Temperature Response of Seed Growth Characteristics of Soybeans1

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Abstract
The growth rate and duration of an individual seed are important parameters in the yield production process of a grain crop. Experiments were conducted in the CSIRO phytotron at Canberra, Australia to investigate the effect of temperature on the rate and duration of seed growth and associated plant characteristics in soybeans [Glycine max (L.) Merr.]. Soybeans (‘Fiskeby V’) were grown at 24/19 C until beginning seed growth and shifted to 18/13, 24/19, 27/22, 30/25, and 33/28 C (day/night temperature, 8‐hour day) until maturity. Seed growth rates (SGR) were estimated by harvesting seeds at 5‐day intervals, and from the growth rate of excised cotyledons cultured in nutrient media for 96 hours. The SGR of seed developing on the plant increased from 6.1 to 7.9 mg seed−1 day−1 as air temperatures increased from 18/13 to 27/22 C, but there was no further change as the temperature increased to 33/28 C. Excised cotyledons showed a similar growth rate response to temperature. Exposing the plants to high temperatures (33/28 C) during the period of flowering and pod set reduced SGR (36%) regardless of the temperature during the subsequent seed growth period, suggesting that seed growth is sensitive to temperature levels at these early stages. Seed respiration on a dry weight basis (mgCO2h−1 g−1) was not affected by temperature. The duration of seed growth was not affected by temperatures of 24/19 to 30/25 C but was reduced by 3 days at 33/28 C, and this was associated with accelerated leaf senescence as shown by leaf yellowing and reductions in CO2 exchange rate. Final seed size was reduced from 200 to 151 mg seed−1 at 18/13 and 33/28 C. The data suggest that SGR and duration are relatively insensitive to temperatures ranging from 24/19 to 30/25 when these are imposed after flowering and pod development. The reduction in the duration of seed growth at high temperatures may be one mechanism by which high temperatures reduce yield.

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