Botany, Distribution, Constituents, and Importance of the Plant
The genus Melissa belongs to the family Labiatae (Lamiaceae) and comprises erect branched herbs with crenate opposite leaves and a two-lipped corolla. It includes very few species, which chiefly occur in many parts of Europe and Asia. For the European region two individual species are differentiated by the Flora Europaea (): M. officinalis L. (comprising the two subspecies officinalis and altissima () Arcangeli, and M. bicornis Klokov, which may be identical with the subspecies altissima.
In contrast, the Flora of Turkey () specifies only one species (M. officinalis L.), which is subdivided into three subspecies:
b) altissima (Sm.) Arcangeli and
c) inodora (Bornm.) Bornm.
Intermediates between all three subspecies can occur.
In the area of Southern Europe and Middle Asia three Melissa species are characterized by Engler and Prantl (1889): M. officinalis L., M. parviflora Benth., and M. flava Benth. The last two species are also included in the Flora of British India (Hooker 1885).
In the Flora Malesiana the species Melissa axillaris Barkh. f. 1963 is described which includes M. parviflora Benth. and M. hirsuta Blume. The existence of three further East Asian members of this genus (M. yunnanensis Wu and Huang, M. asiatica G. Don and M. rugosa Lour.) as clear individual species is uncertain.
It is noteworthy that up to date nearly exclusively Melissa officinalis ssp. officinalis has been used for various purposes. Only M. axillaris has otherwise been utilized, e.g., in folk medicine. M. officinalis ssp. officinalis is a perennial herb native to the Eastern Mediterranean and is now found almost throughout Europe and also in North Africa and North America. It is often cultivated as a herb and frequently naturalized.
This plant is commonly known as lemon balm due to its citrus aroma. Because of the attractive flavor, the leaves are used for culinary purposes, e.g., in salad, vegetables, dressings, liqueurs, and drinks. The major use of this herb, however, is in preparation of the essential oil, which is of interest in perfumery.
Balm has also been used as a medicinal plant for more than 2000 years and is still of topical interest. This is demonstrated by the fact that beyond the already known pharmacological properties of this plant some interesting new medicinal uses have been established during the last few years. Many of the therapeutic effects of this drug can be attributed predominantly or even exclusively to its essential leaf oil.
Up to date, in the above ground parts of the plant a total of about 250 constituents have been identified. More than 190 of them were compounds of the volatile oil, which chiefly consists of mono- and sesquiterpenoids.
This essential oil is primarily responsible for the use of this plant. Several pharmacological effects can be ascribed to this oil, it has sedative, spasmolytic, bacteriostatic, and antifungal properties. Additionally, for some compounds occurring in balm oil virustatic, immunostimulating, as well as mucolytic and expectorant effects have been described.
On the market, numerous preparations (about 300 in the European region) can be found containing either extracts, tinctures, or essential oil of Melissa ojficinalis. However, in many commercial articles such as toilet soaps, bath preparations, etc., the expensive original balm oil has been replaced by oils from cheaper sources, e.g., citronella and lemongrass oils from several Cymbopogon species. A further important groupof active secondary constituents in balm leaves showing important therapeutic activity are tannins of relatively low molecular weight. These so-called pseudotannins amount to about 5% (in dried leaves) and consist of caffeic and related acids and several derivatives.
This compound group constitutes the main antiviral principle of this drug. Water extracts of Melissa showed antibacterial, antimycotic, antitumoral, and antihormonal, and antihormonal effects. Activities inhibitory to the cleavage of DNA by restriction endonucleases have also been described. Information was also given about hypotensive effects of M. ojficinalis extracts. In a recent study, neurotropic action (sedative, analgesic, sleep induction, sleep-potentialization due to pentobarbital) of a hydroalcoholic balm extract in the mouse has been reported.
Apart from essential oils and tannins, members of several other classes of natural compounds, have been found in balm leaves, e.g., of coumarins, triterpenoids, flavonoids, heterosides, bitter substances, waxes, carbohydrates, organic acids, vitamins, and enzymes. However, as far as is known compounds of these groups are not involved in the therapeutic activity of the drug.
The results of our experiments with tissue cultures of Melissa officinalis L. can be summarized as follows.
