Photoassimilate Supply during Seed Filling Limits Bean Yield and Mineral Composition of Common Beans.

Common bean (Phaseolus vulgaris L.) is a primary source of dietary protein and minerals in sub-Saharan Africa.  There is limited knowledge, however, on the relationship between photoassimilate supply during seed growth and accumulation of these seed important components. This study examined the relationship between assimilate supply per seed and seed yield, yield components, and seed composition of three common bean varieties adapted to sub-Saharan Africa.  Incident radiation to the canopy was decreased by 50% with shade cloth from the beginning of seed filling to physiological maturity to limit assimilate supply per seed. Shaded plants adjusted reproductive sink size by decreasing pods per plant rather than seed weight.   Their seeds had decreased seed oil and starch concentrations and increased seed protein concentration, compared to those of un-shaded plants. Limiting assimilate supply per seed increased seed Zn concentration in two of three growing seasons; seed Fe concentration also increased, but significantly in only one of three years. Variation in seed Fe concentration was negatively correlated with seed oil (r = -0.66, p=0.05) and starch (r = -0.88, p=0.002) concentrations.  Seed protein concentration was positively correlated with Fe (r=0.81, p=0.008) and Zn (r=0.80, p=0.009) concentrations in un-shaded plants and with Fe (r=0.90, p=0.001) in the shaded plants.  These results indicate common beans adjust pod load as well as seed composition in response to a decrease in assimilate supply during seed filling.  The close correlation between seed protein (or N) and Fe concentrations suggests a common mechanism involving assimilate transport to the seeds.