Viola (PROSEA)
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Introduction |
Viola L.
- Protologue: Sp. pl. 2: 933 (1753); Gen. pl. ed. 5: 402 (1754).
- Family: Violaceae
- Chromosome number: x= (5), 10, 12, 13; V. odorata: 2n= 20, V. pilosa: 2n= 20, V. tricolor: 2n= 26
Major species
Viola odorata L., V. pilosa Blume.
Vernacular names
- Violet, pansy (En).
- Violette (Fr).
Origin and geographic distribution
Viola is a large genus of about 400 species, mainly from the northern temperate zone, but also in mountainous regions in the tropics. In Malesia, about 18 species occur, some of them introduced as ornamentals.
Uses
Many Viola species are used medicinally in South-East Asia and China for purifying the blood; the crushed aerial parts are applied to ulcers, sores, swellings and cuts and are also used for rheumatism. The flowers of V. betonicifolia J.E. Smith, V. odorata, V. pilosa and other species are generally taken as a diuretic and expectorant, and the syrup is a favourite medicine for cough, colds, asthma and hoarseness in local medicine throughout the world.
Many Viola species, especially V. odorata, are official in several pharmacopoeias throughout the world.
In large doses, the rhizomes, roots and seeds of V. odorata are poisonous, causing vomiting and affecting circulation and respiration. The rhizome in decoction is a powerful emetic and used in dysentery. An infusion of the fresh leaves or a syrup from the petals is used to reduce pain from cancerous growths, especially in the throat, and as a cure for cancer of the tongue. The fresh leaves are also prepared as a compress for local application, as they are emollient.
In India, V. odorata is found on the market in three forms: the dried flowers, the dried aerial parts including flowers and the dried aerial parts without flowers. The flowers are valued as a diuretic, expectorant, and as a purgative in bilious affections. They are also diaphoretic, and very useful in relieving febrile symptoms and excitement in all forms of fever, particularly in mixtures with other herbs. The seeds are also purgative and diuretic. The fresh flowering herb is applied in the treatment of skin diseases, as well as to infected eyes and ears, because of its antifungal and antibacterial activity. The rhizomes of several other Viola are also considered emetic, the leaves emollient and laxative, and the flowers pectoral, diaphoretic and antiseptic. The commonly cultivated, large-flowered Viola (Viola ×wittrockiana Gams) is a hybrid from unknown parentage, possibly between V. tricolor L., V. lutea Huds. and V. altaica Ker Gawl. In India it is used in the same way as the other Viola species. In China, V. patrinii DC. is especially used in the treatment of cancer. Chinese pharmacies import dried preparations of the leaves and fruit into Peninsular Malaysia. The dried flowers are used as a purgative. In India, the herb is also prescribed for syphilis, scrofula and biliousness.
The juice of V. papuana Becker & Pulle (synonym V. klossii Ridley) is used in Papua New Guinea for toothache. In Vietnam, V. inconspicua Blume is used for conjunctivitis, pimples, inflammation of the breast, a sore throat, yellow fever and diarrhoea with vomiting. Young parts are eaten as a vegetable.
Some Viola, including V. odorata and V. tricolor, are widely cultivated as ornamentals or for their essential oil (flowers or leaves) which is used in perfumes, while their medicinal value is of secondary importance in some regions. V. odorata has been planted in the hills of Java to check erosion of borders of terraces. The flowers are often candied, and are used to decorate cakes, puddings, and ice cream, while the fresh flowers are put in salads. The flowers may also be processed into syrup, jellies and marmalade, and can be added to gelatins, ice, vinegar, honey, wine and salad dressing. They can have a slight laxative effect.
V. betonicifolia is sometimes cultivated as a pot herb, while a decoction of the flowers of V. pilosa is taken to improve the complexion.
Production and international trade
In India, annual production of fresh V. odorata herb varied between 2.7-6.5 t in the 1950s, but no recent statistics are available.
Properties
The leaves of V. odorata contain a highly fragrant essential oil. This steam-volatile oil consists for 30-50% of 2,6-nonadien-1-al (the violet-leaf aldehyde), 2,6-nonadien-1-ol, n-hexanol, n-hexenol, n-heptenol, n-octenol, traces of eugenol, as well as several acids. Volatile components of V. odorata leaves can also be extracted with 1,1,2-trichloro-1,2,2-trifluoroethane as well as hexane. More than 100 compounds were separated, of which 23 were identified, representing 95% of the total: 1-dodecanol, pentadeca-5,10-dien-1-ol, pentadec-5-en-1-ol, 1-octadecene, 1-eicosene and octadeca-9,12-dienoic acid, whereas the terpene friedelin has been recorded as well.
