Relationship between assimilate supply per seed during seed filling and soybean seed composition

Developing soybean cultivars with high seed protein concentration has been hampered by the negative correlation between seed protein and seed yield. While previous in vitro studies have documented the impact of assimilate supply to the seed in determining seed protein, in planta studies generally have failed to link seed protein accumulation directly with assimilate supply per plant during seed filling. It may be possible to reconcile this apparent contradiction by expressing the relationship between seed protein and assimilate supply in planta on a per seed basis. We evaluated the association between seed composition and assimilate supply per seed in closely related experimental lines varying in seed protein concentration and in several elite varieties from the Iowa State University breeding program. High seed protein content was associated with greater leaf area per seed at R5.5, which was a consequence of fewer seeds set per plant. The more favorable source/sink ratio provided greater assimilate per seed during grain filling, but limited the yield potential of the high protein lines because of reduced seed set. Depodding during grain filling increased seed size of low protein lines and increased seed protein concentration to levels comparable to those in the untreated high protein lines. Seed size was far less responsive to depodding in the high protein lines. These results suggest that high protein lines maintain assimilate supply per seed at or near saturating levels during seed filling. Improving seed protein levels in high yielding varieties will require increasing assimilate supply per seed without sacrificing seed numbers.