Phenology and Yield Responses of Soybean to Elevated Air Temperatures in a Temperate Region

Abstract

Increased temperature means and fluctuations associated with climate change are predicted to exert profound effects on the development, growth, and yield of soybean. Experiments were conducted to evaluate the impacts of global warming on the phenology, yield-related traits, and yield of two determinate soybean cultivars in a temperate region (37.27°N, 126.99°E

Suwon, South Korea). These two soybean cultivars, Sinpaldalkong [maturity group (MG) IV] and Daewonkong (MG VI), were cultured on various sowing dates within a four-year period, under no water-stress conditions. Soybeans were kept in greenhouses controlled at the current ambient temperature (AT), AT+1.5°C, AT+3.0°C, and AT+5.0°C throughout the growth periods. Growth periods (VE–R7) were significantly prolonged by the elevated temperatures, especially the R1–R5 period. Cultivars exhibited no significant differences in seed yield at the AT+1.5°C and AT+3.0°C treatments, compared to AT, while a significant yield reduction was observed at the AT+5.0°C treatment. Yield reductions resulted from limited seed number, which was due to an overall low numbers of pods and seeds per pod. Heat stress conditions induced a decrease in pod number to a greater degree than in seed number per pod. Individual seed weight exhibited no significant variation among temperature elevation treatments

thus, seed weight likely had negligible impacts on overall seed yield. The number of pods, the main yield determinant, was determined by flower number and pod set ratio. Regardless of cultivars or temperature treatments, the temporal distribution of flowering showed a bimodal temporal distribution. The latter peak was delayed and increased with temperature elevation above ambient in both cultivars, indicating that elevated temperatures increased the flowering duration and the number of flowers differentiated lately. The relative frequency of early flower abscission and the proportion of flower abscission to pod abscission increased with temperature rise, indicating that the increase of pod set failure would be attributed to non-viable pollen or a reduction in photosynthate supply under high temperature stress. Temperature elevation significantly decreased the pod set in hot summer (the period of flowering and pod setting) years, 2013 and 2015, but not in cool summer year, 2014 without a temperature × cultivar interaction. Regardless of cultivars, pod number variations were more closely associated with pod set percentage variations rather than with flower number variations in hot summer years (2013 and 2015), whereas the opposite associations were observed in cool summer year (2014). Using the boundary line analysis with quadratic and beta function, cardinal temperatures (Topt, Tmax) for pod set were estimated at 34.8, and 42.9°C for the Sinpaldalkong cultivar and 34.5, and 41.6°C for the Daewonkong cultivar, respectively. The temperature response of pod set was shown to be similar to the reported temperature response of pollen germination or pollen tube growth

suggesting that pod set would be strongly related to pollen viability. Although the temperature rise could lead to the increase of flower production, the temperature above 35°C would sharply decrease the pod set ratio in soybean. A boundary line analysis (using quantile regression) estimated optimum temperatures for seed number at 26.4 to 26.8°C (VE–R5) for both cultivars

the optimum temperatures (R5–R7) for single seed weight were estimated at 25.2°C for the Sinpaldalkong smaller-seeded cultivar, and at 22.3°C for the Daewonkong larger-seeded cultivar. The optimum growing season (VE–R7) temperatures for seed yield, which were estimated by combining the two boundary lines for seed number and seed weight, were 26.4 and 25.0°C for the Sinpaldalkong and Daewonkong cultivars, respectively. Considering the current soybean growing season temperature, which ranges from 21.7 (in the north) to 24.6°C (in the south) in South Korea, and the temperature response of potential soybean yields, further warming of less than approximately 1°C would not become a critical limiting factor for soybean production in South Korea. However, further warming of more than 1°C may have adverse effects on the soybean yield from southern region in South Korea, and temperature above 35°C would cause severe damage to the formation of pod number in soybean.