The Volatile Phytochemistry of Monarda Species Growing in South Alabama

The genus Monarda (family Lamiaceae) contains 22 species of which three are native to southern Alabama, M. citriodora, M. fistulosa, and M. punctata. Several species of Monarda have been used in traditional medicines of Native Americans, and this present study is part of an ongoing project to add to our understanding of Native American pharmacopeia. Plant material from M. citriodora, M. fistulosa, and M. punctata was collected in south Alabama and the essential oils obtained by hydrodistillation. The essential oils were analyzed by gas chromatographic techniques to determine the chemical compositions as well as enantiomeric distributions. The compounds thymol, carvacrol, p-cymene, and their derivatives were the primary terpenoid components found in the essential oils. The known biological activities of these compounds are consistent with the traditional uses of Monarda species to treat wounds, skin infections, colds, and fevers.

Several Monarda species have been used by Native Americans as medicinal plants [3]. For example, M. fistulosa was used by the Blackfoot, Navajo, Lakota, and Winnebago people to treat boils, cuts and wounds; the Cherokee, Chippewa, Flathead, Ojibwa, and Tewa used the plant to treat colds, fever, and influenza; the Crow, Lakota, Menominee, and Ojibwa used the plant for coughs, catarrh, and other respiratory problems. Monarda punctata was used by the Delaware, Mohegan Nanticoke, and Navajo tribes to treat colds, fever coughs, and catarrh.
Both M. citriodora and M. fistulosa are popular ornamentals and have been introduced to temperate locations around the world [4][5][6]. Geographical location likely plays an important role in the phytochemistry of Monarda species. To our knowledge, however, there have been no previous examinations of M. citriodora, M. fistulosa, or M. punctata growing in their native range of south Alabama. In this work, we have examined the chemical compositions and enantiomeric distributions of essential oils of the three Monarda species from south Alabama.

Monarda Citriodora
The M. citriodora essential oils were obtained as clear orange oils. The essential oil yields for M. citriodora aerial parts essential oil were 1.59% and 1.79% for samples #1 and #2, respectively, while the root essential oil was obtained in 0.879% yield. The chemical compositions of the essential oils from the aerial parts and the roots of M. citriodora cultivated in south Alabama are summarized in Table 1. The essential oils were dominated by the phenolic monoterpenoids thymol (RIdb = 1289) and carvacrol (RIdb = 1296). The other major components were p-cymene (RIdb = 1024) and thymol methyl ether (RIdb = 1239).
The enantiomeric distributions of terpenoids in M. punctata essential oils were analogous to those observed for M. citriodora and M. fistulosa oils with the exception of limonene, which was virtually racemic in sample #1, but 100% (−)-limonene in sample #2.

Discussion
Monarda citriodora and M. fistulosa have been introduced throughout temperate regions of the world as popular herbal medicines as well as ornamentals [4][5][6]. The volatile phytochemistry has shown wide variation depending on geographical location ( Table 4). The essential oils of M. citriodora in the present study were rich in both thymol and carvacrol, whereas essential oils from Europe and Asia were dominated by thymol with much lower concentrations of carvacrol. Monarda fistulosa, in particular, showed wide variation with at least three different chemotypes (carvacrol-rich, thymol-rich, and geraniol-rich, see Table 4). The essential oils of M. fistulosa (samples #1 and #2) in this study fit into the thymol-rich chemotype. Interestingly, there was a high concentration of thymoquinone in M. fistulosa sample #3, with concomitant lower concentrations of thymol and carvacrol. Thymol was reported as the major component of M. punctata in two old reports [11,12]. Consistent with these reports, a floral essential oil of M. punctata from China was rich in thymol (75.2%), which is in agreement with the aerial parts essential oils from Alabama.  The high concentrations of thymol, carvacrol, and p-cymene are consistent with the traditional uses of Monarda spp. to treat skin infections, wounds, fevers, and respiratory problems. Thymol [31], carvacrol [32], and p-cymene [33] have demonstrated antibacterial and antifungal activities [34,35], as well as wound-healing activity [36]. Thymol [37] and carvacrol [38], in addition to thymoquinone [39], have shown antitussive effects. Thymoquinone has also shown wound-healing properties [40]. Furthermore, both thymol [41] and carvacrol [32] have shown analgesic and anti-inflammatory activities [42].

Materials and Methods
Plants were identified by S.K. Lawson and a voucher specimen of each plant was deposited in the University of Alabama in Huntsville Herbarium (HALA); voucher numbers for M. citriodora (SKL61820), M. fistulosa (SKL72020), and M. punctata (SKL9620). The Monarda plant materials were allowed to dry in the shade for several days, the airdried plant materials were pulverized and subjected to hydrodistillation using a Likens-Nickerson apparatus with continuous extraction with dichloromethane (Table 5).

Gas Chromatographic Analysis
The essential oils were analyzed by gas chromatography-mass spectrometry (GC-MS), gas chromatography with flame ionization detection (GC-FID), and chiral GC-MS as previously reported [52].

Conclusions
This study presents, for the first time, analyses of the essential oils of three species of Monarda growing in south Alabama. In addition, the enantiomeric distribution of terpenoids was also carried out. This work illustrates the wide variation in essential oil compositions based on geographical location as well as variations in enantiomeric distribution. It would be interesting to compare enantiomeric distributions for Monarda essential oils from other geographical locations and for other Monarda species. Nevertheless, the phenolic monoterpenoids thymol and/or carvacrol were found to dominate the compositions of M. citriodora, M. fistulosa, and M. punctata and support the traditional medicinal uses of these plants.