Terpene chemistry of lemon verbena (Aloysia citriodora): natural variation and response to ecological and agricultural variables
Chapman, Rebekah Elizabeth
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Lemon verbena (Aloysia citriodora Paláu), Verbenaceae, is an aromatic, perennial shrub native to South America. The essential oil extracts and leaf material are valued medicinally, in tincture or tea form, and for their aromatic properties in culinary and cosmetic industries. Mono- and sesquiterpenoids are the primary constituents of essential oils, and their volatility is responsible for the characteristic odors of this and many other aromatic plant species. In addition to being valued economically, the terpenes play important ecological roles including plant defense, attracting pollinators, deterring microbial infections, and even ameliorating the effects of abiotic stresses for the plants that produce them. The dominant monoterpenes of a common U.S. cultivar of lemon verbena were the cis- and trans-isomers of citral (neral and geranial), followed by δ-limonene, β-ocimene, and δ-+-3-carene; while the dominant sesquiterpenes were germacrene-D, trans-caryophyllene, bicyclogermacrene, and α-curcumene. Qualitative terpene composition did not change significantly throughout the harvest period (July – August). In agricultural study plots, mono- and sesquiterpene concentrations increased until peak flowering and then declined throughout the remainder of the growing season. Plant biomass was inversely related to plant density, though density did not significantly affect terpene yield per plant or per kilogram of plant material. Water stress significantly increased terpene concentration for multiple mono- and sesquiterpenes (e.g., α-pinene, β-phellandrene, α-cubebene and caryophyllene) and significantly reduced foliar biomass yield per plant. Exclusion of ambient UVB radiation increased foliar concentrations of geranial (trans-citral) and β-bourbonene but did not significantly affect other terpenes. Mono- and sesquiterpene emissions were not affected by the exclusion of UVB radiation or water stress. International trade in aromatic plants, such as lemon verbena, is increasing in both volume and value. These species, and the essential oils they contain, have been identified as a renewable resource and source of economic stimulus in many rural areas and developing nations. Implementing good agricultural and collection practices is critical to creation of a sustainable market while preserving and protecting native species of plants. Thin layer chromatography (TLC) was tested as a low-cost, rapid way to assess the monoterpenes of wild populations of aromatic species of plants using A. citriodora as a test species. It was possible to accurately identify the majority of the monoterpenes present with TLC and to distinguish chemotypes that clustered according to population in the different provinces sampled (Salta, Catamarca and La Rioja). Chemotypes containing deleterious compounds (i.e. thujone) could be avoided and those containing positive markers (i.e. citral) could be selected for collection and cultivar development through the use of TLC as a prescreening tool.