Composition and antimicrobial activity of the essential oils from the Phebalium squamulosum species complex (Rutaceae) in New South Wales, Australia

•Most subsp. of Phebalium squamulosum produce unique essential oils, qualifying as new chemotypes.•Chemotype variation complements work presently aimed at species revision using morphology.•For example; phytochemical evidence suggests subsp. verrucosum is taxonomically misplaced.•For example; phytochemical evidence suggests subsp. squamulosum is a taxonomic mess.•Some essential oils produced high antimicrobial activity against pathogenic bacteria. Essential oils have been hydrodistilled and characterized from 21 populations of taxa currently assigned to the endemic Australian species Phebalium squamulosum (Rutaceae: Boronieae) using GC–MS, NMR and quantified using GC–FID. Essential oils were further examined using principle component analysis to distinguish chemotypes, then screened for antimicrobial activity using broth dilution and TLC-bioautography. Collections of subspecies of P. squamulosum, namely subsp. coriaceum, subsp. gracile, subsp. lineare, subsp. squamulosum, subsp. ozothamnoides and subsp. verrucosum, were made from the wild and one from a cultivated plant of known provenance within New South Wales. Results demonstrated considerable intra- and interspecific essential oil component variation, suggesting the existence of distinct chemotypes and supporting previously observed segregate species based on morphological evidence. Antimicrobial testing revealed moderate to high activity for all essential oils dominated by sesquiterpene alcohols; elemol and eudesmol isomers. Conversely, very low antimicrobial activity was observed from essential oils dominated by monoterpenes. This study constitutes the most exhaustive investigation of essential oils from P. squamulosum subspecies to date and provides the first report of antimicrobial activity.