Phenotypic Recurrent Selection for Modified Reproductive Period in Soybean
Seed-filling duration has generally shown a positive association with seed yield in soybean [Glyelne max (L.) Merr.], but results have been inconsistent among studies. Four phenoWpic recurrent selection programs were completed, increased flowering.period duration (FPD) vs. increased seed-filling duration (SFD) and long vs. short reproductive period duration (RPD), with the objective of investigating the effects of these modifications on seed development and seed yield. Selection for average physiological maturity (RT) maintained R7 maturities among populations. Selection for increased RPD through FPD or SFD increased genetic correlations between reproductive stages. The major responses to selection occurred in the first cycle due in part to the 3-yr evaluation, but high correlations between stages also restricted progress. Manipulating RPD modified primarily FPD. The SFD seemed to have a minimum threshold, but responded to selection pressure. Divergent selection for RPD created a 10-d difference in RPD, but the selection had minimal effects on seed yield potential. The soybean has the capacity to achieve seed yield independent of changes in duration of reproductive stage. Increasing RID through increased SFD within a broad-base population had an associated increase in seed set and in seed yield for a set of favorable environments, but this yield advantage was not expressed for a set of environments with drought stress during the reproductive period. Increased SFD was associated with increasing potential number of seed set and, as has been shown, decreasing assimilate uptake rate. Selection for increased SFD would be important for populations where SFD could be limiting. The effects of modifying seed-filling duration are not clear-cut, but manipulating facets that affect it can create unacceptable responses.
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