concludes that NO(x) evolution by the nr(1) mutant was due to nonenzymic, chemical reactions between plant metabolites and accumulated NO(2) (-) 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.