Zinc oxide nanoparticles mitigate drought stress and enhance secondary metabolite production in Hypericum perforatum

Drought stress represents a major abiotic factor responsible for reducing the growth, yield and quality of important plants. This research was investigated the potential of ZnO Nano particles (ZnO NPs) to alleviate drought stress in Hypericum perforatum (common St. John's Wort). A factorial experiment using randomized complete block design (RCBD) was performed with 3 replications and 4 different irrigation treatments (100%, 70%, 40%, and 25% of field capacity) combined with 4 different concentrations of ZnO NPs (0, 10, 50, and 100 mg L^- 1). Drought stress had a significant negative impact on different morphophysiological characteristics, including plant height, root length, leaf area, chlorophyll concentration, relative water content, and photosynthetic efficiency (Fv/Fm). Additionally, drought stressed plants exhibited significantly higher activity of catalase (CAT) and peroxidase (POD), proline content, total phenolic content, content of essential oils, and accumulation of hypericin than non-stressed plants. These biochemical adjustments indicate the activation of osmotic regulation and antioxidant defense mechanisms that contribute to drought stress tolerance. The application of ZnO NPs via foliar application reduced the adverse effects of drought and significantly improved growth characteristics, levels of photosynthetic pigments, and plant water status under drought conditions. Moreover, in combination with the application of ZnO NPs (especially at concentrations of 50 and 100 mg L⁻¹), moderate drought stress (40-70% FC) resulted in significantly higher essential oils and hypericin accumulation compared to all other treatments. Principal component and correlation analysis confirmed the relationships between physiological performance and stress-related metabolites. Overall, application of ZnO NPs represent an innovative opportunity for mitigating drought related stress in H. perforatum.