Synergistic effects of temperature and salt stress on seed germination and the antioxidant enzyme system of okra

Introduction: Against the backdrop of global warming and intensifying soil salinization, the interactive effects of temperature and salt stress are reshaping physiological regulation during crop germination; however, their underlying synergistic mechanisms remain unclear. As a typical thermophilic crop, okra exhibits high sensitivity to thermal environments and water-salt fluctuations during seed germination, making it an ideal model for elucidating multi-factor stress response mechanisms. Methods: Through a two-factor controlled experiment comprising five temperature gradients (20-36°C) and three salt-induced composite stress intensity levels equivalent to -0.05, -0.1, and -0.2 MPa, this study systematically evaluated the coupled responses of germination traits and antioxidant systems in okra seeds. Results: The results demonstrate that temperature dominates germination metabolism, with 24-28°C identified as the optimal germination window. Conversely, the salt-induced composite stress acts as the primary limiting factor, with the -0.1 MPa equivalent intensity established as the critical physiological inflection point below which germination capacity is severely inhibited. Further analysis revealed that temperature significantly modulates the effects of salinity by regulating redox homeostasis, where optimal temperatures centered at 28°C alleviate cellular degradation, whereas high temperatures >32°C amplify salt-induced oxidative damage. Discussion and conclusion: Multivariate analysis elucidated a distinct metabolic cost and passive biomass loss mechanism. Under high temperatures, the combined constraint drives the accumulation of reactive oxygen species ($ROS$), manifested by increased MDA content. This severe physiological stress led to a profound cellular dysregulation accompanied by the upregulation of SOD, POD, and CAT activities, which represented an insufficient compensatory response that ultimately constrained germination potential. In conclusion, this study uncovers the core mechanism by which temperature modulates redox homeostasis to amplify salt stress effects. These findings provide novel empirical evidence for combined stress theories, offering a quantitative basis for the precise management of temperature and salinity in okra cultivation within saline-alkali regions.