Elevated temperature and water deficit are the major abiotic factors restricting plant growth. While in nature these two stresses often occur at the same time; little is known about their combined effect on plants. Therefore, the main objective of the current study was to observe the effect of these two stresses on phenology, dry matter and seed yield in soybean. Two soybean genotypes JS 97-52 and EC 538828 were grown under green-house conditions which were maintained at different day/night temperatures of 30/22, 34/24, 38/26 and 42/28 °C with an average temperature of 26, 29, 32 and 35 °C, respectively. At each temperature, pots were divided into three sets, one set was unstressed while second and third set were subjected to water stress at vegetative and reproductive stage, respectively. As compared to 30/22 °C increase in temperature to 34/24 °C caused a marginal decline in leaf area, seed weight, total biomass, pods/pl, seeds/pl, harvest index, seeds/pod and 100 seed weight. The decline was of higher magnitude at 38/26 and 42/28 °C. Water stress imposed at two growth stages also significantly affected dry matter and yield. The highest average seed yield (10.9 g/pl) was observed at 30/22 °C, which was significantly reduced by 19, 42 and 64% at 34/24, 38/24 and 42/28 °C, respectively. Similarly, compared to unstressed plants (11.3 g/pl) there was 28 and 74% reduction in yield in plants stressed at vegetative and reproductive stage. Thus, both temperature and water stress affected the growth and yield but the effect was more severe when water stress was imposed at higher temperatures. JS 97-52 was more affected by temperature and water stress as compared to EC 538828. Though drought is the only abiotic factor that is known to affect the water status of plants, but the severity of the effect is highly dependent on prevailing temperature.

High temperature is foremost abiotic stress and there are inadequate studies explicating its impact on soybean. In this study, a pot experiment was done in a greenhouse maintained at a day/night temperature of 42/28 °C with a mean temperature of 35 °C to examine the effects of high temperature in soybean plants inoculated with and without arbuscular mycorrhizal fungi (AMF).Various parameters were taken in soybean plants treated with AMF (+) and AMF (−) such as growth analysis, chlorophyll content, canopy temperature, number of stomata, gas exchange, chlorophyll fluorescence, seed yield, and its attributes. It was observed that growth parameters like leaf area, stem height, root length, shoot and root dry biomass were increased in AMF (+) as compared to AMF (−) plants. Chlorophyll content, the number of stomata, photosynthesis rate, stomatal conductance, transpiration rate, and water use efficiency increased in AMF (+) as compared to AMF (−) plants. Chlorophyll fluorescence parameters such as Fv/Fm, Fv/Fo, PhiPSII, fluorescence area, performance index, photochemical quenching, linear electron transport rate, and active reaction centres density of PSII were also found to be enhanced in AMF (+) plants. However, canopy temperature, intercellular CO₂, Fo/Fm, and non-photochemical quenching were higher in AMF (−) as compared to inoculated plants. An increase in growth and photosynthesis ultimately enhanced the seed yield and its attributes in AMF (+) as compared to AMF (−). Thus, AMF (+) plants have shown much better plant growth, photosynthesis parameters, and seed yield as compared to AMF (−) plants under high temperature. Thus, it is concluded that heat stress-induced damage to the structure and function of the photosynthetic apparatus was alleviated by AMF inoculum. Therefore, AMF can be used as a biofertilizer in alleviating the adverse effects of heat stress in soybean.

The study used the DSSAT model to assess potential soybean yields in different regions of India and validated it under diverse agroecological conditions. The average simulated yield under irrigated conditions was 3794 kg ha⁻¹ relative to the simulated average rainfed yield of 2446 kg ha⁻¹, showing a 35.52% reduction in grain yield due to adverse moisture conditions under rainfed conditions. Relative to simulated yield, the average observed (actual) rainfed yield across 43 districts of India was 1025 kg ha⁻¹, which was 2769 and 1421 kg ha⁻¹ lower than irrigated and rainfed potential yield, respectively. A significant positive correlation was observed between simulated water non-limited yield and solar radiation (R² = 0.55, p ≤ 0.05). The simulated rainfed grain yield (R² = 0.66, p ≤ 0.05) had a significant, positive, and curvilinear relationship with growing season rainfall. On the other hand, the actual yield (R² = 0.008) showed a non-significant relationship with mean crop seasonal rainfall across locations. The gap between simulated yield under irrigated and rainfed conditions is huge at locations with low seasonal rainfall and narrows with increasing rainfall. In addition, the gap between actual yield and simulated yield under rainfed conditions was larger, even in high seasonal rainfall areas. The yield gap under rainfed conditions is due to the non-adoption of improved crop management practices and could be reduced with proper interventions. This includes adapting drought-resistant varieties, conserving rainwater, changing land configuration, and adopting waterlogging-tolerant varieties using improved technology to reduce the soybean yield gap.