To clarify the effect of water stress after reproductive growth stage on the seed size (weight of 100 seeds) and the seed coat cracking ratio (percentage of the seeds that have cracked seed coat) of soybean, we conducted pot experiments. Soybeans were direct-seeded (cv. ‘Shurei’ in late June in 2013 and 2014, and cv. ‘Sato-no-hohoemi’ in late May and late June in 2015) to 1/5000 a plastic pots filled with heavy clayey soil in a plastic film house. Water was automatically supplied to the soil surface of each pot by drip irrigation and, in the control plot, soil water was kept to the level which was lower than the field capacity but applied no water stress to the plants. Irrigation amount at a time was set to 0.1 L/pot in all plots before flowering. After flowering, water stress treatments were incorporated by reducing single irrigation amount to 0.07 L/pot. The treatments were incorporated from flowering to early seed filling stage in a ‘former stress (FS) plot’ and from early seed filling stage to harvesting in a ‘latter stress (LS) plot’. As little drainage from pot was occurred, the amount irrigated should be nearly equal to evapotranspiration amount. Seed size was largest in the FS plot and smallest in the LS plot. The seed coat cracking ratio did not significantly differ among plots, but differ among cultivations for each cultivar. The seed coat cracking ratio was not significantly correlated with the average seed size of each plot. The seed coat cracking ratio was smaller in 2014 for ‘Shurei’ and in later seeding in 2015 for ‘Sato-no-hohoemi’, possibly because temperature, vapor pressure deficit, solar radiation and transpiration requirement after pod setting were smaller in those cultivation periods compared with the other cultivation periods for each cultivar.
