Lower total soluble sugars in vegetative parts of soybean plants are responsible for reduced pod number under shading conditions

Much of the variation in soybean yield is associated with changes in pod number per plant, however physiological regulation of pod number is not well understood. Three determinate soybean cultivars, Hai339 (H339), Heinong35 (HN35) and Kennong18 (KN18) were grown for 2 years at 27 plants m in a field experiment and exposed to shading and light enrichment. The data of total soluble sugar (TSS) in leaf, stem and pod wall samples at middle nodes collected every 10 days from 20 days after flowering (DAF) (beginning of the linear seed filling period) to 60 DAF (physiological maturity) were analyzed. The average pod number was reduced by 45% in 2007 and 40% in 2008 with shading, while the average pod number was increased by 49% in 2007 and 70% in 2008 with light enrichment. Shading significantly decreased leaf TSS concentration by 16~39% in 2007 and 14~43% in 2008 and decreased stem TSS concentration by 27% for H339, 19% for HN35, 40% for KN18 across the two years. At 20 DAF, shading decreased the pod wall TSS concentration by 44% for H339, 36% for HN35, 25% for KN18. Shading effect on stem TSS concentration was much greater than leaf and pod wall TSS concentration. The TSS concentration in vegetative parts was not affected by light enrichment. Our results suggested the reduction of TSS concentration was directly responsible for lower pod number under shading conditions, while the increase in pod number by light enrichment is more likely regulated by some other mechanism.