Assimilation of15N2and15NO3−by Partially Nitrate-Tolerant Nodulation Mutants of Soybean

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Abstract
Growth-chamber studies were conducted to evaluate nitrogen assimilation by three hypernodulated soybean [Glycine max (L.) Merr.] mutants (NOD1–3, NOD2–4, NOD3–7) and the Williams parent. Seeds were inoculated at planting and transplanted at day 7 to nutrient solution with 1 mol m–3 urea (optimizes nodule formation) or 5 mol m–3 NO3– (inhibits nodule formation). At 25 d after planting, separate plants were exposed to 15NO2 or 15NO3– for 3 to 48 h to evaluate N2 fixation and NO3– assimilation. Plant growth was less for hypernodulated mutants than for Williams with both NO3– and urea nutrition. The major portion of symbiotically fixed 15N was rapidly assimilated (30 min) into an ethanol-soluble fraction, but by 24 h after treatment the ethanolinsoluble fraction in each plant part was most strongly labelled. Distribution patterns of 15N among organs were very similar among lines for both N growth treatments after a 24 h 15N2 fixation period; approximate distributions were 40% in nodules, 12% in roots, 14% in stems, and 34% in leaves. With urea-grown plants the totalmg 15N fixed plant–1 24 h–1 was 1·18 (Williams), 1·40 (N0D1-3), 107 (NOD2-4), and 0·80 (NOD3-7). The 5 mol m-3 NO3- treatment resulted in a 95 to 97% decrease in nodule mass and 15N2 fixation by Williams, while the three mutants retained 30 to 40% of the nodule mass and 17 to 19% of the 15N2 fixation of respective urea-grown controls. The hypernodulated mutants, which had restricted root growth, absorbed less 15NO3- than Williams, irrespective of prior N growthcondition. The 15N from 15NO3- was primarily retained in the soluble fraction of all plant parts through 24 h. The 15N incorporation studies confirmed that nodule development is less sensitive to external NO3- in mutant lines than in the Williams parent, and provide evidence that subsequent metabolism and distribution within the plant was not different among lines. These results further confirm that the hypernodulated mutants of Williams are similar in many respects to the hyper- or supernodulated mutants in the Bragg background, and suggest that a common mutational event affectingautoregulatory control of nodulation has been targeted.

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