Hormonal and physiological mechanisms underlying genotypic cross-tolerance to combined heat and drought stress in soybean

Combined heat and drought stress represents a significant threat to global soybean production, yet the physiological and hormonal mechanisms conferring genotypic cross-tolerance remain poorly understood. We hypothesized that tolerant genotype leverage specific hormonal regulation to maintain physiological processes and mitigate yield loss under combined stress. This study evaluated the responses of a heat-tolerant (DS25-1) and a heat-sensitive (DT97-4290) genotype to heat (HS), drought (DS), and their combination (HS + DS) during the reproductive stages (R1–R6). While individual stress significantly impaired photosynthetic pigments and gas-exchange parameters, HS + DS synergistically exacerbated these effects, resulting in an 8 °C increase in canopy temperature. Yield analysis revealed that HS + DS induced more severe losses than single stress, with DT97-4290 experiencing a 10% greater reduction compared to DS25-1. Stress-induced metabolic shifts were observed in seed composition: protein content increased under DS, while oil content rose under HS and HS + DS. Fatty acid profiles were differentially modulated, with oleic acid increasing under HS and linolenic acid under DS, particularly in the sensitive genotype. Hormonal profiling revealed a key divergence stress- signaling pattern in DS25-1, characterized by significant abscisic acid accumulation under DS and elevated trans-zeatin riboside across all stress treatments. These findings indicate superior osmotic adjustment while maintaining plant growth. Conversely, the jasmonic acid precursor 12-oxo-phytodienoic acid increased under DS but was suppressed under HS + DS. These findings provide insights into the hormonal crosstalk and physiological traits that underpin stress resilience, offering potential targets for breeding resilient soybean cultivars.