Influence of Soybean Seed Size and Position on the Rate and Duration of Filling1

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Abstract
Information on factors influencing the rate and duration of seed growth is needed to understand completely yield production. This study was undertaken to determine the relationship of seed size and the timing of pod development to the rate and duration of seed growth in soybeans (Glycine max (L.) Merrill). Field studies were conducted for 2 years (1974‐75) on a Maury silt loam soil (Typic Paleudults) utilizing five cultivars of varying seed size: ‘Kanrich’, 234 mg/seed; ‘Williams’, 171 mg/seed; ‘Cutler 71’, 165 mg/seed; ‘Custer’, 129 mg/seed; and ‘Essex’, 114 mg/seed. Each year the rate of dry matter accumulation in the seed was measured on the first and the last pods to develop on each cultivar. Kanrich had the highest seed growth rate (7.96 mg seed−1 day−1); Essex the lowest rate (3.64 mg seed”1 day1), and the other cultivars had rates intermediate between Kanrich and Essex. Seed growth rates were relatively constant across the early and late pods. The duration of the effective filling period (EFP) was less for the late pods on the indeterminate cultivars (Kanrich, Williams, Cutler 71, and Custer), and this resulted in slightly smaller seed. There was no difference in the length of the EFP among the five cultivars used. Air temperatures during the filling period were lower in 1974 than in 1975, and the low temperatures were associated with low seed growth rates, a long EFP and large seed. These data suggest that the rate of seed growth is partially determined by the genetic makeup of the seed. The direct relationship between seed growth rates and seed size provides an explanation of the negative correlations frequently observed between seed size and the number of seed produced by the plant.