Nitrogen fixation potential and agronomic evaluation of three hypernodulating mutants of soybean

The three hypernodulating mutants (NOD1-3, NOD2-4, and NOD3-7) of soybean (Glycine max (L.) Merr.) and the parent (Williams) were evaluated for short-term NO$\sb3\sp-$ effect on N$\sb2$ fixation, measured by an acetylene (C$\sb2$H$\sb2$) reduction assay and by the relative ureide abundance technique. Nitrate was supplied to 3-week-old, hydroponically grown plants with fully developed nodules. Application of 5 mM NO$\sb3\sp-$ resulted in inhibition of nitrogen fixation of all soybean lines, Williams being most affected. Of all mutants tested, NOD1-3 was most tolerant of NO$\sb3\sp-$ in terms of C$\sb2$H$\sb2$ reduction activity, and also showed highest relative ureide abundance in dry tissue extracts. It was found, however, that the C$\sb2$H$\sb2$ reduction activity per gram of nodules was lower for NOD1-3 than for Williams, in both the presence and absence of NO$\sb3\sp-$. NOD1-3 had higher nodule ureide concentration and had similar glutamine synthetase activity in nodule tissue. Nitrate nutrition was also found to result in ureide accumulation in all plant parts of both NOD1-3 and Williams. The data obtained from young plants indicated that although the hypernodulating mutants of soybean possess appreciable NO$\sb3\sp-$ tolerance in terms of nodule initiation and formation, N$\sb2$ fixation was still inhibited by the presence of NO$\sb3\sp-$.The three hypernodulating mutants were also compared with Williams and one non-nodulating mutant (NN5) in a two-year field study. In the absence of N fertilizer, two-year data based on the C$\sb2$H$\sb2$ reduction assay indicated that all hypernodulating mutants fixed more N$\sb2$ than did Williams. NOD1-3 was found to have the highest nodule activity. Based on relative ureide abundance, there were no differences among soybean lines tested in 1988. Both the relative ureide abundance assay (comparing NN5 with other lines in 1989) and C$\sb2$H$\sb2$ reduction assay in 1989 confirmed that nodulation of Williams was completely inhibited by 200 kg N ha$\sp{-1}$ through the initial sampling date. In the absence of N fertilizer and at later stages of plant development, N input from N$\sb2$ fixation by Williams was very similar to that of the mutants. Overall these hypernodulating mutants appear to have a limited N$\sb2$ fixation advantage in the field, but only at early growth stages. Yields from the mutants were, however, 10 to 30% less than that of Williams in both years. Seed protein and oil concentrations were not different among mutants and the Williams parent in 1988. NOD1-3 actually had lower protein concentrations in 1989. Collectively these results indicate that the hypernodulating mutants are inferior to Williams from an agronomic standpoint, although backcrossing them with Williams is expected to improve their performance.U of I OnlyETDs are only available to UIUC Users without author permissio