Chemical composition and antimicrobial activities of essential oils of Ruta graveolens plants treated with salicylic acid under drought stress conditions

This study aims to investigate the effect of foliar application of 0.0 and 1.5 mM salicylic acid (SA) under normal [100% field capacity (FC)] and reduced irrigation (75 and 50% FC) on the yield, chemical composition and antimicrobial activities of essential oils (EOs) obtained from leaves and flowers of Ruta graveolens L. in field condition. The highest yields of EOs (0.366 and 0.215% v/w for leaves and flowers, respectively) were obtained from plants grown in 50% FC treated with 1.5 mM SA. Similarly, EOs from the flowers of plants maintained under the same conditions provided the highest percentage of long chain methyl ketones (94.2%). Moreover, the percentage of 2-undecanone and 2-nonanone, the two major components of the EOs, varied between 52.52-58.10% and 18.00–25.14% for leaves as well as 70.22–84.57% and 3.00–7.44% for flowers, respectively according to the treatment. On the other hand, the obtained EOs exhibited significant antimicrobial activities with minimum inhibitory concentrations (MIC) of 3.5–7.94 μg/ml for the tested bacterial strains; Staphylococcus aureus, Pseudomonas aeruginosa, Escherichia coli and Klebsiella pneumonia, and 1.14–2.5 μg/ml for the tested Candida species. Interestingly, a direct correlation between the percentage of long chain methyl ketones and the antimicrobial efficiency of EOs was observed. Furthermore, the most potent EO exerted concentration-dependent antimicrobial effects against all the bacterial and fungal strains in the killing kinetics studies. It also showed considerable morphological alterations on Candida albicans, including cell surface abnormalities, deformation, disruption, and prevented the germ tube production using scanning electron microscopy and germ tube test.