Abstract
Heavy rain during the growth stage of soybean has been occurring once every two years in Sakata city, Yamagata prefecture since 2000. The frequency of submergence was higher after 2000 than before 2000. Since submergence damage occurred in soybean in Shonai district, Yamagata prefecture due to heavy rain from Aug 14 to 15 in 2008, we investigated the damage by submergence in variety ”Ryuho” at the ripening stage R4∼R5. In this investigation, the pod number per plant did not vary greatly with the investigation point, and the pod number showed a normal distribution with the highest number in the middle part of the plant. Therefore, it was presumed that the occurrence of blossoms falling off and pod shedding due to submergence was low. The sterile pod number per plant and sterile pod ratio (sterile pod number / whole pod number) increased and the pod number per plant decreased with increasing submergence time. The ratio of sterile pods and small grains (less than 5.5mm) was higher at lower nodes than at upper nodes. This is because the lower part of the plant was exposed to oxygen deficiency for a longer period and was damaged more than the upper part, resulting in earlier defoliation in the lower part. There was no significant correlation between the submergence duration and small grain ratio. The whole grain ratio was decreased greatly by occurrence of damaged grains (wrinkled grain, rotten grain, and skin sliced grain). However, there was no significant correlation between the rate of damaged grains and the pod position on the stem, and the ratio of whole grain was below 50% at all positions on the stem. Therefore, we considered that the soybean method in the field with submergence-damaged should be harvested without mixing with that in the non-submerged field.
