Heritability and Repeatability of Enhanced N2 Fixation in Early and Late Inbreeding Generations of Soybean

Abstract

Moderate to high levels of soil nitrate suppress nodulation and N2 fixation in soybean [Glycine max (L.) Merr.], resulting in some instances in a net loss of N from the soil when the high‐protein seed is harvested. Development of nitrate tolerance, i.e., the ability to nodulate and fix N2 in the presence of soil nitrate, in cultivars of soybean is desirable. Our objectives were to (i) combine, through hybridization, the reported nitrate tolerance of Korean genotypes 464, 466, 468, and 469 (Commonwealth of Australia Plant Introduction nos CPI66126, CPI66141, CPI66157 and CPI66159, respectively) with the agronomic and seed traits of cultivars Valder, Reynolds, Forrest and Bossier; (ii) assess heritability and repeatability of N2 fixation between individual F2 plants and F2‐derived F3 lines, and F3‐derived lines in the F6 and F7 generations; and (iii) examine correlations of N2 fixation with agronomic and seed traits. Individual plants and lines were evaluated during five seasons between 1986–1987 and 1990–1991 on alkaline vertisol soils (Typic Pellustert) at Breeza and Narrabri, Australia, and in glasshouse culture at Narrabri. The xylem ureide technique was used to estimate N2 fixation activity of individual F2 plants, and of F2‐derived F3 lines and F3‐derived F6 and F7 lines. Data from F2 and F3 generations showed that relative ureide‐N values for the individual F2 plants was of low heritability. We concluded that single‐plant selection for N2 fixation in the F2 generation was largely ineffective. Data from F6 and F7 plot trials, however, indicated that repeatability on a plot basis for relative ureide‐N was about 0.5 (P< 0.01). Phenotypic correlations between relative ureide‐N in xylem sap and other plant and seed traits were variable and generally non‐significant. The selection of site was vital for detecting enhanced N2 fixation. Sites with low soil nitrate did not allow discrimination among genotypes. Three F3‐derived lines, D22‐8, K78‐1, and A46‐4, which were identified in the F6 and F7 trials at the high‐nitrate sites, had the N2 fixation characteristics of the Korean parents and near‐commercial yields (75–83% of that of the highest yielding cultivar, Forrest.