1. Undifferentiated callus cultures are capable of producing minute amounts of a complex and heterogeneous mixture of volatiles. Compared with the essential leaf oil of the plant, the qualitative and quantitative composition of these culture hydrodistillates was entirely different.
The oil of the intact plant harvested during July consisted of 79.6% mono-terpenes, 18.6% sesquiterpenes, and about 2% miscellaneous compounds. In contrast, the callus hydrodistillates contained 1% (0.4) monoterpenes, 21.5% (40.7) sesquiterpenes, 0.5% (20.5) diter-penes, and 77% (30.4) miscellaneous compounds, with the quantitative composition being strongly dependent on the type of phytohormone added.
Two dimethylcyclohexanones, eremophilene, some oxygenated sesquiterpenes, and diterpene hydrocarbons, all absent in the aboveground parts of Melissa officinalis, are the most characteristic members of the callus volatiles. Altogether, leaf and callus oil have only few trace compounds (limonene, l-octene-3-ol, linalool, jft-elemene) in common.
2. These results are in agreement with the findings of numerous other studies, showing that generally the mixture of volatiles produced by undifferentiated cell cultures is quite different in composition from that of the parent plant. In some cases, however, a high degree of correspondence has been observed:
Particular interest should be paid to the fact that undifferentiated callus cultures of Mentha piperita have been found to biosynthesize essential oil comparable in quantity and composition to that of the intact plant.
It must be emphasized that these results are contradictory to the situation met in unorganized in vitro systems of all the other species (including Melissa officinalis) of the Labiatae family investigated so far. None of these exhibited such a high degree of similarity between volatiles accumulated in glandular trichomes and those derived from unorganized cell cultures lacking differentiation.
3. As already discussed, for several reasons the occurrence of volatile terpene hydrocarbons in balm callus cultures is of considerable interest. These compounds could not be detected in the aboveground parts of Melissa plants and therefore the situation appears comparable to that of Thuja occidentalis (). The diterpenes identified in the cell suspension cultures of this gymnospermous plant were obviously also not present in the intact plant. Conversely, callus and suspension cultures of Andrographis paniculata failed to produce the characteristic diterpenes (andrographolides) of the intact plant. On the other hand, however, like balm callus, these cultures biosynthesized some new oxygenated sesquiterpenes (paniculides) not occurring in the parent plant.
4. While only four trace compounds could be detected in both callus and leaf hydrodistillate, callus, and underground parts of the intact balm plant have at least 25 compounds in common. To what extent this similarity can be ascribed to the primarily heterotrophic nature and the lack of specialized oil-secreting and -accumulating cells that both biological systems have in common, must be left open, since this matter is still under investigation.
5. Unfortunately, only very small quantities of volatiles (about 0.001% calculated on fresh weight basis) are produced in vitro, which is approximately, l/50th of the oil yield of balm leaves. A reason for this situation may be that we did not succeed in inducing the formation of differentiated accumulation sites for these lipophilic secretes in callus cultures of Melissa.
There are numerous examples supporting the existence of a connection between the accumulation of appreciable amounts of essential oil and the differentiation of specialized storage sites. This can be explained by the fact that many essential oil compounds are cytotoxic to plant tissues, and thus cultures lacking proper accumulation sites will suffer autotoxicity.
However, in vitro cultures of Perilla frutescens var. crispa () and Mentha piperita () have, interestingly, demonstrated that in rare cases appreciable quantities of essential oil can be accumulated in cell cultures of labiates devoid of any storage sites like glandular trichomes, oil cells, etc.; unfortunately, this does not apply to Melissa.
6. In recent years, the number of publications dealing with Melissa officinalis has clearly increased and the number of constituents identified has been growing rapidly. About 384 individual compounds have been described so far in this plant. A survey of the distribution of all these individual components over the different compound classes in tabular form is therefore presented here.
7. To sum up, our investigations have shown that in the near future, callus cultures of Melissa officinalis will not gain industrial importance for the alternative production of lemon balm oil. This is mainly due to the very poor yield of volatile oil and its composition being completely different from that of original lemon balm oil. However, cell cultures of M. officinalis have proved to be an interesting experimental model in basic research for studying questions of secondary metabolism, e.g., formation and storage of volatiles with distinct accumulation sites being absent, influence of different phytohormones on sesqui- and diterpene biosynthesis, relationship between metabolism of the heterotropic systems, callus, and roots of the intact plant.