Salicylic Acid and Chitosan Elicitors: Pre‐Harvest Foliar Applications Improve Antioxidant Activity by Increasing Phenolic Compounds in <i>Achillea gypsicola</i> Hub‐Mor

Salicylic acid (SA) and chitosan (CH) play an important role in the defense mechanism of plants by increasing secondary metabolite production in plants. In this study, their effects on volatile oil composition, phenolics, and antioxidant activity of Achillea gypsicola were investigated. Treatments included control (water + ethanol), 0.5-, 2-, and 8-mM SA, and 2, 4, and 8 g L^-1 CH. The process is completely randomized with four replications, with two harvests between 2021 and 2022. As a result, 4 g L^-1 CH significantly increased 1,8-cineole and borneol production. The highest total phenolic content (TPC) and total flavonoid (TFC) were obtained from the 8 g L^-1 CH treatment. LC-MS/MS analysis revealed a 2-fold increase in chlorogenic acid, 1.01-fold increase in gallic acid, and 0.56-fold increase in protocatechuic acid from 4 and 8 g L^-1 CH treatments. Strong positive correlations were found between DPPH radical scavenging assay and iron ions chelating assay (0.803**), TFC (0.556**), and TPC (0.562**).