The flowers of V. odorata contain traces of an essential oil, and compounds were identified as linalool, terpineol, benzyl acetate, methyl salicylate, eugenol, pentadecanoic acid ethyl ester, pentaoxahexadecan-1-ol, tetraoxahexadecan-1-ol, octadecadienal, octadecatrienoic acid ethyl ester, pentaoxanonadecan-1-ol and hexadecanoic acid. In general, ketones are responsible for the characteristic odour of the flowers. Besides volatiles, the flowers contain an emetic compound called violin, and several flavonoids such as viola-quercetrin and rutin. The roots and rhizomes contain saponins, which are active as an emetic and expectorant, and also salicylic acid, methyl-salicylic ester and gaultherin, a glycoside of methyl salicylic acid. In addition they contain an essential oil closely resembling that of the flowers, and an alkaloid, odoratine, which has a marked hypotensive activity.
The chemopreventive potential of V. odorata on 7,12-dimethyl benz[a]anthracene-induced skin papillomagenesis was assessed with Swiss albino mice. Application of an acetone extract of V. odorata at 2 mg and 5 mg/kg, significantly reduced the papilloma incidence to 75% and 65% respectively in the anti-initiational experiment, but in the case of anti-promotional phase the papilloma incidence was reduced to 75% and 50% respectively. Continuous treatment of 5 mg/kg of the acetone extract after a period of 20 weeks showed complete tumour regression in mice. The modulatory effects were assessed by the significant increase in the level of sulphhydryl groups, activity of glutathione-S-transferase, glutathione reductase, glutathione peroxidase and glucose-6-phosphate dehydrogenase activity followed by a significant decrease in the activity of cytochrome P-450 and a decrease in the malonaldehyde level (lipid peroxidation).
Significant oral antipyretic activity in rabbits was exhibited by hexane-, chloroform- and water-soluble extracts of V. odorata, which were comparable in potency to aspirin. Pyresis was induced by subcutaneous yeast injections. Antipyretic activity was more prominent in the hexane-soluble portions of these plants. No obvious toxic effects were noted for plant extracts up to doses of 1.6 g/kg. An infusion of the leaves is reported to have diuretic activity when administered to rabbits by gastric intubation at a dose of 2 g/animal.
From the green parts of V. tricolor during flowering, caffeic-, protocatechuic-, genistic-, p-hydroxybenzoic-, 4-hydroxyphenylacetic-, p-coumaric- (trans and cis forms), vanillic- and salicylic acids were extracted. An extract was tested for 9 plant pathogenic fungi, but was found to be effective only against Trichophyton mentagrophytes. The content of the flavonoid rutin was found to be highest in petals of yellow-flowered V. tricolor, compared to their aerial parts. Rutin and its semi-synthetic derivatives have a wide application in the treatment of some venous diseases.
Adulterations and substitutes
Ipecac (Psychotria ipecacuanha (Brot.) Stokes) can be used as a substitute for the rhizome of V. odorata, as an emetic. In India, dried V. odorata is commonly adulterated with other Viola species.
Description
- Perennial herbs (in Malesia), rhizomes and stolons absent or present.
- Leaves simple, alternate, suborbicular to linear-lanceolate, margin serrate or subentire, often glandular in indentations; petiole present; stipules free or adnate to the petiole, persistent, often conspicuous, usually serrate or fimbriate.
- Flowers bisexual, 5-merous, often fragrant, axillary, solitary, peduncle with a pair of bracteoles; sepals equal, entire to dentate, prolonged into appendages below the point of insertion; corolla zygomorphic, petals unequal, the lower saccate or spurred, usually broader than the others, the lateral pair smaller than the upper pair, often bearded inside; androecium shorter than the petals, filaments connivent around gynoecium, anthers 5, 2-celled, subsessile, the 2 lower ones with appendages projecting into the spur, connective produced into an apical appendage; ovary superior, glabrous or pubescent, with 3 placentas, ovules numerous, style straight, curved or geniculate, filiform to clavate, often lobed at apex.
- Fruit a 3-valved loculicidal capsule, globose to cylindrical, subtended by the dried calyx, 4-16 mm long; valves glabrous or hairy, boat-shaped, usually with rigid keels and thin sides, so that on drying they contract and forcibly discharge the seeds; seeds numerous.
- Seed ellipsoidal, glabrous, testa leathery, usually with terminal elaiosome.
