Will Future CO2 Concentrations Safeguard Soybean Production to Heat Wave Events?

Background: A key challenge across agro-ecosystems is to maintain rates of productivity that will keep pace with global demand. Currently, atmospheric CO₂ concentrations [CO₂] are expected to reach 600 ppm by 2050. Climate models also project more frequent heat waves. These abrupt periods of excessive temperature could impact crop yield beyond increases in mean temperature alone, because heat waves may negate the mitigating effect of higher [CO₂] on primary production and surpass the temperature threshold for reproductive development. In this study we quantify the effect of 5-day heat wave events on photosynthesis, respiration and yield of Glycine max (soybean) grown under ambient (400 ppm) and elevated (600 ppm) [CO₂]. Methods: This study was located at the Soybean Free Air CO₂ Enrichment site (Savoy, Illinois, USA). Each heat wave was implemented using overhead heating units that increased canopy temperature 6 ^oC above the 30 year mean growing season temperature. Heat wave events occurred during critical developmental stages for yield in Glycine max, R2 (full bloom) and R5 (pod filling). Treatments included: 1) aCO₂ (ambient [CO₂]), 2) eCO₂ (elevated [CO₂]), 3) aCO₂ x Hw (Heat wave), and 4) eCO₂ x Hw. Results: Irrespective of [CO₂], midday soil moisture was significantly lower in heated plots. This corresponded with increased plant water stress and a decrease in photosynthesis by 14% (aCO2 x Hw) and 28% (eCO2 x Hw) at R2 and 30% and 20% at R5 compared to soybean in aCO2 and eCO2 plots, respectively. Nevertheless, the total number of flowers and pods produced was lowest in aCO2 x Hw plots. This may be attributable to an increase in water-use efficiency of soybean in eCO2 x Hw plots where leaf area index was maintained despite the reduction in photosynthesis. Similar to photosynthesis, leaf respiration initially decreased in response to heat waves. Conclusions: The Midwestern U.S. produces 38% of the world’s soybean in mostly rain-fed croplands that are susceptible to changes in climate patterns. Our results show that heat waves during the R2 and R5 stages of development will impact soybean production. Understanding how climate influences the relationship between physiology and crop yield is essential because productivity is expected to decrease in response to heat waves in this region.