Comparison between NOx evolution mechanisms of wild-type and nr1 mutant soybean leaves

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 NOx (NO + NO2). It was tested for possible nonenzymic NOx formation and evolution because of known chemical reactions between NO2- and plant metabolites and the instability of nitrous acid. It did not evolve NOx during the in vivo NR assay, but intact leaves did evolve small amounts of NOx under dark, anaerobic conditions. Experiments were conducted to compare NO3- reduction, NO2- accumulation, and the NOx 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. NOx 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 NOx evolution. Lag times and threshold NO2- concentrations for NOx evolution were similar for the two genotypes. While only 1 to 2% of NOx from wild type is NO2, the nr1 mutant evolved 15 to 30% NO2. The kinetic patterns of NOx evolution with time were 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 NOx evolution by wild type is primarily an enzymic conversion of NO2- to NO. However, this report concludes that NOx evolution by the nr1 mutant was due to nonenzymic, chemical reactions between plant metabolites and accumulated NO2- and/or decomposition of nitrous acid. Nonenzymic NOx evolution probably also occurs in wild type to a degree but could be easily masked by high rates of the enzymic process.