Effects of Sowing Time on the Yield and the Occurrence of Delayed Stem Senescence in Soybean in South Kanto

The effects of sowing time on the yield and the occurrence of delayed stem senescence in soybean cultivar Enrei and Tamahomare were examined from 2006 to 2008, in the field at the experimental farm of Nihon University and in pots in 2009. Both cultivars were sown in mid-May and mid-June at two planting densities (60cm and 30 cm row space×15 cm intrarow spacing). In both cultivars, the leaf area index, the top dry weight and the yield were higher in mid-May sowing than in mid-June sowing. In Tamahomare, the yield in mid-June sowing was the same as that in mid-May sowing, and was higher at the higher planting density. However, in Enrei, the yield and the number of branch nodes in mid-June sowing were not increased at the higher planting density. In Tamahomare, the number of branch nodes and pods on the branch were increased by increasing the planting density. No significant difference in the occurrence of delayed stem senescence was observed between the mid-May sowing and mid-June sowing in Tamahomare. On the other hand, in Enrei, the occurrence of delayed stem senescence was increased by seeding in mid-May. In Tamahomare sown in mid-May and mid-June and in Enrei sown in mid-June, the amount of cytokinin supplied from root to top decreased at the seed filling stage, but, in Enrei sown in mid-May the cytokinin supply did not decrease at the seed filling stage. In conclusion, Enrei must be sown in mid-June to obtain high quality and high yield soybean in south Kanto. In Tamahomare, the occurrence of delayed stem senescence was not changed by changing the sowing time from mid-May to mid-June. Moreover, in mid-June sowing a high planting density 30 cm row space×15cm intrarow spacing is necessary to obtain a high yield.