Response of nodulating and non-nodulatingPisum sativum L. to nitrate

This study examined whether ‘Risnod2’ and ‘Risnod27’ non-nodulating mutants of pea (Pisum sativum L.) provided with increasing concentrations of nitrate could achieve a growth and nitrogen accumulation comparable to their parental N₂-fixing cv. Finale. In the cv. Finale, nodule number, nodule dry mass accumulation, total C₂H₂-reducing activity of nodulated roots (TAR) and estimated N₂ fixation were considerably inhibited at 5.0 and 10.0 mM root medium NO₃ ⁻ concentrations. In contrast a 0.63 mM level stimulated both the nodule dry mass and TAR. The cv. Finale N₂-fixing plants grown on 0 to 2.5 mM NO₃ ⁻ levels had higher shoot N concentrations than the Nod⁻ mutants, but within the 5.0 to 10.0 mM levels the Nod⁻ mutants approached or even overtopped the N concentration of the cv. Finale plants. Compared with a high positive correlation found in the Nod⁻ mutants, shoot N concentration in the cv. Finale was negatively correlated with the root medium NO₃ ⁻ concentration. The pattern of nitrogen content in shoot dry mass was very similar to that seen in the shoot dry mass accumulation. The Nod⁻ mutants grown on the 5.0 and/or 10.0 mM NO₃ ⁻ level had plant dry mass, shoot nitrogen concentration, shoot nitrogen content, and root/shoot dry mass ratio comparable with those of the nodulating cv. Finale grown on the same nitrate levels.