Variability of Soybean Genptypes in Yield Response to Increased Temperature.

High temperature in the summer season negatively affects soybean yield in the temperate region including Japan. We evaluated 24 divergent cultivars from Japan, USA, Thailand and Indonesia (MG III to approx. VII) for their yield and its components under ambient and increased air temperatures (CT and HT). Two soil culture beds (1.5m by 22m) each in a film-tunnel (2.5 m by 25m, and 2.5 m height) with temperature control were used as two reps. for each of CT and HT. The high temperature treatment was applied during 37 days from 30 days after emergence. One cultivar was planted in 12 hills of 0.3m by 0.3m spacing and the soil was kept at optimal moisture content.  The mean max/min air temperatures during the treatment period were 32.2/21.9 in CT and 35.8/23.7 in HT, respectively. The seed weight (g/m2) in HT relative to that in CT ranged among cultivars from -35% to +50%. There was a slight tendency that the earlier maturing cultivars showed more negative responses. The response was examined for yield components, total dry matter at maturity (TDW), total node number (NN) per TDW, pod number (PN) per node, seed number (SN) per pod, and single seed weight. Among these components, the cultivar variation in yield response was associated most closely with response of TDW, followed by those of PN per node and SN per pod. There seems to exist a considerable genotypic variation in response to increased temperature through different responses in biomass accumulation and pod and seed development.