No abstract

Soybean [Glycine max (L.) Merr.] seed protein concentration has been declining over the past decades with protein values falling below the ∼40% (dry weight base) required for optimal industrial processing. Further refinement of management practices is needed to help maintain concentrations above market standards. The objective of this work was to evaluate the impact of N fertilization during late reproductive stage on soybean seed protein on two cultivars differing in seed protein concentration in order to untangle the role of biological nitrogen fixation (BNF) to mediate seed protein responses. In experiments at two planting dates, late N fertilization was applied as either a foliar or soil-applied fertilizer and consisted of applying 80 kg ha -1 of N from R3 to R6 with four 20 kg ha -1 N applications. Both cultivar and planting date affected seed yield and protein concentration, ranging from 3,242 to 3,949 kg ha -1 and 34.8 to 38.4 g 100 g -1 , respectively. Soil and foliar N fertilizations had no effect on yield or N uptake but had positive effects on seed protein concentration (+2 g 100 g -1 ) only in the late-planted experiments. Applying N in the soil inhibited BNF. The amount of N 2 fixed during the seed-filling period, expressed per unit seed, is a relatively good predictor of the response of seed protein to N fertilization suggesting a critical role of BNF during this stage for securing a high concentration of seed protein.

Traditionally, commercial soybean breeding has focused on increasing seed yield by crossing elite cultivars and limiting the genetic diversity within commercial germplasm. Wild and ancestral soybean genotypes have higher seed protein concentrations than commercial ones. Different seed protein concentrations and compositions result in diverse functional properties of soybean meal, in particular solubility is important for beverages and protein isolates production. The objectives of our study were (i) to characterize seed protein concentration and composition in genotypes from different soybean domestication periods (types) and (ii) to evaluate the protein concentration and solubility profiles of the defatted meals obtained from these genotypes. Variation within seed and meal protein concentration, composition, and solubility was evident along the domestication process. Wild relative (G. soja) and Elite genotypes had the maximum and minimum seed protein concentrations, respectively (42.9 and 36.3 g 100 g−1). Soybean meal protein concentrations were 55.1, 47.7, 48.4 and 44.1 g 100 g−1 for Wild relative (G. soja), Asian landraces, North American (Nam) ancestors and Elite, respectively. Ample genotypic variation was observed for β‐conglycinin components, such as for β, α, and α′ subunits and for total glycinin and its components. Asian landraces had the highest protein solubility. Wild and ancestral germplasm are a reservoir of useful traits to improve soybean seed quality. This study opens the gates to the introduction of ancestral germplasm to breeding programs focused on protein quality and functionality.