Effect of rising temperature on flowering, pod set, dry-matter production and seed yield in soybean

The effects of rising temperature on growth, yield and dry-matter production of soybean (cv. Enrei) growing in a temperature gradient chamber (TGC) during the whole growth season were investigated from 2002 to 2004. The experimental plots, E1 (control), E2, E3 and E4 (high-temperature plots), were arranged along the temperature gradient in TGC. The average mean air-temperature in E4 was 1.0°C, 1.0°C and 1.6°C, higher and the maximum air-temperature was 2.0°C, 2.0°C and 3.4°C higher than that in E1, in 2002, 2003 and 2004, respectively. Total number of nodes per m² in 2002 and 2003 tended to increase in the high-temperature plots especially due to the increased number of racemes with compound leaves in 2002, and increased number of branches in 2003. The duration of flowering period in E2 and E4 was one and three days shorter than that in E1, respectively, in 2002 and that in E4 was two days shorter than that in E1 in 2004. In 2002 and 2004, the total number of flowers per plant in the high-temperature plots decreased 18.7-31.0% as compared with that in E1 due to the decrease in the number of flowers on the upper order racemes. In 2002 and 2003, plant dry-weight in the early ripening period was heavier in the high-temperature plots than in E1 especially in E3, and the rate of increase in E4 was lower than in E3. In 2004, the higher the temperature, the lighter the plant dry weight in the early ripening period, i. e., lightest in E4. The change in the photosynthetic rate in the fully expanded upper leaf was not different among the plots. Seed yield in E1 was the highest in 2003 and lowest in 2002. A close correlation was observed between plant dry weight and seed yield over three years. In the normal year (2002) and low-temperature year (2003), the seed yield was higher in high-temperature plots especially in E3 than in E1, but in a high-temperature year (2004) the higher the temperature, the lower the seed yield and the yield was lowest in E4. This seed yield decline in the high-temperature plots was due to the decrease in both the number of flowers and pods per m². The relationship between seed yield and mean temperature during the ripening period was different with the year. It was expected that the higher the temperature within a range of 27-31°C, the higher the seed yield, but a further temperature rise restricts the seed yield.