The effect of biological treatments of putrescine and salicylic acid on enhancing rosemary resistance to salinity through activation of secondary metabolites

Background and Objective: Soil salinity is one of the most important abiotic stresses that restricts plant growth and yield by disturbing ionic balance, reducing water absorption, and increasing the production of reactive oxygen species (ROS). Rosemary (Rosmarinus officinalis L.), as a medicinal and aromatic plant, has a high capacity for producing secondary metabolites such as phenols, flavonoids, and essential oils, which play an essential role in plant defense against stresses. Recent studies have shown that the application of growth-regulating compounds such as putrescine (a natural polyamine) and salicylic acid (a key phytohormone) can enhance plant resistance to unfavorable conditions by stimulating defense pathways and increasing the production of antioxidant compounds. Therefore, the aim of this research was to evaluate the effect of biological treatments with putrescine and salicylic acid on rosemary tolerance to salinity through the induction of secondary metabolites and the improvement of defense mechanisms.
Methodology: In this study, the effects of putrescine (at concentrations of 0, 0.5, 1, and 2 mM) and salicylic acid (at concentrations of 0, 0.5, 0.7, and 1 mM) under salinity stress (0, 75, 125, and 150 mM NaCl) were evaluated on the physiological traits, biochemical parameters, and metabolite variations of rosemary. The experiment was conducted as a factorial in a completely randomized design with three replications.
Results: The results showed that with increasing salinity concentration, proline content in the plant increased significantly. Under salinity stresses of 75, 125, and 150 mM, treatments with putrescine at 2 mM and salicylic acid at 1 mM increased proline content by 0.138, 0.166, and 0.218 mg g⁻¹ fresh weight, respectively, corresponding to increases of 78.98%, 82.53%, and 59.91% compared to the control. In addition, salinity increment caused a significant increase in carbohydrate content, and putrescine treatment at 0.5 mM under non-stress conditions enhanced carbohydrate accumulation. Under 75 and 125 mM salinity, the highest carbohydrate content was observed in the salicylic acid treatment at 1 mM, reaching 3.13 and 3.38 mg g⁻¹ fresh weight, which corresponded to increases of 138.93% and 77.89% compared to the control. Under 150 mM salinity, the combined treatment of putrescine (0.5 mM) and salicylic acid (0.7 mM) resulted in the highest proline content (3.3 mg g⁻¹). Under non-stress conditions, putrescine at 0.5 mM increased peroxidase activity by 86.53%. At 75 mM salinity, putrescine at 1 mM enhanced peroxidase activity by 23.47%, whereas at 150 mM salinity, the combined treatment of putrescine (0.5 mM) and salicylic acid (0.7 mM) increased peroxidase activity by 116.1%. Furthermore, the analysis of defense-related compounds showed that the combined treatment of putrescine (2 mM) and salicylic acid (1 mM) resulted in the highest rosmarinic acid content under 150 mM salinity. This treatment also significantly increased terpenoid compounds such as α-pinene, 1,8-cineole, borneol, and camphor. The increase in 1,8-cineole under this treatment indicates its positive effect on the synthesis of volatile compounds with antioxidant and antimicrobial properties.
Conclusion: The findings of this study demonstrated that biological treatments with putrescine and salicylic acid play a significant role in enhancing the salinity tolerance of rosemary plants. These compounds improved the plant’s physiological balance under salt stress by increasing the production of secondary metabolites such as phenols and flavonoids, reducing oxidative damage, and activating antioxidant-related defense pathways. Therefore, the application of these treatments can be recommended as an eco-friendly, low-cost, and effective strategy to improve the tolerance of medicinal and aromatic plants to environmental stresses, especially salinity, within sustainable agriculture programs and the production of high-value medicinal crops.