Global warming and increasing soybean production in warm regions require a quantitative understanding of how yield-formation processes of this crop are affected by high temperatures. The response of soybean growth to increased temperature was examined under a controlled environment that mimics field conditions using a temperature gradient chamber (TGC). In 2009 and 2010, the (cv) Enrei cultivar was grown in soil culture beds (1m×24m) in two TGCs. Three temperature treatments, T a (near ambient temperature), T a+1 (ambient temperature +1°C) and T a+3 (ambient temperature +3°C), were established by dividing the rows along which the temperature gradient was created. In 2009, only cvs. Ryuho and Suzuyutaka were grown under two temperature regimes. The mean temperatures in 2009 and 2010 for the entire growth period ranged from 25.9 to 28.7°C and from 27.1 to 30.1°C for T a and T a+3, respectively. The individual seed growth rate (SGR) was determined as the linear coefficient of the single-seed weight vs. time (d), based on periodic plant harvesting. The effective seed-filling period (EFP) was calculated by dividing the final single-seed weight by the SGR. The flowering date was affected by increased temperature only in 2010. The days to beginning of seed fill and maturity were longer under higher temperatures. The SGR was slower at T a+3 (7.1 and 5.9mg seed -1d -1 in 2009 and 2010, respectively) than at T a (8.5 and 7.5mg seed -1 d -1 in 2009 and 2010, respectively), whereas the EFP was longer under high temperatures than at near ambient temperature. The final single-seed weight was reduced by increased temperatures. The temperature had no significant effect on cell volume, but the number of cells per cotyledon was smaller at T a+3 (3.4×10 6 and 2.6×10 6 in 2009 and 2010, respectively) than at T a (5.0×10 6 and 4.3×10 6). The results indicated that an increase in temperature by 3°C in warm regions decreased the seed size of soybean by decreasing the cell number and SGR.
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