Exogenous ascorbic acid enhances drought tolerance in Hypericum perforatum L. by modulating antioxidant defense and osmotic adjustment.

Drought stress poses a significant threat to the cultivation of Hypericum perforatum L. (St. John’s wort), a valuable medicinal plant. While ascorbic acid (AsA) is a known mitigator of abiotic stress, its protective role and underlying mechanisms in H. perforatum remain unexplored. This study elucidates the biochemical and physiological basis of exogenous AsA-induced drought tolerance in H. perforatum subjected to full irrigation (100% FI), moderate (75% FI), and severe (50% FI) deficit irrigation. Severe drought significantly inhibited growth, reducing biomass, chlorophyll content, relative water content, and leaf area. Foliar application of AsA, particularly at 400 mg L− 1, markedly ameliorated these inhibitory effects. The AsA-mediated mitigation was mechanistically linked to a robust enhancement of the plant’s antioxidant defense system, evidenced by significantly increased activities of ascorbate peroxidase, catalase, and superoxide dismutase, alongside elevated levels of endogenous AsA, total phenolics, and osmolytes (proline and soluble carbohydrates). Consequently, AsA-pretreated plants exhibited substantially reduced oxidative damage, with lower levels of malondialdehyde and electrolyte leakage under severe stress. Our findings demonstrate that exogenous AsA fortifies drought tolerance in H. perforatum primarily by orchestrating a synergistic enhancement of osmotic adjustment and reactive oxygen species scavenging capacity. This effective and practical strategy highlights the potential of AsA application to sustain the production of this economically important species in water-limited environments.