Yield trade-off and the role of parental selection based on seed size when breeding for soybean seed protein

Alternative physiological strategies can increase protein concentration in soybean: (i) more-than-proportional increases in seed protein content (mg seed(-1)) relative to increases in carbohydrate and oil content in large-seeded genotypes or (ii) more-than-proportional reductions in carbohydrate and/or oil content relative to protein content reductions in small-seeded genotypes. Because these strategies differentially affect crop growth and development, we hypothesized that populations developed from high-protein (HP) parents with contrasting seed sizes will present differences in how seed yield and protein concentration correlate. To test this, three breeding strategies were developed by mating high-yielding cultivars and HP ones that differ in seed sizes, reflecting the alternative strategies mentioned above. Neither tested crossing strategies showed differences in their correlation values between seed yield and protein concentration, as initially expected. Nevertheless, populations developed from crossing a HP-small-seeded parent to a HP-large-seeded one showed the highest number of transgressive segregants for protein yield. Our results showed that parent selection based on seed size has no beneficial effects on the development of high-yielding, HP soybean populations, but it might affect the number of transgressive segregants for protein yield.