Significance of flower differentiation and development in the process of determining soybean [Glycine max] yield: Effects of shading treatment on the number of floral buds and pod sets

Shading treatment (shading rate 89%) was conducted on a soybean canopy (cv. Tatisuzunari) at successive 7-day intervals during 4 weeks before and after flowering, and the number of floral buds and pod sets were investigated. Shading treatment 28-15 days before flowering (DBF) and that 1-21 days after flowering (DAF) decreased the number of floral buds on a basal order raceme (terminal and first order) and upper order raceme (more than first order), respectively. This implies that soybean floral buds were differentiated according to the raceme order from the basal order to the upper order, and that the buds on the upper order raceme play a sink-adjusting roll after flowering. The number of pod sets on basal order raceme was higher than that on the upper order raceme. Shading treatment 28-15 DBF increased the number of pod sets to compensate the decrease in the number of floral buds, but that after 15 DBF decreased the number of pod sets on both the basal and upper order racemes. It was suggested that unfavorable nutritional conditions for the plants not only during the flowering period but also before flowering increased flower or pod abortion. To clarify the effect of dry matter production on the number of floral buds, three rates of shading treatment (40, 60, 82%) were given during both 30-10 DBF and 1-20 DAF. When increasing the shading rate before and after flowering, the number of floral buds decreased on the basal and upper order rancemes, respectively. A higher correlation was found between the net assimilation rate (NAR) and the number of floral buds. This indicates that the decrease in NAR affected floral differentiation. In conclusion, active and rapid growth at the earlier stage and higher leaf photosynthetic capacity during the flowering period were needed to increase floral bud differentiation