Drought and high nighttime temperature impact on soybean seed yield and quality under ambient and elevated CO2 environments

The increasing prevalence of abiotic stresses, including high nighttime temperatures (HNT) and drought during the reproductive stage, poses a risk to global soybean production. Additionally, the influence of rising atmospheric CO₂ levels must be considered when addressing changes in temperature and drought conditions. In this study, two soybean genotypes, DS25-1 and DS31-243, were grown under control (0.15 m³ m^-3 volumetric soil moisture content (VWC) and 30/22 °C day/night temperature), HNT (30/26 °C day/night temperature), and drought (0.08 m³ m^-3 VWC) conditions at ambient (425 ppm, aCO₂) and elevated (725 ppm, eCO₂) CO₂ concentrations in sunlit plant growth chambers during flowering and pod development stages. The plants exposed to eCO₂ under control and drought conditions showed increased photosynthesis (DS25-1: 55 and 142%, and DS31-243: 77 and 61%) and non-photochemical quenching (DS25-1: 98 and 57%, and DS31-243: 67 and 126%) compared to aCO₂. On average, the pods and seed numbers increased by 60% and 59%, respectively, under HNT and eCO₂ compared to HNT and aCO₂. In contrast, the drought decreased pods and seeds by 43% across genotypes and CO₂ environments. This has resulted in a reduction in seed yield by 62% and 56% in DS25-1 and DS31-243, respectively, under drought conditions compared to the control. Under aCO₂, the seed yield of DS31-243 was reduced by 42% under HNT compared to the control. The seed protein content was reduced under eCO₂, while other treatments did not influence its content. The seed carbohydrate content decreased under the HNT condition, while the drought and eCO₂ did not influence its production. The stress conditions during seed development resulted in reduced polyunsaturation, while the oleic acid content increased. The study highlighted the positive impacts of eCO₂ on physiology and yield.