An experiment was conducted using sunlit plant growth chambers known as Soil-Plant-Atmosphere-Research (SPAR) units located at the Rodney Foil Plant Science Research Center, Mississippi State University, Mississippi State, MS, in 2014. Five SPAR units were used in this experiment. The SPAR units can precisely control air temperatures and chamber carbon dioxide concentration at predetermined set points at near ambient levels of solar radiation. Each SPAR unit consists of a steel soil bin with 1 m deep by 2 m long and 0.5 m wide that houses either rooting medium or pots and a Plexiglas chamber with 2.5 m tall by 2 m long × 1.5 m wide that houses aerial plant system. The Plexiglas chamber is connected to a heating and cooling system and fan connected to air ducts that pass conditioned air through the plant canopy at the rate to simulate the field-level leaf flutter. The environmental monitoring and control systems are supporting the SPAR units by a network to store the data and provide automatic acquisition of data at 10 s intervals throughout the day and night. More details about the operations and controls of SPAR chambers have been described by Reddy et al. [21] . During the experiment, the daily solar radiation from 285 - 2800 nm, outside the SPAR units, was recorded using pyranometer (Model 4-8; The Eppley Laboratory Inc., Newport, RI) and ranged from 17 to 28 MJ∙m⁻²∙d⁻¹ with an average of 21 MJ∙m⁻²∙d⁻¹ (Table 1). To eliminate the need for border plants, variable density shade cloths that were designed to simulate canopy spectral properties placed around the edges of the plant canopy were raised regularly to reach canopy height.

In the returning path of airline ducts, a humidity and temperature sensor (HMV 70Y, Vaisala Inc., San Jose, CA) was installed to monitor the relative humidity (RH) of each chamber, From these measurements, the vapor pressure deficits (VPD) in the units were estimated as per Murray [22] (Table 1). For the

†Values in each column followed by the same letter are not significantly different (P ≤ 0.05) according to Fisher’s LSD.

duration of the treatment period, evapotranspiration rats (ET) was monitored and expressed on a ground area basis (L∙d^?1) for each SPAR unit. The ET was calculated as the rate at which condensate was removed by the cooling coils at 900 s intervals by measuring the volume of water in collecting devices connected to a calibrated pressure transducer (Table 1; [23] ).

Two soybean cultivars, Asgrow AG 5332 (AG) with indeterminate and Progeny P5333 RY (PR) with determinate growth habits, from Maturity Group V were used for this experiment. Four seed were planted in 90 PVC (polyvinyl chloride) plastic pots (15.2 cm diameter and 30.5 cm height) at 1.5 - 2 cm soil depth. Pots were filled with topsoil: sand (1:3 by volume) medium classified as a sandy loam (87% sand, 2% clay, and 11% silt) with 0.5 kg gravel at the bottom. Each pot had a hole (0.5 cm diameter) at the bottom to allow the drainage of extra water and nutrients. The plants were irrigated and fertilized with full-strength Hoagland nutrient solution [24] by an automated drip irrigation system delivered three times a day at 0700, 1200, and 1700 h. Each SPAR was assigned to 1 temperature treatment, 18 pots, nine pots for each cultivar, arranged in six rows, three pots per row in a completely randomized design with a 5 × 2 factorial arrangement.

Five day/night temperature treatments, 20˚C/12˚C, 25˚C/17˚C, 30˚C/22˚C, 35˚C/27˚C, and 40˚C/32˚C, imposed from seeding were maintained within ±0.5˚C of the treatment set points measured with aspirated thermocouples (Table 1). The daytime temperature was initiated at sunrise and returned to the nighttime temperature one h after sunset. The chamber [CO₂] was measured and maintained at 420 µmol∙mol⁻¹ with a dedicated infrared gas analyzer (LI-COR, Model LI-6552, Lincoln, NE) from a gas sample that was drawn through the lines underground from each SPAR unit to the inside the laboratory. Pure CO₂ was supplied from a compressed gas cylinder through a system that included a pressure regulator, solenoid and needle valves and a calibrated flow meter to maintain chamber [CO₂] at desired set points [20] . To remove moisture from the gas sample, the sample lines were run through refrigerated water (4˚C) that was automatically drained and through a column of Mg (ClO₄)₂. The environmental data for mean daytime CO₂ concentrations, which were not significantly different among the temperature treatments, were 420 µmol∙mol⁻¹ for the experimental period (Table 1).

Four seed per pot were sown and time for 50% emergence (t50%) was recorded when the two cotyledons appear above the soil surface in each pot as described by Fehr et al. [9] . Seed emergence rate (SER) was calculated as a reciprocal to t50%. The data were subjected to analysis of variance [25] with completely randomized design considering cultivars and temperature treatments as sources of variance. Replicated values for seed emergence data were analyzed using one-way ANOVA of the general linear model, PROC GLM, in SAS [25] to determine the effect of temperature on the measured parameters. Data were tested for differences among the treatments for the parameters measured using Fisher’s protected least significant difference test at P ≤ 0.05 and the standard errors of the means were calculated. To determine the best-fit equations between temperature and seed germination parameters, R² was used. Graphical analysis was carried out using SigmaPlot 13.0 (Systat Software Inc., San Jose, CA).

A series of short-term pot-culture experiments were conducted in outdoor conditions from 24 March to 22 July 2015 to generate data needed to validate the model. The same two cultivars, AG and PR that used in the first experiment were used in these experiments. The cultural aspects including soil and nutrient conditions were similar to those described in Experiment I. Five pots were used for each cultivar as replications. Four seed were seeded in each pot. The air temperature was recorded using a WatchDog sensor (B100, Spectrum Technologies, Inc., Plainfield, IL). The seed emergence, t50%, as defined in Experiment 1, was recorded in all pots and treatments and SER was estimated from those measurements.

This experiment was conducted in the SPAR units with similar management and cultural practices as described in Experiment I. In this experiment, we evaluated 64 soybean cultivars belonging to MG III, IV, and V that are commonly grown in the US Midsouth under three different day/night temperature treatments, low―20˚C/12˚C, optimum―30˚C/22˚C, and high―40˚C/32˚C. The experiment was organized in a completely randomized design with two-factor interactions, three levels of temperature × 64 cultivars, replicated three times such that each cultivar and temperature treatment appeared in each SPAR unit once, whereas replications represented as nine different SPAR units. Treated seeds of those 64 soybean cultivars were sown in 576 PVC plastic pots (0.1 m diameter and 0.45 m tall), each filled with sandy soil and 0.250 kg of gravel at the bottom. The temperature treatments were imposed soon after sowing and continued until 20 days after sowing.

Four seed were seeded in each pot, and t50% was recorded in each pot. Then, SER was calculated as described earlier. Two-way analysis (ANOVA) was performed on the replicated values of the measured parameters using the PROC GLM procedure in SAS [25] to determine the effect of temperature, cultivar, and their interaction. Post ANOVA means comparison was made using the least significant difference (LSD = 0.05). Models performance statistics were carried out as described by Loague and Green [26] , Mitchell and Sheehy [27] Reddy et al. [28] and Reddy and Bonne [29] .