Submitted to: Plant Growth Regulation Society of America Quarterly
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/12/1996
Publication Date: N/A
Interpretive Summary: Soybeans produce many more flowers than ever develop into seed-bearing pods. Each flower is fertilized and begins to produce a pod with seeds. However, most pods fail to develop for more than a few days. This high level of pod abortion is thought to be a major limiting factor in soybean seed production. While it has been possible to decrease the level of abortion at individual flowering nodes by applying an artificial plant hormone, benzylaminopurine (BAP for short), directly onto flowers when they open, it has not been possible to increase or stabilize soybean yield consistently with BAP in the field. Therefore, we examined how the response to BAP under field conditions was affected by genotype and environment. Our results show that soybean genotypes vary widely (some positively, some negatively) in their response to BAP application. Ranking of genotypes for response to BAP was not affected by environment. BAP clearly altered the level of pod set. But it also increased flowers per node, decreased seeds per pod, and decreased seed size, even at nodes not treated with the hormone. We conclude that response to BAP is highly dependent on genotype and dosage. The potential benefits of decreased pod abortion often are offset by negative adjustments in seed number and size resulting in little or no yield advantage.
Technical Abstract: Cytokinins can enhance pod set in soybean when applied directly to flowering racemes. This effect could lead to a growth regulator method to determine whether natural flower abortion limits seed yield. However, response to cytokinin application has been variable under field conditions. Therefore, we conducted two field experiments to evaluate genotypic and environmental effects on the yield response to cytokinin application during flowering. In Exp.I, 1.0 mM benzylaminopurine (BAP) was applied to racemes of 30 progeny lines from crosses involving IX93-100, a genotype having many flowers per raceme and known to be responsive to BAP. Mean pod set across genotypes and environments increased by one or two applications per raceme. Genotypes varied in response, but rankings for one application were similar in two environments. IX93-100 was intermediate in response. In Exp.II, 1.0 mM BAP was applied to racemes of three progeny lines selected as highly, moderately, and non-responsive to BAP application in Exp.I. Seed yield of the highly-responsive genotype was increased by two BAP applications to one raceme per plant. Yield of all three genotypes decreased with BAP application to all racemes. Yield-influencing effects of BAP, in addition to changes in pod set, included an increase in flowers per raceme, a decrease in seeds per pod, and a decrease in seed size. These effects occurred on both treated and untreated racemes. We conclude that BAP treatment during flowering can increase seed yield in soybean, but the response is highly dependent on genotype and dosage. Final seed production in BAP treated plants reflects complex adjustments in reproductive development that may offset any increase in pod set at an individual raceme.