Effect of Seed Position Within Pod on Seed Size and Its Spatial Distribution Across Main Stem in Soybean

Seed size in different seed position and its spatial distribution across main stem were investigated by glyphosate-resistant soybean cultivar SV19-U2 grown under both ambient and light-enriched conditions at low and high densities.Obvious effect of seed position on seed size was found.Seed size of first-seeded position within several-seeded pod was minimal.In two,three and four-seeded pod,the seed size of second-seeded position was 10.6%,14.8% and 17.3% larger than that of first-seeded position.Light enrichment and high density increased seed size of first-seeded position by 17.7%-27.9%.The increment in seed size from first-seeded position was greater than that from second-,third-and fourth-seeded position,while the inherent seed size character in different position was not changed.The spatial distribution of seed size from different seed position across main stem showed the spinning shape from top to bottom.Seed size at bottom node position was smaller,and was larger at middle node position.The curve of seed size's spatial distribution across main stem was even and seed size in one-seeded pod was much sensitive to light and density conditions.Under high density,light enrichment imposed more influence on seed size at top node position than at lower node position.The more seed number in a pod could decrease the increment of single seed size.It is proposed that seed size at different position might not be directly related to the sequence of seed filling,and seed size is modulated by assimilate distribution mechanism within pod.