Physiological and Biochemical Resilience of Rosemary to Combined Drought and Salinity: A Two-Year Study of Biostimulant-Mediated Stress Mitigation

Rosemary (Rosmarinus officinalis L., syn. Salvia rosmarinus Spenn., Lamiaceae) is a Mediterranean perennial herb with high tolerance to environmental stress, but its long-term response to combined drought and salinity and the potential of biostimulants to modulate acclimation remain insufficiently understood. This study evaluated the effects of chronic drought and salinity, applied individually or in combination, on rosemary growth, water status, ion homeostasis, physiological stability, and essential oil composition, and assessed the mitigating effects of foliar nano-chitosan and seaweed extract. The experiment was conducted under arid Mediterranean conditions over two consecutive years (2024–2025) with four harvests, using a split–split–plot randomized complete block design with three irrigation levels, three salinity levels, and three biostimulant treatments. Drought, salinity, and their interaction significantly affected all evaluated traits. Severe combined stress reduced dry weight by 33.3%, while overall physiological performance remained relatively stable. Under severe combined stress, B2 treatment increased relative water content and the K+/Na+ ratio by 14.9% and 66.7%, respectively, and reduced Na+ accumulation by 17.9%. B2 showed a consistently higher Biostimulant Efficiency Index than B1, indicating a greater response to the higher-concentration combined formulation. Moderate drought with B2 increased essential oil contents of camphor, a-pinene, and 1,8-cineole. Overall, rosemary maintained relatively stable physiological performance despite significant stress-induced changes in individual traits, indicating a degree of physiological resilience to chronic combined stress while the nano-chitosan–seaweed formulation was associated with improved water relations and ionic balance under severe combined stress, although these changes did not translate into biomass recovery.