Analysis of factors of cold-weather tolerance to low temperatures during flowering stage, and application to breeding in soybeans [Glycine max] in Hokkaido [Japan]

Hokkaido, which is the largest crop production area in Japan, is located in northern Japan. Cool summers often do the damage to soybeans as well as rice plants in Hokkaido. However, studies related to cold weather-damage and the tolerance in soybeans are much fewer than those in rice plants in our country. Poor growth due to low temperature in the beginning of growth, abscission of flowers and pods due to low temperature during the flowering stage and insufficient grain filling due to low temperature in the pod-filling stage are the three major factors reducing soybean yields affected by cold weather. The most common damage by low temperature is poor growth, but abscission of flowers and pods causes the most severe damage in soybean yield. There are few effective cultivating measures against cold-weather damage, and breeding is the best countermeasure in soybean. Although detail studies have done on breeding for cold-weather damage due to poor growth, very few researches which play a part in breeding for the damage due to flower and pod abscission have been made. In 1993, very severe low temperature during flowering season ruined soybean production in Hokkaido. Thereafter, it has been recognized that improving cold-weather tolerance during the flowering stage is dispensable to stabilize soybean cultivation. Accordingly, it is very important to conduct researches which contribute to soybean breeding for cold-weather damage caused by flower and pod abscission. In this thesis, the author analyzed the soybean growth focusing on environmental factors in regard to cold-weather damage. Response to pod setting under low temperature and mechanisms of the tolerance in varieties with different levels of cold-weather tolerance were clarified. The relations between the cold-weather tolerance and agronomic characteristics were investigated in order to develop a cold-weather tolerant variety by an indirect selection.