The Mentha comprise a genus of the Labiatae (Lamiaceae) that are widely distributed in the north and south temperate zones of Eurasia and Africa, and members of which have been extensively introduced into the Americas. Up to some 25 species have been characterised, but the genus is extremely complex taxonomically and much phenotypic plasticity and genetic variability occurs. Diversity in Europe appears to be at the species level whereas that in central Asia mainly involves variation within one species, i.e. M. sylvestris ().
Most of the species can hybridise to yield numerous varieties that are widespread in nature and can be recognised by their intermediate appearance and general sterility – although fertile hybrid swarms are known. Consequently, the ancestry of several “species” and varieties is uncertain – especially so as several have been widely cultivated as culinary herbs and many cultivars have escaped into the environment. This variation may be responsible for differences in secondary metabolism that have often been recorded in nominally the same species. Thus it is essential that fully documented voucher specimens be deposited in herbaria when studies are carried out on the genus.
Table Classification of certain Mentha species
|Chromosome no.||Carvone group||Menthone group||Miscellaneous group|
|20-36||M. niliaca Jacq.||M. pulegium L.||M. sylvestris L.
M. rotundifolia (L.) Huds.
|48||M. spicata L.
M. crispa L
|72-96||M. cardiac Ger. and Baker||M x piperita L
M. arvensis L.
|M. citrata Ehrh
M. aquatica L.
M. gentilis L.
All the Mentha species predominantly occur in damp or wet habitats -ditches, water-meadows etc. – and they possess a creeping rhizomic rootstock and propagate mainly vegetatively. Most are perennial and vary in height from 4-6 cm (M. pulegium) to 30 to 60 cm (M. piperita). The purple, lilac or, less usually, pink or white flowerheads occur in mid-summer to autumn on axillary whorls in terminal spikes and both the foliage and flowers (but not rootstock) contain the characteristically pungent oils. A classification of the most common and well-studied species is in Table Classification of certain Mentha species although other schemes and systems of nomenclature have been proposed. Hybridisation in the M. spicata group is especially prevalent and chromosome doubling with introgression occurs between the species in many other examples. In particular, detailed studies of arvensis x spicata hybrids (= M. gracilis) have been published. In view of the differences and discrepancies in nomenclature and of characterisation of species, hybrids and varieties, the names chosen by the original authors are retained in this post: no attempt has been made at rationalisation or systemisation. A wide range of Mentha germplasm is available as tissue cultures, rhizomes, cuttings and seeds.
Fresh and dried foliage, infusions, syrups, essences and other concoctions of members of the Mentha – especially of the various forms of M. spicata (spearmint) and M. piperita (peppermint) – have been long used in folk medicine and as flavouring materials. M. piperita is one of the most widely used of such plants as an insect repellant, antispasmodic, antiseptic, anaesthetic (cf. cooling effect, see later), anthelmintic, expectorant and diuretic: extracts have also been useful for the relief of rheumatic pain and have been claimed efficacious in the treatment of flatulence, gastritis and other intestinal disorders. A similar array of properties have been recorded for M. pulegium (pennyroyal) and for M. spicata as well as several other species: thus various Mentha oils have anti-bacterial or fungitoxic properties, oil of M. crispa is a main component of a medication claimed to be effective against inflammation and a variety of oils have found cosmetic uses. Mentha oils have also been implicated in plant-insect interactions. Foliage or oils of M. spicata and M. piperita are used commercially in herbal products (teas etc.), as additions to tobacco and snuff and in culinary sauces as well as in flavourings for mouthwashes and toothpaste and in confectionary (e.g. chewing gum), liqueurs and medicines. Their potency is such that 1 part in 300 can disguise and dominate the flavour of other materials. Components of the oils, e.g. menthol 16, which induce a refreshing and cooling sensation, are widely used in ointments, inhalants and as a perfume and cosmetic base in addition to providing raw materials for systematic modification to produce novel (i.e. non-naturally occurring) flavour and fragrance materials. These modern commercial uses have led to extensive cultivation of a few Mentha species, whereas the rest have remained as kitchen herbs. The importance of mint oils in flavouring and fragrance has led to the class being third in the world-rankings of value of commercial flavouring materials behind vanilla and pepper. Commercial applications have been recently reviewed.
