Ecological Preferences and Diversity of Essential Oil Composition in Endangered Wild-Growing Populations of Sideritis sipylea Boiss. (Lamiaceae) of the East Aegean Islands (Greece): Evidencing Antioxidant Potential, Antimicrobial and Cytotoxic Activities

Plants from the genus Sideritis (Lamiaceae) have been widely used in folk medicine for a long time and consequently are a focus of the scientific community. Despite this interest, explicit data about the essential oils (EOs) of the Endangered Sideritis sipylea have not been readily available to date. In this study, we investigated the ecological preferences of Greek S. sipylea and the chemical composition of the essential oils of wild-growing S. sipylea populations from two Greek islands (S1: Samos, S2: Lesvos); we explored concomitant associations with environmental factors; and we assessed their (i) antioxidant potential (two tests), (ii) antimicrobial activity against six microbial and two fungal strains, and (iii) cytotoxic effect in two human and one murine cell lines. We compiled an ecological profile in R based on all known Greek localities of S. sipylea, outlining for the first time its preferences regarding temperature (3.48 ± 1.53 °C to 30.70 ± 1.11 °C) and the precipitation regimes (5.92 ± 2.33 mm to 136 ± 11.43 mm) shaping its natural occurrence in the wild. The chemical analysis (42 compounds in total) confirmed the domination of monoterpene hydrocarbons in both samples (with quantitative and qualitative differences) and identified 12 new constituents reported in S. sipylea for the first time (e.g., Bicyclogermacrene and Cumacrene). Dominant compounds in S1 (39 constituents) were β-Myrcene (20.4%) followed by β-caryophyllene (11.8%), bicyclogermacrene (7.1%), β-pinene (6.3%), carvacrol (6.2%) and α-terpinene (6.1%), whereas in S2 (26 constituents) the main ones were α-pinene (37.3%), β-pinene (15.1%) and sabinene (12.1%), followed by β-caryophyllene (5.6%) and bicyclogermacrene (5.5%). The strong antioxidant capacity and cytotoxic activity of S. sipylea EOs are reported herein for the first time, while new insight is provided regarding their effect on bacterial and fungal strains (four ones originally tested herein). The biological activity analysis demonstrated variation among samples, with S2 being more potent than S1. Altogether, the results of the present study demonstrate the high biological potential of S. sipylea EOs with an interesting antioxidant capacity and antimicrobial and cytotoxic effects and reveal associations of natural chemodiversity with climatic factors.