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].

As far as we are aware, this work presents the first chiral analysis of terpenoid constituents of Monarda species. Several investigations on the enantiomeric distributions in other members of the Lamiaceae have been reported in the literature, however. There seems to be much variation in the enantiomeric distribution of monoterpenoids across the family. Consistent with what was observed in Monarda essential oils, (+)-α-pinene was the major enantiomer found in Coridothymus capitatus [43], Rosmarinus officinalis [44], Lepechinia heteromorpha [45], Ocimum canum, and Ocimum kilimandscharicum [46]. Likewise, (+)-β-pinene predominates over (−)-β-pinene in C. capitatus [43] as well as the Monarda essential oils. On the other hand, (−)-β-pinene dominates in R. officinalis [44] and Lepechinia mutica [47]. The essential oils of peppermint (Mentha × piperita) and spearmint (Mentha spicata) have shown nearly racemic mixtures of α- and β-pinenes [48]. (+)-α-Phellandrene and (−)-β-phellandrene were the dominant enantiomers in the Monarda essential oils. In marked contrast, however, (−)-α-phellandrene and (+)-β-phellandrene predominated in L. mutica essential oil [47]. (−)-Limonene predominates in M. fistulosa essential oil, peppermint (M. piperita) and spearmint (M. spicata) essential oils [48] whereas (+)-limonene is the major enantiomer in C. capitatus [43], O. canum, and O. kilimandscharicum [46], and a nearly racemic mixture was found in rosemary (R. officinalis) essential oil [44]. (+)-Linalool was the predominant enantiomer in C. capitatus [43], Salvia schimperi [49], Pycnanthemum incanum [50], O. canum, and O. kilimandscharicum [46], whereas (−)-linalool was the major stereoisomer in Lavandula angustifolia [51] and R. officinalis [44].