Biodiversity and Phytochemistry of Lemon Balm
Lemon balm (Melissa officinalis, Lamiaceae) is a plant native to the eastern Mediterranean and western Asia that has been used traditionally as a spice, tea, and medicine. Modern pharmacological studies have reported lemon balm to possess antioxidant and antimicrobial properties. The purpose of this study was to investigate how essential oil composition, especially rosmarinic acid content and antioxidant activity, changes between lemon balm subspecies, plant parts, and harvests.
Central European climate conditions allow for permanent lemon balm fields with multiple yearly harvests. The 28 lemon balm accessions analyzed in this study comprised two subspecies, officinalis (MOFF) and altissima (MALT). They were planted in March 2008 at the Leibniz Institute of Plant Genetics and Crop Plant Research (IPK) in Gatersleben, Germany, and then harvested in June and August 2013. To procure samples, the plants were dried, separated into leaves and stems, and then hydrodistilled. Gas chromatography/mass spectrometry (GC/MS) and gas chromatography/flame ionization detection (GC/FID) were used to analyze the samples’ essential oil content. High-performance liquid chromatography (HPLC) was used to measure total phenolics and rosmarinic acid content. The 2,2-diphenyl-1-picrylhydrazyl radical (DPPH) test and ferric reducing ability of plasma (FRAP) assay were used to measure antioxidants. Hierarchical cluster analysis (HCA) and principal component analysis (PCA) were used to study oil patterns. PCA was also used to study rosmarinic acid, total phenolics, and antioxidant activity.
Fifteen MOFF and 13 MALT accessions were evaluated. Table 1 details their morphological differences. In general, MALT had more rounded leaves, longer shoots, and more space between nodes. Table 2 details the essential oil content in the leaves and stems of MOFF and MALT. MOFF had significantly more oil than MALT in both the first (P = 0.004) and second (P < 0.001) cuts. More oil was in the leaves than stems and in the second cut (harvest after regrowth) than the first. Also, “oil content increased with the year of cultivation.”
The essential oil components were highly variable. The oil content changed between cuts for both subspecies, and MALT had more variability than MOFF. Geranial and neral (monoterpene aldehydes) were characteristic of MOFF leaf oil, while MOFF stem oil contained no more than 35% geranial, neral, and citronellal in the first cut. In the second cut, however, the stems and leaves had “geranial and neral as main oil compounds.” The sesquiterpenes beta-caryophyllene, caryophyllene oxide, and germacrene D were characteristic of MALT. MALT leaves also contained monoterpenes; germacrene D was a “major compound” in MALT stem oils. MOFF first cuts had geranial as the primary component plus neral, citronellal, and caryophyllene oxides. Second cuts shifted to mostly geranial and neral (80–90%). Several second cuts of MALT were rich in the monoterpenes alpha-pinene, beta-pinene, and sabinene, which were nearly absent in MOFF.
In MOFF leaf oils, geranial and neral correlated positively with each other, and negatively with citronellal, caryophyllene oxide, germacrene D, and beta-caryophyllene. Caryophyllene oxide correlated positively with citronellal and germacrene D. In MALT leaf oils, alpha-pinene, beta-pinene, and sabinene correlated positively with each other and negatively with caryophyllene oxide. Caryophyllene oxide also negatively correlated with germacrene and beta-caryophyllene.
Rosmarinic acid is a characteristic polyphenol found in lemon balm. The Pharmacopoeia Europaea requires that the dried lemon balm leaves contain at least 1% rosmarinic acid. In this study, MOFF leaves had more rosmarinic acid than MALT leaves (P = 0.038), and the first cut had more than the second cut (P = 0.020). Rosmarinic acid positively correlated with caryophyllene oxide, and negatively correlated with sabinene, alpha-pinene, and beta-pinene in MALT leaf oils.
The leaves were richer than the stems in total phenolics which ranged from 47.3 to 104.2 mg/g for the leaves, and 26.7 to 57.6 mg/g for the stems. MALT stems had significantly less total phenolics than MOFF stems (P < 0.001). The major polyphenol was rosmarinic acid. DPPH measurements revealed that leaves had more Trolox equivalents than the stems and activity was higher in the second than first cut (P < 0.001). MOFF stems were higher in antioxidants than MALT stems (P = 0.04). For the FRAP assay, the leaves had higher values than the stems, and the first cut had higher levels than the second cut (P = 0.021). Correlations between rosmarinic acid, total phenolics, DPPH, and FRAP antioxidant activity were significant (P < 0.001).
In conclusion, MOFF and MALT have distinct essential oil profiles which change between harvests and over time. In the first cut, MOFF contained citronellal, geraniol, and neral while the second cut contained 80-90% geranial and neral. Sesquiterpenes were primarily found in the first cut of MALT, with the addition of sabinene and pinenes to the second. The first cut of MOFF leaves had the most rosmarinic acid. This information could inform agricultural practices that select for desired compounds.
The authors declare no conflicts of interest.
Chizzola R, Lohwasser U, Franz C. Biodiversity within Melissa officinalis: Variability of bioactive compounds in a cultivated collection. Molecules. January 2018;23(2):E294. doi: 10.3390/molecules23020294.