Pod and Seed Development in Soybean Cultivars with Differences in Seed Size

The accumulation of dry weight by the pod is an important process during reproductive growth in soybeans [Glycine max (L.) Merr.]. Thus, field experiments on Maury silt loam (Typic Paleudalfs) and Egam silt loam (Cumulic Hapludolls) with soybean cultivars differing in seed size were conducted for 2 years to investigate pod and seed development characteristics and associated changes in seed moisture content. Pods were tagged and random samples from different plants were taken at weekly intervals. Measurements included: pod length and width (1979 experiment only), pod wall and seed dry weight, seed moisture percentage, and water content (mg . seed⁻¹). Cultivars differed in maturity and final seed size which ranged from 92 to 262 mg . seed⁻¹. Pod length and width in all cultivars in 1979 were at a maximum when the seeds averaged 4% of their maximum dry weight. Length and width were positively correlated with final seed size. In most cultivars sampled in both 1975 and 1979 there appeared to be little redistribution of dry matter from pod walls prior to seed physiological maturity. However, some redistribution was apparent in ‘Emerald’ and ‘Essex’ in 1979 and ‘Cutler 71’ in 1975. Seed of all cultivars showed rapid water uptake during early seed development and water content (mg . seed⁻¹) reached a maximum before seed physiological maturity. Moisture percentage of the seed was greater than 80% when the seed started rapid accumulation of dry matter and declined progressively during seed development, reaching less than 60% as the seed approached physiological maturity. These changes in percentage moisture during seed development were consistent across cultivars in both years, regardless of seed size. Water uptake and moisture content seemed to he directly related to the stage of seed development and were not affected by genetic differences in seed size.