Comparing essential oils from Australia's ‘Victorian Christmas Bush’ (Prostanthera lasianthos Labill., Lamiaceae) to closely allied new species: Phenotypic plasticity and taxonomic variability

Abstract

Prostanthera lasianthos (Lamiaceae) is commonly referred to as the ‘Victorian Christmas Bush’, a name that derives from the floral display it gives in the Christmas period. However, botanists recognise P. lasianthos as a heterogeneous species aggregate that includes several putative new species that are confined to disjunct locations. Recently one taxon was revised to P. eungella B.J.Conn & K.M.Proft. In the current study the chemistry of essential oils was determined for 25 specimens, representative of 8 taxa (including P. eungella). Chemical relationships were investigated using two types of multivariate analysis and several distinct chemical profiles were identified. One taxon (P. sp. Wollomombi Gorge) was sampled nine times from three different locations during the year to determine the effects of seasonal variation and subjectively-assessed soil moisture content. It was demonstrated that the chemistry of this taxon is strongly influenced by soil moisture independently of growth stage, but possibly influenced by periods of dry or wet weather. Two distinct chemotypes and one intermediate type were identified. These chemotypes differed by mediated expression of linalool and hence linalyl acetate. These metabolites were also present in specimens of P. sp. Bald Mountain, but as minor components. This latter taxon could be reliably distinguished by higher relative abundance of butanoic acid, 1-methylbutyl ester. As judged by a smaller sampling size for the other taxa, evidence that this pattern of mediated expression between terpene species was found. However, the identity of terpenes that demonstrated this mediated expression were different for each taxon; for example, volatiles from P. eungella demonstrated mediation between α-pinene and caryophyllene. Nevertheless, it is possible that chemical expression of all taxa in the heterogeneous species aggregate is strongly mediated by the moisture or nutrient content of soils. Thus, the dataset produced during this study can be used as a reference against other chemical data to investigate taxonomic placement in members of this group.