Cymbopogon Spreng. (Aromatic Grasses)
Distribution and Importance of the Plant
Cymbopogon Spreng. is one of the major aromatic plant genera, belonging to the tribe Andropogoneae of the family Poaceae. The number of species recognized by different workers in this genus varies from 55 to over 100. With the exception of the cultivated and introduced species, Cymbopogon occurs only in the Old World tropics and subtropics. Like other members of the tribe Andropogoneae, Cymbopogon is adapted to hot moist conditions. The species are more or less evenly distributed in the area, but several rather diffuse centers of diversity such as Indo-China, India, East Africa, and Queensland can be recognized. The species of Cymbopogon are either densely or loosely tufted plants ranging in height from 20 cm to 3 m. They are mostly perennial; culms of most species are erect and unbranched.
Most of the species of Cymbopogon can be clearly distinguished from the related genera in the tribe by their aromatic smell. The species like C. nardus (L.) Rendle, C. winterianus Jowitt, C. flexuosus (Nees) Wats., C. citratus (DC.) Stapf and C. martinii (Roxb.) Wats, have been cultivated for a long time for their essential oils.
Lemongrass oil, citronella oil, palmarosa oil, and gingergrass oil are the names of commercially important essential oils extracted from the genus. In addition to the main aromatic constituents listed in Table 1, more than 50 other aromatic components have been reported to be present in the oils in varying concentrations. These essential oils have traditionally been regarded as large-volume oils used in a wide variety of perfumery products and to a much smaller extent in flavoring. They are used either as perfumery oils per se or as sources of individual chemical constituents or their derivatives, which are themselves important materials in the perfumery or flavoring industries.
Lemongrass oil per se is used as an ingredient of aerosol deodorants, floor polishes, household detergents, and a whole range of domestic and industrial products in which a pleasant fresh lemon fragrance is desired, whether in its own right or as a mask for the unpleasant odors of certain ingredients. From citral, the major constituent of lemongrass oil, ionones (β-ionone, methyl ionone, etc.), an important group of perfumery isolates and finally vitamin A and E are manufactured.
The range of products in which citronella oil is used for its fresh fragrance is very large and includes household and toilet soaps, many types of detergents, various household and industrial polishes, cleaning compounds, aerosols, bath preparations, and so on. Geraniol and citronellal are important isolates of citronella oil. Citronellal is important as a source of hydroxy-citronellal, which is of great importance in its own right, possessing a fragrance reminiscent of lily-of-the-valley. From hydroxy-citronellal it is also possible to manufacture 1-menthol. Geraniol and its derivative citronellol (the latter can also be manufactured from citronellal) both possess fragrances reminiscent of rose and are in fact known as rose alcohols. They are both of considerable importance in an extensive range of perfumery products. Derivatives of citronellol include esters such as formate and acetate, the former having applications in floral composition of rose and lily-of-the-valley type, and the latter when a more fruity character is required. Other products from geraniol and citronellol include nerol, laevo citronellol etc., each of which has a special use in different branches of perfumery.
Palmarosa oil is rich in geraniol and is an important raw material in perfumery, extensively used for imparting a rose-like aroma to wide range of soaps, cosmetic products, toiletry goods, tobacco products, and many others. Geraniol isolated from palmarosa oil can also be used to synthesize the perfumery compounds mentioned in the previous paragraph.
Gingergrass oil is closely related to palmarosa oil and in it perillyl alcohol exists along with geraniol. Gingergrass oil is used as a cheap substitute for palmarosa oil. Perillyl alcohol has a powerful warm woody and herbaceous floral odor, more like linalol and terpineol. It blends well with green floral notes in narcissus and lily, imparting a petal-like effect to rose perfumes. Perillyl acetate, having an odor reminiscent of dill and spearmint, can be synthesized from perillyl alcohol.
C. jwarancusa yields an aromatic pale yellow oil possessing a minty odor. The oil has been used medicinally for all kinds of fever. Piperitone, which is the major constituent of the oil, has recently been used as an asthmolytic. Further, it can be synthetically converted into menthol and thymol, with great pharmaceutical utility.
Several Cymbopogon species are also used in native medicine. The essential oil and infusions of these aromatic grasses are used to treat a number of human diseases like leprosy, gout, rheumatism, sprain, coughs, cold, fever, and various stomach troubles including cholera.
Production and Demand for Essential Oils on the World Market
The major producers and the main consumers on the world market of important essential oils of Cymbopogon species are given in Table Major producers of important essential oils of Cymbopogon species, respectively.
