Yield response of field-grown soybean exposed to heat waves under current and elevated [CO₂ ]

Elevated atmospheric CO₂ concentration ([CO₂ ]) generally enhances C₃ plant productivity, whereas acute heat stress, which occurs during heat waves, generally elicits the opposite response. However, little is known about the interaction of these two variables, especially during key reproductive phases in important temperate food crops, such as soybean (Glycine max). Here, we grew soybean under elevated [CO₂ ] and imposed high- (+9°C) and low- (+5°C) intensity heat waves during key temperature-sensitive reproductive stages (R1, flowering; R5, pod-filling) to determine how elevated [CO₂ ] will interact with heat waves to influence soybean yield. High-intensity heat waves, which resulted in canopy temperatures that exceeded optimal growth temperatures for soybean, reduced yield compared to ambient conditions even under elevated [CO₂ ]. This was largely due to heat stress on reproductive processes, especially during R5. Low-intensity heat waves did not affect yields when applied during R1 but increased yields when applied during R5 likely due to relatively lower canopy temperatures and higher soil moisture, which uncoupled the negative effects of heating on cellular- and leaf-level processes from plant-level carbon assimilation. Modeling soybean yields based on carbon assimilation alone underestimated yield loss with high-intensity heat waves and overestimated yield loss with low-intensity heat waves, thus supporting the influence of direct heat stress on reproductive processes in determining yield. These results have implications for rain-fed cropping systems and point toward a climatic tipping point for soybean yield when future heat waves exceed optimum temperature.