Seed Protein Content and Delivery of Assimilates to Soybean Seed Embryos

Increasing seed protein content in soybean [Glycine max (L.) Merr.] is associated with reduced seed yields. The objectives Of this study were to investigate the effects of seed protein content on the capacity to deliver dry matter and amino acids to the seed embryo and to identify possible yield‐limiting factors. Fifteen soybean genotypes representing 0.42 to 0.51 g g‐1 seed protein content were grown in the greenhouse during the winter season. High seed‐protein content did not limit specific dry matter accumulation rates. Removing side leaflets to reduce assimilate availability reduced assimilate uptake rates 21%. The reduced uptake rates were not associated with seed protein content. The modified seed structure associated with increased seed protein content did not limit rate of assimilate uptake by the seed. Thus, high seed protein content did not reduce sink activity. Genotypes with high seed protein content had higher rates of increase in seed density than the cultivar Ransom, indicating possible increased seed maturation rates for these genotypes. Rates of amino acid unloading from opened seed coats into a collection medium were associated With the seed protein content. The ratio of rate of amino acid unloading from opened seed coats to rate of dry matter delivery to the embryo was similar to the seed protein content. On the average, selecting for high seed protein content increased the capacity for amino acid production and transport and for amino acid unloading from the seed coat; however, additional factors were involved in the control, of seed protein content. A possible yield‐limiting factor, which is supported by the data on changes in seed density, is the capacity for amino acid unloading from the seed coat.