Intra-raceme variation in pod-set probability is associated with cytokinin content in soybeans

A large proportion of flowers abscise during development in soybean (Glycine max L. Merr.). A reduction in this abscission might increase pod and seed number, and thereby can lead to an increased yield. Previous studies showed that pod-set probability was greater at proximal positions of individual racemes, and that the probability was enhanced by the exogenously applied cytokinins. However, whether intra-raceme variation in the pod-set probability relates to endogenous cytokinin levels remains unknown. To address this question, intra-raceme variation in cytokinin content and pod-set probability was investigated. A soybean genotype IX93-100, which has long racemes, was grown in an environmentally controlled chamber (30/20°C day/night temperature, 15 h day length, 600 μmol m^-2s^-1 photosynthetic photon flux density). Flowers, which were divided into three floral positions (proximal, middle, distal) on individual racemes, were sampled at intervals after anthesis. The cytokinins in the samples were identified by gas chromatography-mass spectrometry (GC-MS) and further quantified by enzyme immunoassay (EIA). The GC-MS analysis revealed that cis-zeatin riboside (c-ZR) and isopentenyl-adenosine (iPA) were predominant forms of cytokinin in soybean racemes. The total amount of these cytokinins in racemes, which was monitored by EIA, peaked one to two weeks after the first flowering on a raceme, when pod development was initiated. Within individual racemes, the total cytokinin concentrations were greater at more proximal floral positions, as was the probability of pod set. Removal of proximal flowers at anthesis enhanced both cytokinin concentrations and pod set at middle positions on the raceme. Thus, pod-set probability was significantly associated with the cytokinin concentration at different floral positions within individual soybean racemes.