Impact of Low- and High-Temperature Stress on Soybean Early Seedling Growth.

Growth and yield of soybean are detrimentally affected by high and low temperatures during seedling stages, depending on the sowing dates. The objectives of this study were to evaluate 64 soybean cultivars representing maturity group III, IV, and V for temperature tolerance and to classify them in to different temperature tolerant groups. An experiment was conducted in sunlit controlled chambers by imposing three temperature regimes 20/12, 30/22, and 40/32 °C during seed germination and seedling establishment under optimum moisture and nutrient conditions. Physiological data including canopy temperature and SPAD were measured before harvest. Plant height, number of leaves on main stem, leaf area, and plant-component dry weights were measured at 17 days after planting. Several root morphological traits were assessed using winRHIZO root imaging system. Soybean cultivars varied significantly for many shoot and root parameters measured, particularly plant component weights and root morphological parameters among all temperatures. Cumulative low and high temperature response indexes, developed by summing individual response indices for each vegetative and physiological parameter, were used to classify the cultivars as sensitive, moderately sensitive, moderately tolerant, and tolerant. A strong and positive correlation (R² = 0.96) was observed between high and low temperature response indexes and implied that phenotypic characterization would be applicable and behaved almost similarly under both low and high temperatures. The identified heat- and cold-tolerant cultivars are potential candidates in breeding programs and would provide an option for the soybean producers to select cold and heat tolerant cultivars for the early and late planting systems.