Soybean root and nodule nitrate reductase

Abstract

Nitrate reductase (NR) activity was followed in root and nodule from Glycine max (L.) Merr. (Cv. Tracy) inoculated with Rhizobium japonicum. Initially, a plus NO3‐ in vivo assay was used. When chlorate‐resistant mutants were used as inoculum, nodule NR activity was reduced by about 90%. indicating that the bacteroid accounts for much of the normal nodule's NR. With plants 3 to 15 weeks of age nodule NR activity (g fresh weight)‐1 was highest in young plants and root activity highest in old plants. Root and nodule total NR activity increased with plant age and were often not greatly different. Root NR activity correlated with plant NO3‐ supply and increased from 0.8 to 11.4 μmol plant‐1 h‐1 as NO3‐ was increased from 0 to 3 mM. In contrast, nodule NR activity was high in plants grown without NO3‐ and did not appear to increase as nitrate supply to the plant was increased. Nodule activity was 6 to 14 μmol NO2‐ plant‐1 h‐1. Use of a minus NO3‐ in vivo assay had little affect on root NR activity, but greatly reduced nodule activity. Root tissue was found to have 5 to 38 times more NO3‐ than nodule tissue. It is concluded that low nitrate levels within the nodule limit NR activity and that it is improbable that the nodule is a major site of plant nitrate reduction.