The present study investigated the effect of 14 treatments consisting yeast extract (YE) (0, 0.5, 1.0, 1.5, 2 g/l), salicylic acid (SA) (0, 40, 80, 160, 320 mg/l) and YE (1 and 1.5 g/l) in combination with SA (80 and 160 mg/l) foliar application on essential oil content and constituents of lemon balm (Melissa officinalis L.). The experiment was conducted in a completely randomized design with three replications under greenhouse conditions. Essential oils analyzed by GC/MS and a total of 39 compounds were identified that the major constituents were citronellol, trans-carveol, γ-3-carene, linalool, citral and carvacrol acetate, respectively (42.8 to 48.0% in total). Citronellol was the main constituent of essential oils with 11.05%. SA and YE significantly altered the amount of 23 constituents of lemon balm essential oil at 1% probability level (P˂0.01). The highest citronellol, linalool and citral (14.50, 7.9 and 8%, respectively) production was obtained at 1.5 g/l YE+160 mg/l SA treated plants that was 103, 88 and 203% higher than control plants, respectively. The highest essential oil content (0.336% v/w) that was 49% higher compared to control was achieved by 1.0 and 1.5 g/l YE+160 mg/l SA treatments. The principal component analysis (PCA) and heatmap indicated that the content of compounds varied with different treatment and also revealed a clear separation between control and treatments groups. The results suggested that SA, YE and SA in combination with YE had the considerable ability to stimulate the production of major constituent such as citronellol, citral, and linalool in this plants.
