Essential oil composition and leaf trichomes of Pegolettia baccharidifolia and Pegolettia retrofracta (Asteraceae)

Abstract

The essential oils obtained by hydrodistillation from Pegolettia baccharidifolia Less. and P. retrofracta (Thunb.) Kies were studied for the first time, using GC/MS and GC/FID techniques. A total of 178 components were identified, representing between 853% and 100.0% of the total oils. Some differences were observed in the composition of the essential oils of the two species but pronounced differences were also observed between different provenances of P. baccharidifolia from geographically separated populations. Five compounds, delta-cadinene, caryophyllene oxide, T-cadinol, T-muurolol and delta-cadinol (= alpha-muurolol, torreyol) were present in almost all Pegolettia samples. Of interest was the discovery of thymyl and isothymyl derivatives in two populations of P. baccharidifolia from the Vanwyksdorp area which were mainly in a dormant growth stage. However, 8,9-dehydrothymol was also present in two samples of P. baccharidifolia (0.1 and 0.2% of total yield) that were taken from actively growing plants, and 8,9-dehydrothymol isobutyrate was present in P. retrofracta at 2.1%. All other thymyl and isothymyl derivatives (thymol isobutyrate, 8,9-dehydrothymyl isobutyrate, 8,9-dehydrothymol, isothymyl valerate, thymyl valerate, isothymol, 8,9-dehydroisothymol isobutyrate, 10-isobutyryloxy-8,9-dehydrothymol isobutyrate, and 10-acetoxy-8,9-epoxythymol isobutyrate) were found only in the two Vanwyksdorp populations. The two species differ markedly in the shape and margins of the leaves, as well as in their surface appearance (glossy and bright green in P. baccharidifolia, dull green in P. retrofracta). However, the leaf trichorhes (multicellular glands, probably the site of essential oil production and/or accumulation) are very similar in both species.