Chemical polymorphism of populations of Thymus caespititius grown on the islands Corvo, Flores, São Miguel and Terceira (Azores) and on Madeira, assessed by analysis of their essential oils

The composition of the essential oils isolated from 24 populations of Thymus caespititius collected on Corvo, Flores, São Miguel and Terceira (Azores) and on Madeira were studied by GC and GC-MS. All the oil samples analysed were dominated by their monoterpene fraction (66-89%). In the Azorean populations, the proportion of the oxygenated monoterpenes (51-79%) was higher than that of the monoterpene hydrocarbons (8-27%). In contrast, the monoterpene hydrocarbons and the oxygenated monoterpenes represented 35-44 and 42-43%, respectively, of the total oils from the populations grown on Madeira. Cluster analysis of the identified components with a concentration ≥1% grouped the oils into three main clusters that corresponded with their main components: carvacrol (41-65%), thymol (35-51%) and α-terpineol (33-37%). Although the populations collected on Madeira were grouped in the same cluster, the chiral analysis of sabinene, terpinen-4-ol and α-terpineol showed that there was a clear chemical polymorphism. Actually, in the oils from two populations (−)-sabinene, (−)-terpinen-4-ol and (+)-α-terpineol were the predominant enantiomers while in that from the third population an opposite ratio was found. The chemical polymorphism of the essential oils from T. caespititius may result either from the genetic variability of the populations or from the influence of edaphic factors.