Effect of soaking exogenous abscisic acid on Onobrychis viciifolia seed germination and physiological mechanism under drought stress

Drought is one of the major environmental stresses affecting forage planting and productivity in the alpine areas. Abscisic acid (ABA), a plant hormone, plays a critical role in regulating plant growth, development, and responses to environmental stresses. In this study, seeds of the alpine forage species Onobrychis viciifolia were subjected to different concentrations of ABA soaking treatments and then exposed to drought stress simulated by 10% polyethylene glycol (PEG6000) in a controlled indoor environment. The germination indices and physiological characteristics of early seedlings were measured and analyzed to investigate the effects of exogenous ABA soaking on seed germination and early seedling growth under drought conditions. The results indicated that drought stress significantly inhibited the germination rate, germination potential, germination index, and vigor index of O. viciifolia seeds. Pretreatment with low-concentration ABA, particularly 5 mg·L^–1 and 10 mg·L^–1 significantly improved these germination parameters. Additionally, pre-soaking seeds with 10 mg·L^–1 ABA significantly increased the primary root elongation and root-shoot ratio. The activity of antioxidant enzymes, such as superoxide dismutase, peroxidase, and catalase, was significantly increased in the 10 mg·L^–1 ABA-treated group, indicating enhanced antioxidant defense mechanisms. The content of osmotic regulatory substances, including proline also increased, suggesting improved osmoregulation and cellular protection under drought stress. However, higher concentrations of ABA (>20 mg·L^–1) did not show any significant positive effects on seed germination or early seedling growth under drought conditions. In some cases, higher concentrations of ABA even slightly inhibited germination, possibly due to overstimulation of stress responses or toxicity. In conclusion, appropriate concentrations of ABA soaking can enhance the germination characteristics of O. viciifolia seeds under drought stress and effectively mitigate oxidative damage to early seedlings. The results of this study provide a theoretical basis for promoting the production of high-quality forage grasses in arid regions.