Night Temperature Effects on Morphology, Phenology, Yield and Yield Components of Indeterminate Field-Grown Soybean

Small changes in geography and topography can result in great differences in night temperature. Indeterminate soybeans [Glycine max (L.) Merr. ‘S09-90’] of Maturity Group 0 were grown under field conditions in 1981 and 1982 to assess the effect of low night temperature on growth and seed yield. The soil was a Woodburn silt loam (fine-silty, mixed, mesic Aquultic Argixerolls). Mean minimum night temperature treatments were: Check (uncontrolled, ca. 10°C), 16±1°C and 24±2°C. Elevated night temperatures were achieved by enclosing the plots at night in polyethylene-covered chambers and increasing chamber temperature with electric heaters. The chamber covers were removed each morning to provide natural field conditions during the daylight hours. Treatments were applied from 2 weeks after crop emergence until physiological maturity. Higher night temperatures enhanced vegetative growth. However, morphological characteristics, e.g., plant height, number of nodes, and number of auxiliary branches were not significantly (P < 0.05) different among the treatments at maturity since the growth of these components also terminated earlier for plants receiving higher night temperatures. Warmer night temperatures considerably shortened the time period between plant emergence and appearance of first flower (R₁) and appearance of flowers at the node immediately below the uppermost node (R₂), while they had little additional effect on the subsequent reproductive development. Physiological maturity (R₇), however, was hastened by higher night temperatures. Accumulated growing degree-days appeared to be of value for the prediction of all developmental stages and were a good indicator of the time period between emergence and physiological maturity of plants within treatments. Plants with warmer nights, however, accumulated more heat units than the check for the onset of all developmental stages. Seed weight/plant increased above the check by 27 and 37% for 16°C, and 20 and 23% for the 24°C treatments in 1981 and 1982, respectively. The growth of individual seeds appeared to be most responsive to night temperatures, and increased seed size (1000-seed weight) accounted for the seed yield increase of both treatments. Final numbers of seeds/plant and pods/plant were not significantly affected by the treatments. The number of seeds/pod for the 24°C treatment was, however, significantly greater than the check. The data indicate that the genetic potential for seed yield of indeterminate soybeans is not realized under low night temperatures. This study emphasizes the importance of night temperatures as a potential criterion for developing new soybean cultivars.