Soybean Seed Emergence and Seedling Growth and Developmental Responses to Temperature.

Planting soybean early in the season can increase yield by avoiding mid- and late-season drought and high temperature conditions during flowering in the US Midsouth dryland production systems. Understanding soybean seed emergence and early-seedling growth and development including roots will be helpful to make planting decisions. The objectives of this study were to quantify temperature effects on soybean seed emergence and early-seedling growth including roots. Two experiments were conducted to evaluate wide range of temperatures on soybean using sunlit plant growth chambers. In Experiment 1, temperature (20/12, 25/17, 30/22, 35/27, and 40/32°C) effects on seed emergence and seedling growth were determined on two soybean cultivars, AG 5332 (Indeterminate) and Progeny 5333 (Determinate) for plants grown at optimum water and nutrient conditions. In Experiment 2, temperature effects (20/12 to 40/32°C) on seedling growth, including root growth and development, were determined. Dates of seedling emergence and plant growth including root growth was determined in both the experiments. Seedling root growth and development rates were measured using the winRHIZO root image analysis system at 21 days after planting in Experiment 2. Time to emergence and the rate of seed emergence development, the reciprocal of time, showed quadratic responses with temperature. The seed emergence rate increased with temperature up to 31°C, and declined slightly at the highest temperature tested. There were no differences between the cultivars for these two responses. Root and shoot growth and developmental responses also showed similar responses with temperature as that of seed emergence rates. Shoot growth, stem length, leaf area and whole plant- and plant-component rates increased temperature up to 31°C and declined at the high temperature tested. Cultivars differed showed similar responses to temperature. Stem lengths and leaf numbers were typically higher in the AG5332 across temperatures compared to Progeny 5333. Root growth traits such as volume, length, surface area, and root weight and developmental aspects such as root numbers, forks, tips, and crossings increased with temperature up to 28°C and declined at the two high temperature tested. This indicates that temperature optima for many root parameters was 3°C lower than the shoot parameters. Information on soybean seed emergence, seedling growth and developmental responses to temperature will help producers planting decisions in the field based current and projected weather conditions.