Seasonal Variations in the Chemical Compositions of Essential Oils of Selected Aromatic Plants Growing Wild in Turkey

The summer leaves of Thymbra spicata var. spicata and Satureja thymbra, two Labiatae aromatic shrubs growing wild in the East Mediterranean region of Turkey, and, in addition, the leaves of Salvia fruticosa, Mentha pulegium, Laurus nobilis, and Inula viscosa were collected in 4-week intervals to follow plant development and essential oil composition. The essential oils extracted from leaves (and flowering tops) were estimated during the growing season by means of GC−FID, and 19 terpenic constituents were identified. The changes in the essential oil content (quantity and composition) varied for all six plant species, according to corresponding environmental and growth factors and the major adaptive strategy toward summer drought that each plant species has evolved. The concentration of the fungitoxic components in the essential oils of Thymbra spicata and Satureja thymbra, the phenolic constituents carvacrol and thymol, were low in the early phenological stage and increased gradually with plant development. The maximum was reached in June/July, shortly after flowering. Taking this into account, the harvest of these two species in order to obtain their essential oils offers acceptable economic possibilities for their use as a natural fungicide. The main compounds of the essential oils found in Salvia fruticosa, Laurus nobilis, and Mentha pulegium were the oxygen-containing monoterpenes 1,8-cineole and pulegone, respectively, and a periodic increase and decrease in their concentrations was observed. The essential oil of Inula viscosa contained only small amounts of some of the investigated components. The best time of harvest to gain the essential oils with the highest active ingredients is July for Thymbra spicata, Satureja thymbra, Salvia fruticosa, and Mentha pulegium (Labiatae) and September for Laurus nobilis (Lauraceae). Keywords: Essential oils; seasonal variations; thymol; carvacrol; Thymbra spicata; Satureja thymbra