Role of Chitosan in Enhancing Secondary Metabolism and Stress Tolerance in Rosemary under Drought/Heat Combined Stress

Abiotic stresses, particularly drought and elevated temperatures, are becoming increasingly frequent and severe due to climate change, substantially impacting plant physiological, biochemical, and metabolic processes. This study assessed the effects of drought, heat and their combination on Salvia rosmarinus (formerly Rosmarinus officinalis), while also evaluating the potential of foliar-applied chitosan as an elicitor and stress-mitigating agent. Key biochemical markers were analyzed, including photosynthetic pigments (total chlorophyll and carotenoids), osmoprotectants (soluble sugars and proline), and indicators of oxidative stress (hydrogen peroxide and lipid peroxidation). Secondary metabolism was evaluated through quantification of phenolic and essential oil profiles, as well as the antioxidant activity of green phenolic extracts. Combined drought and heat stress significantly increased oxidative damage and reduced chlorophyll content. The accumulation of osmoprotectants, particularly under drought and combined stress conditions, played a crucial role in stress mitigation. Notably, chitosan application alleviated pigment degradation, promoted the accumulation of soluble sugar, and substantially reduced oxidative damage. Multivariate analyses revealed that specific classes of secondary metabolites are differentially associated with each stress condition, suggesting that rosemary dynamically modulates both phenolic and essential oil composition in response to environmental cues. Under combined drought and heat stress, chitosan-treated plants exhibited enhanced antioxidant activity, with notable increases in rosmarinic acid—the major phenolic compound—and monoterpene hydrocarbons. Conversely, the biosynthesis of sesquiterpene hydrocarbons, oxygenated monoterpenes, and oxygenated sesquiterpenes was less responsive to both the combined stress and chitosan treatment. Overall, these findings underscore the potential of chitosan as a sustainable elicitor that not only enhances the phytochemical profile of rosemary by increasing key bioactive metabolites under stress but also improves abiotic stress tolerance, particularly under compounded environmental conditions.