Thermal sensitivity analysis of soybean reproductive stages under projected climate scenarios

Introduction

Soybean growers in Iran face a problem that won't disappear on its own: average summer temperatures across the central plateau have climbed by 1.4°C since 1990, and climate models project another 1.8-4.2°C of warming by mid-century depending on the emission pathway [1,2] . Soybean (Glycine max L.) is a warm-season crop, but its reproductive phases particularly flowering, pod initiation, and seed filling carry strict upper thermal limits beyond which pollen viability collapses, pod abortion rises, and seed weight drops sharply [3,4] . Most thermal sensitivity research on soybean has focused on the U.S. Corn Belt and Brazilian Cerrado, where growing-season temperatures and humidity regimes differ from Iran's continental semi-arid conditions [5,6] . The few Iranian investigations available tested only one or two climate scenarios and didn't separate the effects by individual reproductive stage [7] . This gap matters because adaptation strategies whether shifting sowing dates, selecting heat-tolerant lines, or applying deficit irrigation during critical windows depend on knowing which stage is most at risk and by how much [8] . Crop simulation models such as CROPGRO-Soybean can fill this gap by isolating stage-specific temperature responses under controlled virtual experiments that would take decades to replicate in the field [9,10] . When properly calibrated against local field data, these models allow systematic testing of multiple climate projections without the confounding effects of year-toyear weather variability [11] . This research calibrated CROPGRO-Soybean with two years of field data from Tehran, then ran it under six CMIP5 climate scenarios to quantify yield sensitivity at each reproductive stage and identify the critical thermal thresholds that trigger yield loss.

Projected Seasonal Temperature Shifts Downscaled GCM outputs for Tehran showed that mean daily maximum temperatures during the soybean reproductive window (July-September) are projected to increase by 1.2°C (RCP 2.6), 2.4°C (RCP 4.5), and 4.1°C (RCP 8.5) by the 2070 horizon relative to the 1990-2020 baseline. The warming isn't uniform across months: August, which overlaps with seed filling, shows the steepest rise (+4.7°C under RCP 8.5), while July temperatures increase by 3.6°C under the same scenario. Nighttime minimum temperatures are projected to rise even faster, potentially disrupting the diurnal cooling that soybean needs during grain loading.

Material and Methods

Material: Field calibration data came from two seasons (2021-22 and 2022-23) of soybean trials at the Agronomy Research Farm, University of Tehran, Karaj campus (35°49′N, 50°58′E, 1,312 m a.s.l.). Cultivar Williams 82 was grown under standard management: 60 kg N ha⁻¹ (as starter), Bradyrhizobium inoculation, 60 cm row spacing, and four furrow irrigations at R1, R3, R5, and R7. Soil was a calcareous clay loam (Typic Calcixerept) with pH 7.8, organic carbon 0.94%, and available water capacity of 162 mm m⁻¹ [12] . Phenological dates, biomass at key stages, final seed yield, and 100-seed weight were recorded.

Methods

CROPGRO-Soybean (DSSAT v4.8) was calibrated using 2021-22 data and validated against 2022-23 observations. Genetic coefficients were adjusted iteratively until simulated phenology matched observed dates within ±3 days and yield within ±8%. Six climate scenarios were constructed by applying delta-change factors from three CMIP5 GCMs (HadGEM2-ES, GFDL-ESM2M, MIROC5) under RCP 2.6, 4.5, and 8.5 for 2040 and 2070 time slices to the baseline weather [13] . Each scenario was run 30 times with stochastic weather variation. Yield change at each reproductive stage was isolated by applying the temperature increment only during the window of that stage while holding other periods at baseline [14] . Critical thresholds were identified as the maximum daily temperature at which simulated yield first dropped below 95% of the non-stressed potential. R5 had the lowest critical threshold at 32.0°C, and projected temperatures under RCP 4.5 by 2070 exceed it by 3.2°C (Table 2). All four stages face threshold exceedance under mid-range scenarios by 2070. Figure 2 shows that projected temperatures under all three RCP pathways by 2070 exceed the critical thresholds for every reproductive stage. The gap is widest at R5 under RCP 8.5, where projected maximum temperature surpasses the threshold by 6.1°C.

Results

Discussion

The finding that R5 is the most thermally vulnerable stage is consistent with controlled-environment work by Puteh et al. [3] and Hatfield et al. [15] , who showed that temperatures above 33°C during seed filling reduce assimilate translocation to developing seeds and accelerate leaf senescence. The 46.2% yield loss projected under RCP 8.5 (2070) is more severe than the 30-35% estimated for the U.S. Midwest under comparable warming [6] , likely because Tehran's baseline temperatures are already closer to the damage threshold, leaving less thermal buffer.

The low R5 threshold of 32°C compared with 35°C for pod initiation and 36°C for maturity reflects the biochemical sensitivity of starch and lipid synthesis enzymes that operate within a narrow optimum range during grain loading [16] . Even the mild RCP 2.6 pathway pushes 2070 temperatures above this limit, which means that emission reductions alone won't fully protect current soybean genotypes in Iran. Adaptation options include shifting sowing dates earlier by 15-20 days to move R5 into cooler late-July windows, breeding for heat tolerance at the seed-filling stage, and applying supplemental irrigation to exploit evaporative cooling [17] .

Combining early sowing with heat-tolerant lines appears to be the most realistic near-term strategy, as it doesn't require new infrastructure [8,11] .

Limitations

The model was calibrated with only two field seasons from a single location, which limits the robustness of genetic coefficients. CO₂ fertilization effects were held constant across scenarios, though rising CO₂ may partially offset thermal damage. The analysis used delta-change downscaling from three GCMs; dynamical downscaling could capture mesoscale temperature patterns more accurately. Field validation of projected yield losses under actual heat-stress years would strengthen these findings.

Conclusions

This simulation-based research showed that the seed-filling stage (R5) is the thermal weak point of soybean under Iranian semi-arid conditions, with projected yield losses of 12-46% depending on the emission pathway and time horizon. The critical maximum temperature threshold for R5 damage was just 32°C a level that all modeled scenarios exceed by mid-century. Flowering (R1) and pod initiation (R3) were also at risk, but their higher thresholds provide slightly more breathing room. By 2070, even the most optimistic emission pathway pushes temperatures above critical limits at every reproductive stage. Iranian soybean production will need a combination of earlier sowing, heat-tolerant cultivars, and possibly supplemental irrigation during seed filling to maintain current yield levels. These stage-specific vulnerability maps can guide breeding programs toward targeted heat tolerance at R5 and help extension services adjust planting calendars before the projected warming fully materializes.

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