The number of seeds per unit area is an important yield component in soybean [Glycine max (L.) Merr.]; however, the mechanisms responsible for the regulation of this yield component are not well understood. Field experiments were conducted at Lexington, KY (3 yr), and at Taian, China (1 yr), to investigate the relationship between net canopy photosynthesis and seeds per unit area using genotypes with differences in individual seed growth rates (SGR). At Lexington, shades (30 and 63% reduction in insolation) were placed over plots from growth stage R1 until maturity to create differences in canopy photosynthesis. Planting dates (early and late) and row spacing (wide and narrow) were used at Taian to create differences in canopy photosynthesis. Crop growth rate (CGR) was measured between growth stage R1 and R5 as an estimate of net canopy photosynthesis. Yield, seeds per m2, and SGR were also measured. Within each genotype, there was a linear relationship between CGR and seeds per ml across treatments and years. Within an experiment, seeds per m2 at a constant CGR was inversely related to genotypic differences in SGR. A partitioning coefficient was estimated by dividing the total sink demand (seeds per m2 X SGR) by CGR. There were no apparent genotypic differences in partitioning coefficient; however, partitioning coefficient decreased linearly as CGR increased. The data suggest that the model proposed by Charles-Edwards, which describes seeds per m2 as a direct function of canopy photosynthesis and a partitioning coefficient and in inverse function of assimilate flux to individual seeds, accurately describes the regulation of seeds per m2 in soybean.
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