- Seedling with epigeal germination; cotyledons ovate, leafy; first leaves alternate, margin serrate.
Growth and development
Many Viola species produce both normal, open flowers and closed (cleistogamous), reduced flowers in individual plants; the normal ones are cross-pollinators, attracting insects, and the closed flowers are self-pollinated. In the tropics, V. odorata flowers from May-October, while V. pilosa can be found flowering throughout the year. The seeds of Viola are often dispersed by ants (myrmecochorous), which are attracted by the odorous elaiosome attached to the seed.
Other botanical information
Most Viola species occurring in Malesia, except V. biflora L. and V. tricolor , belong to section Viola . Subsections can be distinguished on the basis of plant habit (stem and stolons present or absent) and method of seed dispersal (seeds dispersed by ants and/or by an explosive dispersal mechanism), but characteristics overlap sometimes and conclusions are not yet final.
The widespread references to V. patrinii DC. in South-East Asian literature probably concern V. betonicifolia and V. inconspicua , while V. patrinii should be restricted to northern East Asia. V. betonicifolia and V. inconspicua differ mainly in the length of the calyx appendages.
Ecology
In tropical regions, Viola occurs generally in open or slightly shaded localities, usually above 1000 m altitude, occasionally descending to 250 m altitude. Viola grows well in cool, moist climates, but exposure to heavy rain hampers flowering. V. odorata grows well in rich, well-drained but moisture-retentive soils.
Propagation and planting
Propagation of V. odorata is by division, cuttings from well-developed runners or seeds. The plants are repotted once or twice a year, and flower profusely in the second year. The old plants are removed after 4-5 years.
In vitro culture of Viola is widely practised and several protocols have been developed. A scheme for obtaining virus free V. odorata plants from meristem tip culture has also been developed: a medium containing kinetin, gibberellic acid and indolylacetic acid for the isolation of the meristem tips, the subsequent propagation through axillary budding by using agar solidified medium containing N-6-benzylaminopurine, the final rooting of plantlets with a low auxin level. Biological and serological tests allowed the identification of two virus-free clones which are intensively multiplied. Subsequently they are transferred to a greenhouse protected against the aphid vectors, to avoid new virus infections.
Diseases and pests
Cultivated Viola is frequently attacked by a complex of three viruses: cucumber mosaic virus, bean yellow mosaic virus and viola mottle virus, with aphids acting as main vectors, but also mealy-bugs. Nematodes are known to attack the roots of several Viola species.
Harvesting
In India Viola is harvested annually, in February-March.
Handling after harvest
The flowers of V. odorata are washed and dried in the shade, or between cloth, in order to retain the fragrance and colour. When crumbly to the touch, the dried petals are placed in airtight containers and stored in a dark place. Other plant parts are simply dried in the shade.
Genetic resources and breeding
In India, natural populations of Viola have been too heavily exploited and have become rather rare. Therefore, there is some danger of genetic erosion. There are no known breeding programmes of Viola for medicinal purposes.
Prospects
Viola is well known in several systems of traditional medicine throughout the world, and different plant parts are commonly applied for a vast array of complaints. However, surprisingly little is known about the phytochemistry (with the exception of the volatile oil) and phytopharmacology of these species. More research will therefore be needed to find evidence for the pharmacological effects reported.
Literature
- Council of Scientific and Industrial Research, 1976. The wealth of India: A dictionary of Indian raw materials & industrial products. Vol. 10. Publications and Information Directorate, New Delhi, India. pp. 514-517.
- Jacobs, M. & Moor, D.M., 1971. Violaceae. In: van Steenis, C.G.G.J. (Editor): Flora Malesiana. Series 1, Vol. 7. Noordhoff, Leyden, the Netherlands. pp. 179-212.
- Khattak, S.G., Gilani, S.N. & Ikram, M., 1985. Antipyretic studies on some indigenous Pakistani medicinal plants. Journal of Ethnopharmacology 14(1): 45-51.
- Okamoto, M., 1987. On the Violet group subsection Serpentes in Southeast Asia. Acta Phytotaxonomica and Geobotanica 38: 113-122.
- Perwaiz, S. & Sultana, S., 1998. Antitumorigenic effect of crude extract of Viola odorata on DMBA-induced two stage skin carcinogenesis in the Swiss albino mice. Asia Pacific Journal of Pharmacology 13(1): 43-50.
- Quisumbing, E., 1978. Medicinal plants of the Philippines. Katha Publishing Co., Quezon City, the Philippines. pp. 624-626.
Authors
G.H. Schmelzer & S.F.A.J. Horsten
