Selection for yield shifted the proportion of oil and protein in favor of low-energy seed fractions in soybean

Owing to the high contents of oil and protein, soybean [Glycine max L. (Merr.)] seeds are energy-rich. Genetic improvement of soybean yield is linked to the dilution in seed protein; changes in low-energy compounds such as carbohydrates have received less attention. We tested the hypothesis that selection for yield in soybean has shifted seed composition in favor of low-energy compounds. We characterized the dynamics of three seed fractions (protein, oil, and residual) during seed filling in seven commercial soybean varieties released between 1980 and 2013 in a two-year field experiment. Genetic gain in yield was linked with reduced seed energy cost: seed protein concentration decreased by 0.06% yr(-1), oil remained stable, and the residual fraction increased by 0.09% yr(-1). Seed accumulation of the residual fraction revealed a shorter lag phase and a longer effective filling period for the modern relative to the older soybean varieties. Seed energetic requirement significantly decreased - 0.06% yr(-1). Resource availability per seed during the seed-filling period was not limiting across all tested varieties. Further research is warranted on the biochemical nature of the residual fraction, and on the implications of seed composition for future breeding, soybean meal and end uses.