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.