Heritabilities and Associations of Nodule Mass and Recovery of Bradyrhizobium japonicum Serogroup USDA 110 in Soybean

Information is needed on the feasibility of breeding to increase the contribution of fixed N₂ to the seed yield and protein content of soybean [Glycine max (L.) Merr.]. The objectives this study were to: i) determine the heritabilities of nodule mass as formed by indigenous bradyrhizobia and of percent recovery of Bradyrhizobium japonicum (Kirchner) Buchanan serogroup USDA 110, ii) examine the relationship betweenodulation ability with indigenous bradyrhizobia and the ability to selectively form nodules with an inoculant strain of B. japonicum, and iii) determine the influence of nodule mass and percent recovery of serogroup USDA 110 on seed yield and other agronomic traits. Four selected plant introductions were crossed to form three populations. Random F₃-derived, F₅ and F₆ lines (F₃:₅, F₃:₆) from these populations were grown in the field at three locations in Minnesota during 1982. Heritability estimates for indigenous nodule mass, on an entry mean basis over locations, ranged from 54 to 67% for the three populations. This result suggests progress in selecting for increased nodule mass with native bradyrhizobia should be possible. A positive relationship of nodule mass to seed yield was observed indicating that further selection for increased nodulation with the native bradyrhizobia may be warranted. Heritability estimates over locations for percent recovery of serogroup USDA 110 ranged from 26 to 52% for the three populations. Inoculation with strain USDA 110 increased seed yield at two locations and nodule mass at one location. The percentage recovery of serogroup USDA 110 was significantly increased (P = 0.05) from a background level of 1.6% to average of 9.9% in the inoculated rows. However, percent recovery of serogroup 110 was not correlated with seed yield or seed protein. Ability of a line to nodulate preferentially with serogroup USDA 110 was not dependent on the line's relative ability to nodulate with the indigenous strains.