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Heritability of Total and Fixed N Content of the Seed in Two Soybean Populations¹

Improved seed yield in the soybean [Glycine max (L.) Merr.] may result from selection to increase the N₂-fixing ability of the crop. To investigate that possibility, broad-sense heritabilities of total and fixed N in the seed were estimated for two F₂ soybean populations. One population was derived from a cross of high (‘Ware’) and low (D66-5566) N₂-fixing parents and was grown years. The second population, grown in 1 year only, was derived from a cross of intermediate N₂-fixing parents (‘Williams’ and ‘Calland’). Both populations and their parents were grown in ¹⁵N-labeled soil in the field along with ‘Clark’ rj₁rj₁ nonnodulating plants as controls. The amount of fixed N in the mature seed at harvest was determined by the ¹⁵N-dilution method for individual plants. Broad-sense heritability estimates of fixed N content of the seed ranged from 0.53 to 0.60. Estimates for percentage of fixed seed N varied from 0.12 to 0.43. Estimates for total seed N [as a weight (from 0.49 to 0.61) and as a percentage (from 0.20 to 0.87)], for seed yield (from 0.52 to 0.59), and for plant weights (from 0.57 to 0.62) are also reported. strong positive association was noted between seed dry weight and fixed N content of the seed (r = 0.99). These results suggest that improving N₂ fixation in the soybean is feasible through selection based on the mass of fixed N in the seed. Selection for high seed yield will probably likewise result in the identification of plants with a high rate of N₂ fixation.

Key Words: Glycine max (L.) Merr. • Rhizobium japonicum (Kirchner) Buchanan • ¹⁵N-dilution method • Nitrogen fixation • Seed yield • Plant weight

² Former graduate student (now assistant plant breeder, North American Plant Breeders, Brookston, IN 47923); associate professor of crop breeding; associate professor of crop breeding, Dep. of Agronomy, Univ. of Maryland, College Park, MD 20742; and Soil scientist, USDA-ARS, Beltsville Agric. Res. Center, Beltsville, MD 20705, respectively.

Received for publication July 5, 1983.