Some of the alleged properties have been pharmacologically assessed and a few substantiated. Although many extracts can cause irritation, they are generally non-toxic. Any pharmacological effect must be a consequence of the secondary metabolites stored in the plants and the active constituents of the extracts appear to be terpenoids, especially the monoterpenoids, that overwhelmingly predominate in the essential oils. However, few attempts have been made to characterise the putative physiologically active compounds. In one such study, carvone (5), menthol (16) and menthone (12) were all found to be more effective antiseptics than phenol as assessed by the Rideal-Walker test, although they did not achieve the potency of thymol; and these monoterpenoids also showed marked carminative effects and had mild anti-fungal properties. Pennyroyal (M. pulegium) was known as an abortifacient in ancient and medieval times (): the active ingredient has been claimed to be pulegone (8) and recent studies confirm that this compound can induce abortion in both animals and humans. Pennyroyal tea, rather than the active principle or the oil, may be preferable for this purpose, as the monoterpenoid is extremely toxic. Oils from M. arvensis and M. piperita have been shown to be potent cockroach repellents: the active compounds were ( — )-limonene (2), ( — )-menthol (16) and ( — )-menthone (12) or ( — )-carvone (5) and ( + )-pulegone (8), respectively, in the two species. The enantiomers of these compounds showed no activity (): this latter finding is consistent with the view accepted over the last few years that drug and organoleptic activity is generally restricted to a particular enantiomer or diastereoisomer when a chiral molecule is involved.
Generally, the volatile oils from Mentha species are obtained by steam distillation to yield ca. 0.3% fresh mass (FM) from foliage or flowerheads; although commercially selected cultivars of M. piperita can yield up to three fold these levels, corresponding to 2 to 3 tonnes of oil per acre. A very efficient extraction on the laboratory scale can be achieved by use of supercritical C02. The experience of commercial growers is that the yield of essential oil, together with the pattern of biosynthesis and subsequent metabolism of the monoterpenoids that yield the spectrum of products characteristic of a desirable oil, is influenced by environmental factors. Thus, acceptable oils can be obtained only in certain climates and sites, and the components of the oil from the same stands may vary from harvest to harvest. Since all commercially used plants of M. piperita are derived by vegetative propagation of a strain that originated at Mitcham, Surrey (UK), it is presumed that genetic variation is not an important factor in governing the composition of the oil, and indeed both photoperiod and temperature are known to be important factors in controlling the growth and flowering of the plant and the yield and composition of the essential oil derived therefrom. Laboratory studies have shown that application of cytokinins can increase the yields of oil from M. piperita by up to two fold – apparently due to an enhancement of the levels of the appropriate biosynthetic enzymes. Virtually all commercial production of oil from M. spicata (spearmint) occurs in the USA, which also accounts for 90% of oil of M. piperita (peppermint) – now with contributions from Bulgaria, the UK, Italy, the former Soviet Union, China and Brazil. US production of peppermint oil increased from 2260 to 3350 tonnes p.a. from 1978 to 1993 and that ofM. spicata was 1370 tonnes in the former year. In 1992, samples of peppermint oils from China and the USA typically sold at a wholesale price of ca. $7 and $4.9 kg respectively and the latter source amounted to a total world market of ca. $95 million. M. arvensis is the only other member of the genus that is grown on a commercial scale, mainly in Brazil, Japan, China and Paraguay), and is important as a source of menthol 16,.
Conclusions and Prospects
1. Callus of a variety of Mentha species has been easily established from shoot or root explants and cell suspensions, and immobilised cultures can be readily derived therefrom.
2. The secondary metabolism of the parent plant is almost always markedly changed under the conditions of culture, both qualitatively and quantitatively. Monoterpenoids can be accumulated in selected cell lines but the levels rarely approach those in the parents. Biotransformations can also be carried out by cell suspensions when treated with exogenous substrates. However, none of these in vitro processes seem likely to be of commercial development. More significant from the latter aspect is the discovery of the production of yellow pigments in most Mentha cultures: these pigments form intensely blue complexes with Fe(II) and may be of use in commerce as naturally-occurring colouring materials.
3. The readily available cultures provide excellent biomass for studies on the biosynthesis of the terpenoid-components of the Mentha and for the factors which regulate the formation of these compounds. Such biomass is also invaluable for studies on the purification of the enzymes of the terpenoid pathway which could lead to further investigations of gene transfer to produce desirable secondary metabolites in previously inactive species.
4. The enol esters which result in the blue pigmentation (see 2 above) seem to be characteristic chemotaxonomic markers for Mentha species.
5. Micropropagation methods for the regeneration of plantlet from callus of Mentha species seem straightforward, although careful choice of culture media is necessary to achieve the same composition of oils from plantlets or from their parents.
Selections from the book: “Medicinal and Aromatic Plants IX” (1996).