Major volatile and phenolic compounds in species of Plecostachys and Tenrhynea (Asteraceae) that are used as herbal teas

Plecostachys polifolia, P. serpyllifolia and Tenrhynea phylicifolia (Asteraceae) are aromatic species endemic to southern Africa. Plecostachys serpyllifolia is a traditional herbal tea, while the traditional use of T. phylicifolia as a tea is reported for the first time. The volatile and phenolic compounds of the species are important in the context of their use as medicinal teas. Furthermore, chemical similarities may support the idea, based on molecular phylogenetic evidence, that the two genera are congeneric. Since there are no published chemistry data except for P. serpyllifolia, we studied the variation in volatile chemistry and phenolic chemistry of several populations of all three species, using standard methods: Steam distillation, Gas Chromatography-Mass Spectrometry (GC–MS) and Ultra-High Performance Liquid Chromatograph (UHPLC) with a Quadrupole time of-flight (qToF) Mass Spectrometer (UHPLC-MS). Despite their strong pleasant fragrances, the three species produce very low yields of essential oil (up to 0.114% of dry weight, but usually much lower). The oil composition was similar within and between the two species of Plecostachys and similar when compared to the main compounds in the monotypic Tenrhynea. The dominant compounds were caryophyllene oxide and E-caryophyllene across almost all the populations analysed, followed by sabinene, δ-cadinene and α-phellandrene. Trace compounds include Y-terpinene, linalool, 1-terpineol, cubenene, α-cadinene, pinocarvone, and E-nerolidol, among others. An exploratory study of the phenolic compounds in several populations of the three species showed that they can easily be differentiated by variation of the three main compounds. These show a diagnostically different pattern in all the species studied for multiple replicates. While all three species accumulate 3,4-dicaffeoylquinic acid, a yet unidentified compound is confined to P. serpyllifolia and 1,4,5-tricaffeoyl-3-methoxyoxaloylquinic is unique to Tenrhynea phylicifolia. This is a first comparative chemophenetic study of chemical variation and similarities between the two genera and three species. It supports the proposed sister group relationship (based on DNA evidence) between Tenrhynea and Plecostachys and the idea that the two genera should be combined.