Chitosan‐induced modulation of secondary metabolism and stress tolerance in <scp> <i>Salvia rosmarinus</i> </scp> under combined drought and heat stress

BACKGROUND: Abiotic stresses, particularly drought and elevated temperatures, negatively affect plant physiological and metabolic processes. This study investigated the effects of drought, heat, and combined stress on Salvia rosmarinus (rosemary), and evaluated foliar-applied chitosan as a biostimulant. After 3 weeks, photosynthetic pigments, osmoprotectants (proline, soluble sugars), oxidative stress indicators (hydrogen peroxide and lipid peroxidation), phenolic compounds, essential oils, and antioxidant activity were evaluated. A natural deep eutectic solvent (NADES) was used to enhance phenolic extraction sustainably. RESULTS: Combined stress intensified oxidative damage and reduced photosynthetic pigments, while elevated osmoprotectants and chlorophyll a/b ratio reflected adaptive responses. Chitosan improved pigment retention, boosted sugar levels, and mitigated oxidative damage. Stress-exposed, chitosan-treated plants showed increased rosmarinic acid and key monoterpenes (camphor, α-pinene, 1,8-cineole), indicating enhanced secondary metabolism. Antioxidant assays confirmed superior scavenging and reducing activities in chitosan-treated plants under stress. CONCLUSION: Chitosan modulated physiological and metabolic responses in Salvia rosmarinus, improving resilience and phytochemical profile under abiotic stress. This is the first report demonstrating foliar-applied chitosan's effectiveness in Salvia rosmarinus facing drought and heat. © 2026 The Author(s). Journal of the Science of Food and Agriculture published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.