Impact of 2,4-DP and BAP upon pod set and seed yield in soybean treated at reproductive stages

A large number of soybean (Glycine max L. Merr.) flowers and young pods abscise rather than develop into mature pods. Flower and pod drop or abortion accounts for the majority of total reproductive abscission and influences potential soybean yield. The objectives of this study were to determine the patterns of flower, pod and seed development under treatments with the growth regulators, 2-(2,4-dichlorophenoxy) propanoic acid (2,4-DP) and 6-benzylaminopurine (BAP), applied at the early reproductive stages, and to examine the association of reproductive abscission with growth characteristics and agronomic traits, including seed yield and seed weight. Small seeded [cv Pungsan (11.1 +/- 0.4 g 100-seed-1)] and large seeded [cv Manlee (21.0 +/- 0.5 g 100-seed-1)] genotypes were separately planted in the greenhouse and field, and treated with either 2,4- DP or BAP. 2,4-DP (a synthetic auxin) and BAP (a synthetic cytokinin) were each applied at three concentrations (i.e. high, intermediate or low): 0.12 mM, 0.08 mM, 0.04 mM, and 1.5 mM, 1 mM, 0.5 mM respectively. High and low concentrations were employed for greenhouse experiments to examine the number of flowers per plant in pots. With the exception of low BAP (0.5 mM) treatment in Pungsan, all treatments increased total pod number with various numbers of seeds per pod. Low 2,4-DP (0.04 mM) in both genotypes or BAP (0.5 mM) in Manlee significantly reduced flower abortion and delayed abscission of pods in both genotypes, resulting in increased pod setting rates. Under field conditions using intermediate concentrations, 1 mM BAP significantly increased 100- seed weight to 22.3 g at R1 in Manlee and 11.9 g at R3 in Pungsan. BAP (1 mM) at R3 in Pungsan significantly improved seed yield (40.1 g plant-1). Maturity was not significantly affected by either application in Manlee, but was significantly affected by BAP in Pungsan. In Pungsan, 2,4-DP increased pod number, plant height and node number, but decreased 100-seed weight in Pungsan treated at R1, causing no significant change of seed yield. This study suggested that exogenous regulators significantly influenced reproductive and growth characteristics, and consequently seed yield, but increase of pod number was not always beneficial for seed yield.