A chemosystematic investigation on the mono- and sesquiterpenoids in the genus Origanum (Labiatae)

We have examined the volatiles of Origanum species native to Crete together with two naturally occurring hybrids. The three groups of Ietswaart's clas sification were all represented. Group A, section Amaracus was represented by Origanum calcaratum and by Oliganum dictamnus; group B, section Chilocal yx by Origanum microphyllum; section Majorana by Oliganum onites; group C, section Oliganum by Origanum vulgare ssp. hirtum. Two natural hybrids of O. vulgare ssp. hii.t2lm, with O. microphyllum and Origanum onites L., O. x m inoanum and O. x intercedens were also examined. The main volatiles found w ere compared with all the existing published analyses for Oliganum and the taxonomic significance of volatile oil composition was assessed. Our own re sults and the existing data together show convincingly that most Oliganum s pecies are rich either in sabinyl compounds or cymyl compounds but never bo th. We found that the hybrid between sabinyl-rich O. microphyllum and cymyl -rich O. vulgare ssp. hirtum contains mainly cymyl compounds, the sabinyl b eing suppressed. We suggest that one or more components of the cymyl pathwa y act throughout the Origanum genus, whenever present, to suppress the sabi nyl pathway, which however is never completely absent. Generally, volatile composition is in accord with Ietswaart's classification. Twelve of the 13 species in group A whose essential oil composition has so far been examined were found to be cymyl-rich and to lack significant amounts of acyclic com pounds or sesquiterpenoids. Group B contains a similar number of cymyl-rich and sabinyl-rich species but is the only group with members rich in sabiny l compounds. Group C is the only group rich in acyclic compounds and/or ses quiterpenoids. (C) 1999 Elsevier Science Ltd. All rights reserved.