: Soybean plants differentiate abundant floral buds, but most of them fail to grow pods and abort during development. Many studies indicated promotive effects of exogenously applied cytokinin on pod setting, but the effects of auxin application on pod set are ambiguous. In this study, we examined the changes in the concentrations of endogenous auxin and cytokinin in racemes and the effects of application of the two hormones on pod setting to clarify the role of auxin and cytokinin in soybean pod setting. The longraceme soybean genotype IX93-100 was grown in pots and in the field. The auxin (IAA, indoleacetic acid) concentration in racemes was high for a long period from pre-anthesis to 9 days after anthesis (DAA) of the first flower on a raceme, but the cytokinin concentration was high for a short period, with a peak at 9 DAA. The IAA concentration was higher in distal portions of racemes, but the cytokinin concentration was higher in basal portions of racemes. In pot-grown plants, IAA applied to racemes tended to reduce the number of flowers and pods. In contrast, 6-benzylaminopurine (BA) applied to racemes before anthesis tended to reduce the number of flowers and pods, and that applied around 7 DAA significantly increased the pod-set percentage. However, these effects of IAA and BA application were slight in field-grown plants. These results indicate that the concentration of endogenous auxin and cytokinin in racemes changes in a different manner, and that cytokinins have a positive, and auxin a negative effect on pod setting when respective hormones are applied to racemes after the anthesis stage.

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Supernodulating soybean cultivar "Sakukei 4" has a high nitrogen fi xing activity. The yield of soybean is reduced under high groundwater-level conditions, but the reduction might be mitigated by using such cultivars. In this study, we investigated the yield of Sakukei 4 treated with a high groundwater level at different times, especially at the time of fl owering and seed set. The cultivars Enrei and Sakukei 4 differed in the growth period during which a high ground water level treatment affected the number of fl owers and seed weight. In both cultivars, the number of fl owers and seed weight were decreased by the high ground water level treatment applied during the fl oral differentiation stage. The high ground water level treatment applied during the fl owering time did not promote the falling of fl owers and pods in Enrei, but promoted it in Sakukei 4. The main reason for the decrease in yield of Sakukei 4 was the falling of fl owers and pods in high-order racemes and the decrease in the pod number. In this study, the use of supernodulating soybean cultivar Sakukei 4 did not mitigate the yield reduction under the high ground water level conditions at the time of fl owering. Key words Flowering and seed-setting habit, High groundwater level, Number of pod, Sakukei 4, Soybean (Glycine max (L.) Merr.).