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 classification were all represented. Group A, section Amaracus was represented by Origanum calcaratum and by Oliganum dictamnus; group B, section Chilocalyx 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 minoanum and O. x intercedens were also examined. The main volatiles found were compared with all the existing published analyses for Oliganum and the taxonomic significance of volatile oil composition was assessed. Our own results and the existing data together show convincingly that most Oliganum species are rich either in sabinyl compounds or cymyl compounds but never both. We found that the hybrid between sabinyl-rich O. microphyllum and cymyl-rich O. vulgare ssp. hirtum contains mainly cymyl compounds, the sabinyl being suppressed. We suggest that one or more components of the cymyl pathway act throughout the Origanum genus, whenever present, to suppress the sabinyl 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 compounds or sesquiterpenoids. Group B contains a similar number of cymyl-rich and sabinyl-rich species but is the only group with members rich in sabinyl compounds. Group C is the only group rich in acyclic compounds and/or sesquiterpenoids. (C) 1999 Elsevier Science Ltd. All rights reserved.