RESPONSE OF NODULATING AND NONNODULATING PISUM-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.