Physiological bases of environmental and genotypic effects on soybean seed composition

Seed composition is an important determinant of soybean marketing value. Final seed composition is influenced by genotype and environment during seed filling. Variation in protein or oil concentration may determine years or locations with deficits in soybean oil or meal yield. A general model of environmental effects on seed composition to predict regions and/or environmental conditions that produce consistently high quality soybeans is needed. One way to reduce environmental chances of protein deficits is to use high protein varieties. However, breeding for high protein concentration has been hindered by negative correlations with yield and oil. Discovering the physiological bases for environmental and genotypic variation in seed composition is fundamental to overcome these practical restrictions on improving soybean market value. The thesis is organized in five chapters that describe a series of related studies aimed at understanding the physiological bases of environmental and genotypic effects on soybean seed composition. Chapter 2 presents a meta-analytical approach to synthesize published data from studies addressing the effect of major environmental conditions on seed composition. Results showed that increases in seed component concentration are not necessarily linked to synthesis stimulation. Experiments described in Chapter 3 explore two hypotheses that emerged from the meta-analysis focused on the effect of water stress. Chapter 4 provides definitive evidence that assimilate supply per seed underlies genotypic differences in seed protein concentration. Chapter 5 expands on that discovery by relating variation in assimilate supply with the developmental processes that determine seed composition. Finally, experiments described in Chapter 6 utilize molecular markers to explore the genomic basis for two divergent physiological strategies soybeans use to produce high protein seed.