Comparisons Among Effective Filling Period, Reproductive Period Duration,and R5 to R7 in Determinate Soybeans1

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Abstract
Visual estimates of seed filling duration are important tools in soybean [Glycine max (L.) Merr.] research. Yet little information exists on the correspondence between visual estimates of seed filling duration and direct measurements of seed mass accumulation. The objectives of our study were a) to compare visual estimates of seed filling duration with those obtained from seed growth data to determine if they could be used interchangeably and b) to determine their comparative stability across environments, to assess which estimates of seed filling duration would be a better selection criterion.Five field experiments were conducted in 2 years at Gainesville, FL. Crop growth analyses were performed on 88 random F7 lines from two crosses and their parents in 1982, and on 20 selected lines combined from both crosses in 1983. Genotypes used had determinate growth habit and were in Maturity Groups VII and VIII. The period R5 to R7 was determined additionally on 56 random genotypes in 1983. Effective filling period (EFP) was determined as the time required to achieve final mature plant seed mass using the calculated linear seed growth rate. Reproductive period duration (RPD), defined as the period of linear harvest index (HI) crease, was estimated disregarding fallen leaves.The beginning of EFP and RPD occurred simultaneously while stage R5 occurred 9 to 17 days earlier. The period R5 to R7 was 8 to 13 days longer than EFP, and 1 to 7 days longer than RPD. Associations among EFP, RPD, and R5 to R7 were always positive, but not consistently significant. Therefore, EFP, RPD, and R5 to R7 estimated different periods in the reproductive ontogeny of determinate soybeans. The usefulness of EFP, RPD, and R5 to R7 as selection criteria appears to be limited by the existence of highly significant genotype by environment interactions that were primarily due to changes in the ranking of genotypes across environments. Low broad sense heritability estimates for R5 to R7 were observed.