Prostanthera (Lamiaceae) as a ‘Cradle of Incense’: Chemophenetics of Rare Essential Oils from Both New and Forgotten Australian ‘Mint Bush’ Species

Abstract

The highly aromatic Australian mint bushes from the genus Prostanthera Labill. produce a high yield of essential oil on hydrodistillation. Together with its rich history, horticultural potential, iconic flowers, and aromatic leaves, it achieves high ornamental and culinary value. Species in the genus express highly diverse and chemically unique essential oils that demonstrate intra- and inter-specific patterns that have inspired taxonomic reinterpretation for over a hundred years. Previous studies have conveyed that phenoplastic expression of volatiles creates chemotypes within taxa, adding complexity to chemophenetic exploration. The current study chemically characterised essential oils from 64 highly aromatic specimens, representative of 25 taxa, giving yields as high as >2% g/g. The chemical profiles of essential oils are diverse, but generally include 1,8-cineole and signatory compounds such as sesquiterpene oxides, caryophyllene oxide, kessane and cis-dihydroagarofuran; sesquiterpene alcohols, globulol, epiglobulol, maaliol, prostantherol, spathulenol and ledol; and monoterpene derivatives of common scaffolds, borneol, bornyl acetate, bornanone, linalool and linalyl acetate. As in previous studies, analysis of chemical data confirms that the chemistry strongly agrees with taxonomic classifications. Importantly, as in classical taxonomy, the current chemical study complemented morphological analysis but conveys chemovariation, obscuring the taxonomic agreement. Nevertheless, variation within taxa may be due to environmental factors, meaning that cultivation of species in gardens will create different chemical profiles as compared to those published here.

The Diversity of Volatile Compounds in Australia’s Semi-Desert Genus Eremophila (Scrophulariaceae)

“…Some variation between specimens was evident, but only the relative amounts of components. This type of variation generally occurs as a response to extrinsic cues, such as amounts of rainfall or soil moisture retention, which is argued to influence chemical profiles in the genus Prostanthera [39,40] as well as Eremophila.…”

Purely Australian Essential Oils Past and Present: Chemical Diversity, Authenticity, Bioactivity, and Commercial Value

“…Australian essential oils that have strong antimicrobial effects include TTO [41,42], some chemotypes of Eremophila longifolia [18,19,40], essential oil from some species of Eucalyptus [43], or from some species of Prostanthera [44][45][46]. Although there are many antimicrobial essential oils, the antimicrobial effects can be traced to active ingredients that are either common or are common in Australian species.…”

“…This genus is currently undergoing extensive taxonomic revision, particularly the Prostanthera ovalifolia R.Br. [44,45,52,[91][92][93], and P. lasianthos Labill. [76,77], heterogeneous species aggregates.…”

“…[76,77], heterogeneous species aggregates. In the P. ovalifolia group, several sesquiterpenes are expressed singularly or in combinations, commonly diluted by 1,8-cineole [44]. The sesquiterpenes, such as maaliol, prostantherol, cis-dihydroagarofuran and kessane, have minimal dedicated pharmacological research (Figure 1).…”

Pharmacology of Natural Volatiles and Essential Oils in Food, Therapy, and Disease Prophylaxis

“…It is often the case that the chemical profiles within species are ‘flamboyant’ ( Sadgrove NJ., 2020 ; Sadgrove NJ. et al, 2020 ), i.e., highly variable, which can be caused by abiotic stressors that change expression patterns of volatiles, known as ‘phenoplasticity’, as mentioned above.…”