Pelargonium spp. (Geranium)

2015

The genus Pelargonium is a member of the family Geraniaceae in which grouping are also included the genera Geranium, Erodium, Monsonia and Sarcocaulon. The vast majority of the 250 or so natural species of Pelargonium derive from South Africa, although a few species are native to Australia, East Africa and Syria. The genus is subject to large morphological diversity and, for descriptive purposes, has been subdivided into 15, or sometimes 16, sections (or subgenera) based on leaf and flower characteristics and on habitat. The leaves of many of the species and numerous artificial hybrids are scented, and members of the subgenera Pelargonium, Cortusina and Polyactium appear to be especially rich in essential oil.

“Geranium oil” is the commercial name given to the product obtained by the steam distillation of the green parts of several variants of Pelargonium namely P. graveolens, P. capitatum, P. odoratissimum and P. radula (otherwise known as P. radens or P. roseum). The highest quality oil possesses a delicate rose-like fragrance and is used in perfumes and toilet waters. Lower quality product has been widely used as a general purpose perfume for hand creams, soaps and other toilet requisites, although alternative, synthetic products are now being employed more frequently for this purpose.

World production of geranium oil is estimated at between 250 and 300 tonnes per year, with perhaps 200 tonnes being traded on an international basis and the balance being retained for home consumption. China is now the major producer of medium quality geranium oil, exporting around 60-70 tonnes per year. Good quality oil is produced in North Africa, notably in Egypt, Algeria and Morocco, although only small amounts of oil are regularly exported from the latter two countries. Up until 1983, Egypt was the major supplier of geranium oil, with production of over 100 tonnes per year in the middle 1970’s. During the last decade, however, production has declined significantly from this level and was estimated at around only 40-50 tonnes in 1985. The highest quality oil (so-called Bourbon oil) is from Reunion Island, but here too production has fallen from around 60 tonnes in 1978 to nearer 35 tonnes per year at the present time. A major portion of the oil from this source is exported to southern France to supplement the small quantity of very high quality oil produced in the region of Grasse for use by the exclusive perfumery houses in the area.

Geranium oil is also produced on a relatively small scale in a number of other countries, for example Spain, Hungary, USSR, Yugoslavia, Israel, India, Haiti, Congo, Kenya, Tanzania, Zaire, Madagascar and the Ivory Coast. Production is mostly for internal consumption, but commercial quantities of oil from several of these countries are sometimes available in the market.

Optimal yields of geranium oil are obtainable when plants are grown under a subtropical climate in a light, fertile and well-drained soil. Reasonable rainfall and a humid atmosphere are advantageous to oil accumulation and temperatures must remain above 5°C throughout the year. Plants are generally propagated by taking cuttings from healthy plants and rooting them in nursery beds prior to transfer into the field. Young plants are planted in rows 60-80 cm apart, individual plants being spaced 30 to 40 cm apart. After 6 months growth, plants may be cropped by removing all the aerial material except for one stalk and associated leaves. Two, and sometimes three, crops are possible per year from a mature plantation, and such a plantation can remain economic for up to 6 years (Tropical Products Institute 1961). Yields of fresh plant material vary between 10 and 40 tonnes per year per hectare, but may be as high as 70 tonnes per year per hectare.

The essential oil is obtained by steam distillation of the green material either directly after harvesting, or after 24 h storage in order to reduce the volume. Any further storage of harvested material leads to the onset of fermentation and a deterioration in oil quality. Best quality oil is obtained through distillation from stainless steel vessels with steam supplied from an external source. Yields of oil vary according to the time of cropping and, of course, environmental factors. Maximal yields of up to 0.4% may be obtained from herbage harvested just before and during flowering. Typical overall yield is within the range 15-80 1 of oil per hectare per year. However, yields of up to 185 1 per hectare per year have been obtained, at least on an experimental scale, by selection and subsequent propagation of high-yielding plants.

Commercial geranium oil is a complex mixture of over 120 mono- and ses-quiterpenes and low molecular weight aroma compounds. The main components are citronellol, geraniol, linalol and their methanoyl (and other) esters, and together these components constitute some 60-70% of the total oil. The other significant components in all oil types are isomenthone, menthone, nerol, cis and trans-rose oxides, α-terpineol, α-pinene, myrcene and β-phellandrene. Bourbon and Egyptian oils have relatively low levels of citronellol and its esters and much higher levels of geraniol, geranyl esters and linalol when compared with the Chinese oil. The main difference between North African and Bourbon oils is that the former uniquely contain 10-epi-γ-eudesmol () whilst the latter, and incidentally the Chinese oil, contain the sesquiterpene hydrocarbon 6,9-guaiadiene () ().

Apart from the species of Pelargonium that are cultivated for geranium oil, there are many other scented-leaved variants within the genus whose essential oils display wide chemical diversity. For example, the oil of Pelargonium tomentosum consists almost entirely (96%) of isomenthone () and menthone () whereas Pelargonium Mabel Grey produces an oil containing 88% of the citral isomers, geranial and neral (). The leaves of these variants are used for culinary purposes to impart, respectively, a delicate peppermint and lemon flavour to food. On the other hand, the oil from Pelargonium fragrans is an extremely complex mixture containing many different classes of lower isoprenoids such as fenchone, camphene (), sabinene, 1,8-cineole (), limonene, p-cymene () and farnesene (). The leaves of some scented pelargoniums have even been used as medicinal remedies, for example, the camphoraceous vapour from steamed leaves of Pelargonium betulinum is inhaled for coughs and chest problems, whilst the powdered leaves of Pelargonium inquinans are said to alleviate the symptoms of headaches and colds.

Pelargonium: Future Prospects

It is quite clear that in vitro techniques are unlikely to provide a commercially attractive, alternative source for natural geranium oil. On the other hand, tissue culture techniques can provide an element of variation in essential oil composition (both in vivo and in vitro) that could be exploited in a number of ways. In this context, recent experiments have shown that when suspension cells of P. fragrans are treated over a 5-day period with colchicine, accumulation of essential oil is considerably enhanced and the product profile of the oil is quite variable and very different from that of the untreated cells. Since perception of aroma of an oil does not necessarily depend upon the presence or concentration of a single component but upon the relative amounts of a large number of, perhaps minor, constituents, techniques to induce such variation in oil product could be used to provide novel fragrances or flavours. If this is the case, then the technologies required to produce commercial quantities of these oils are already in place.

On the other hand, formation of calliclones from cell lines with changed oil composition could lead to new varieties of pelargoniums for use in a traditional cropping regime. A further possibility for the production of such new hybrids is provided by the recent demonstration of plant regeneration from culture-derived protoplasts of a number of Pelargonium species. If the reported methods are generally applicable to scented-leaved pelargoniums, then any number of novel genotypes could be produced via somatic hybridization of culture-induced or colchicine-induced aberrant lines.

 

Selections from the book: “Medicinal and Aromatic Plants III”, 1991.