Table Major producers of important essential oils of Cymbopogon species. ()
|Name of essential oil||Country||Quantity produced (in tonnes)|
Lemongrass oil, which ranks among the most important essential oils in terms of quantities used, has been available in two basic forms, namely, “East Indian” oil, produced in South Asia, and “West Indian” oil, produced in Central and South America, parts of Africa, Indo-China, and the islands of the Indian Ocean. The volume of lemongrass oil traded internationally has declined considerably over the years from approaching 1500 tonnes in the late 1960’s to under 500 tonnes at the beginning of the 1980’s. Of this export total, Guatemala accounts for 50% on average, India for 35-40%, and China for most of the remainder.
Citronella oil ranks alongside lemongrass as one of the most widely used of all the natural essential oils, world consumption amounting to several thousand tonnes a few years back. The Ceylon type of citronella oil, produced solely in Sri Lanka, accounts for a very small proportion of world production and exports, the bulk of which is accounted for by the Java type of citronella. At the beginning of the 1980’s, the total trade of citronella oil was around 1750 tonnes/a. Of this total, Indonesia and China each contributed between 40 and 45%, Taiwan and Guatemala each a little under 3%, Sri Lanka 6%, and Brazil 2%.
In recent years, severe competition has arisen for important essential oils of Cymbopogon species in the form of synthetic substrates mostly derived from turpentine (via α-pinene and β-pinene) and petroleum-derived hydrocarbons like acetylene. Lemongrass oil and citronella oil are also facing severe competition from natural sources, namely, oil of Lit sea cubeba and oil of Eucalyptus citriodora, respectively. The current world demand for the essential oils of Cymbopogon species is met by the existing producers and the situation is likely to remain the same in the near future.
Conventional Practices for Propagation and Improvement
Species like C. winterianus, C. citratus, and C. nardus, which are known in cultivation only, are erratic in flowering and seed setting and they are conventionally propagated vegetatively by means of slips (root stocks). C. flexuosus and C. martinii, which show good flowering and seed setting, are grown from seeds in the nursery before transplanting, although often propagated by vegetative methods.
Cymbopogon species are naturally cross-pollinated and highly heterozygous. There have been various reports of natural hybridization between different species of Cymbopogon (). Earlier improvement work in Cymbopogon species was based on selection and introduction from the natural populations. However, in recent years, scientists have resorted to controlled hybridization followed by selection in the progeny for desired gene combinations. This has given encouraging results in developing many improved strains with improved oil quality and strains with altogether new essential oil components different from the traditional strains. Recently, some economically important strains have been developed in species like C. pendulus (Steud.) Wats., C. khasianus (Munro ex Hack.) Bor, C. jwarancusa (Jones) Schult. and introduced to cultivation on a commercial scale. Attempts are also being made to induce variability in Cymbopogon species through induced mutations. The natural diversity present in Cymbopogon species is decreasing at an alarming rate, due to deforestation and overexploitation. The long-term objectives of Cymbopogon improvement cannot be realized unless a lot of genetic variation is generated and conserved. This could, however, be possible, if the conventional practices are combined with some of the recently evolved biotechnology methods.
Conclusion and Prospects
Essential oil of Cymbopogon species is the source of many commercially important aromatic compounds and the quality of the oil is determined by the high percentage of the desirable aromatic constituents and low percentage of undesirable constituents. The major breeding objectives in this genus are: increased herbage and oil yield, high percentage of desired aromatic constituents in the oil, adaptation to wider agroclimatic conditions, resistance to diseases, pests, and stress conditions. It is envisaged that during the next few years biotechnology will play a very important role in the generation of genetic variability and improvement of Cymbopogon species.
The development of cell culture methodology for Cymbopogon species has advanced to the point of reliable callus induction and subculture for important species. The embryogenic cultures regenerate plants via somatic embryogenesis that can be maintained for more than 3 years. Suspension cultures have been obtained and successfully plated to retrieve callus and regenerate plants. Callus and cell cultures can be used for isolation of mutants that are resistant/tolerant to diseases, salinity, herbicides, pests, cold, drought, and other stresses. Plantlets raised from callus cultures of this genus have shown variations in chromosome number, morphology of plants, and quality and composition of essential oil. High yielding strains of Java citronella and lemongrass have been developed througn induction and screening of somaclonal variations. On the basis of stability/agronomic trials of citronella strains over locations, a variety CIMAP-BIO-13 has been released for commercial cultivation. In addition to high initial establishment (80-90%), the variety shows a 50% increase in total herb yield and oil content. Thus it is clear that callus cultures are useful for producing useful variants. Direct sprouting of axillary buds to produce single and multiple shoots through node and rhizome culture can be used for rapid clonal propagation of selected varieties.
Selections from the book: “Medicinal and Aromatic Plants III”, 1991.