Intra-population terpene polymorphism of Thymus pulegioides L.: Evidence for seven chemotypes in a German limestone grassland

For Thymus pulegioides L. (Lamiaceae), occurring in almost entire Europe, about 20 different essential oil chemotypes are described in approximately 25 studies. However, only few studies mention chemotype diversity on population level, describing up to five chemotypes growing together. The aim of the study was to investigate the chemotype diversity within one T. pulegioides population of a limestone grassland in Germany. Essential oil compounds from samples of 93 thyme cushions were extracted with solid phase extraction (SPE) and analysed by GC-MS. Cluster analysis and nonmetric multi-dimensional scaling (NMDS) of the samples’ essential oil composition revealed the existence of seven different chemotypes: linalyl acetate-chemotype (56 individuals), geraniol-citral-linalyl acetate-chemotype (5), geraniol-citral-chemotype (7), thymol-chemotype (8), carvacrol-chemotype (2), linalool-chemotype (1), and β-caryophyllene-germacrene D-β-bisabolene-chemotype (14). The pattern of major monoterpenes from all chemotypes could be explained by the inhibition of specific steps in monoterpene biosynthesis. The sesquiterpene-dominant chemotype might be caused by a block in a very early step of monoterpene production or a regulatory/channeling mechanism. While the geraniol-citral-linalyl acetate-chemotype has been found for the first time, the other chemotypes have been found in other regions and the β-caryophyllene-germacrene D-β-bisabolene-type is similar to other sesquiterpene chemotypes rarely found in other areas. The large intra-population chemical polymorphism in this study raises questions about the chemotype diversity of thyme in other regions of Germany and potential correlations between essential oil composition and abiotic factors or biotic interactions.