Controlled drought stress affects rosemary essential oil composition with minimal impact on biomass yield

Rosemary (Salvia rosmarinus Spenn.), an indigenous medicinal and aromatic plant within the Mediterranean region, yields essential oils renowned for their antimicrobial, antioxidant, anticancer, and anti-inflammatory properties. When subjected to various stress conditions, rosemary intensifies the production of essential oils as a defensive mechanism, as occurs when it is subjected to drought stress, making it a potentially interesting crop for marginal dryland areas of Sicily and other Mediterranean areas. Managing rosemary cultivation through controlled drought stress close to the balsamic period may increase the concentration and quality of essential oils and optimize the use of limited resources like water. We conducted two on-farm field experiments in Sicily, southern Italy, to assess the effect of controlled drought stress on rosemary biomass, essential oil yield, and composition. The hypothesis was that drought could improve the concentration of essential oils and modify their chemical composition by increasing valuable aromatic components without negatively impacting crop biomass production, increasing interest in cultivating this species. The experiments were co-designed with farmers selecting dedicated field experiments, cultivars, and crop management. Drought stress was applied during autumn and spring in correspondence with medium-high temperatures and rainwater scarcity. The irrigation input was reduced for three weeks during the balsamic period for each trail. Under stressed conditions, morphometric traits were measured. At harvest time, essential oils were extracted using microwave-assisted extraction. Under the same farm conditions, the canopy height was significantly higher in ‘Barbeque’ than ‘Tuscan Blue cv. (67.3 cm vs. 43.8 cm) This indicates a genotype influence on stress response. Essential oil yield increased by 30 % in plants of Tuscan blue cv. subjected to stress conditions in one of two farms. Additionally, drought stress altered the aromatic profile of the essential oil, resulting in differing percentages of key compounds between stressed and unstressed plants. Generally, there was a higher α-pinene percentage in unstressed plants and higher bornyl-acetate and β-caryophyllene concentrations in drought-stressed plants. In the Scicli farm, α-pinene decreased in response to water stress, while camphene and myrcene increased. Our results summarized that controlled drought stress had a minimal impact on rosemary growth, increasing the percentages of some terpenes in its essential oils. No severe decrease was observed in the synthesis of essential oils, suggesting that rosemary is suitable for marginal soils in many Mediterranean countries to protect soils and diversify crops in farms.