The Oregano plants of Chios Island (Greece): Essential oils of Origanum onites L. growing wild in different habitats

Oregano herbs have wide culinary uses and industrial applications due to the high antioxidant and other activity of their main essential oil compound, carvacrol. Among the oregano herbs, Origanum onites (known as “Turkish oregano”, “Island oregano” or “Cretan oregano”) is a narrowly distributed East Mediterranean species occurring mainly in Turkey and Greece, which comprises one of the major herbs of the oregano trade worldwide. We examine the essential oils of O. onites plants collected from 42 localities scattered along the total range of the species on Chios, a Greek island bordering with the Turkish mainland. We focus on the essential oil yield and five oil compounds, namely carvacrol, thymol, borneol and the biosynthetic precursors of the first two, i.e., p-cymene and γ-terpinene, in relation to the habitat type, geological substrate, altitude, bioclimatic area and geographical location, where the collected plants were growing on the island. We also discuss our results in relation to the essential oil based distinction between O. onites and another commercially important East Mediterranean oregano herb, namely Origanum vulgare subsp. hirtum (known as “Greek oregano”). The determined essential oil yield (3.0–7.0mL/100g of dry leaves and inflorescences) and carvacrol content (69.0–92.6%) of the examined O. onites plants from Chios are among the highest reported for this species. The essential oil yield and the content of the five compounds were not found to vary in relation to the different environmental factors examined. These findings let us conclude that the wild oregano of Chios is a valuable natural resource with a high potential for industrial use, due to its high essential oil yield and carvacrol content, that are not influenced by environmental conditions. Moreover, based on our results and recent literature we conclude that O. onites can be distinguished from O. vulgare subsp. hirtum based on the relative content of carvacrol, thymol, borneol and p-cymene – primarily borneol – in the two taxa’s essential oils, a useful means of distinction when the botanical identity of fragmented commercial oregano products is questioned.