Alocasia (PROSEA Medicinal plants)
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Introduction |
Alocasia (Schott) G. Don
- Protologue: Sweet, Hort. Brit., ed. 3: 631 (1839).
- Family: Araceae
- Chromosome number: x= 14;A. longiloba: 2n= 28,A. macrorrhizos: 2n= 24, 28, 42
Origin and geographic distribution
Alocasia consists of about 65 species and has an Indo-Malesian distribution. In Malesia, 57 species are indigenous; Borneo is the main centre of diversity with about 23 species, 20 of which are endemic, followed by the Philippines with 14 species, all endemic. A. macrorrhizos is planted and naturalized pantropically, and A. cucullata (Lour.) G. Don is widely cultivated in tropical Asia.
A. macrorrhizos was an important food plant, even a staple food, in various regions, e.g. in India and the Pacific islands, but this declined as Colocasia was introduced. Due to its ability to become naturalized and its value as a food in times of famine, A. macrorrhizos can be commonly found in regions where extensive cultivation was formerly practised. It is still the principal aroid food plant on a few western Polynesian islands.
Uses
Several medicinal applications of Alocasia have been reported for South-East Asia. Boiled stems of A. macrorrhizos are used as a laxative, chopped-up roots and leaves as a rubefacient, and juice from the petiole against cough. The plants are applied for stimulating the skin, e.g. in cases of fever and to remove blotches. The rhizome is sometimes used as a poultice to treat furuncles. The pounded stems are applied as a paste to snakebites and scorpion stings. The irritant juice of A. longiloba is included in dart poisons, as an addition to the really active poison. A. cucullata is used to treat snakebites in China.
The rhizomes, stems and leaves, mainly of A. macrorrhizos , are used as food, vegetable and forage. The rhizome is a source of very white, easily digested starch or flour. Several species are important as ornamentals.
Properties
The tissues contain calcium oxalate crystals, which produce irritation of the skin and inflammations of the oral cavity and mucous membranes. Sapotoxin is also present, and the toxic effects include gastroenteritis and paralysis of the nerve centres. Hydrocyanic acid is often present. A few cases of fatal poisoning following ingestion of A. cucullata fruits have been recorded; the clinical manifestations were similar to these of cyanogenic glycoside poisoning. The poisonous substances can be removed by repeated cooking, but the rhizomes and bases of petioles of A. macrorrhizos which are sometimes used for food usually contain few poisonous substances.
A lectin has been isolated from the rhizome of A. macrorrhizos , which showed potent mitogenic activity on human peripheral blood lymphocytes in the [3H]-thymidine uptake assay. It was a T-cell mitogen and did not induce any appreciable DNA synthesis in B-enriched lymphocytes. This species also contains a protein which inhibits both the enzymes trypsin and chymotrypsin. The seed extract showed antifungal activity. There is also a report on HIV-1 protease inhibitory activity.
Botany
Small to large perennial herbs, with elongate rhizomes. Leaves alternate, simple, sometimes deeply pinnatifid, sagittate to hastate, sometimes peltate, sometimes partly purplish and/or with whitish veins; petiole long, with sheath in basal part. Inflorescence a spadix shorter than or subequal to the spathe, with a zone of female flowers at base, followed by a zone of sterile flowers, then a zone of male flowers and ending in a well-developed appendix. Fruit a red to orange berry, several-seeded, infructescence enclosed in the persistent spathe. Seeds 3-5 mm in diameter, albuminous.
A. longiloba is a variable species, or perhaps a complex of species. Many of the species described in the past are based on cultivated forms, and should be regarded as cultivars or cultivar groups.
Ecology
A. longiloba and A. macrorrhizos prefer moist conditions and occur in a wide altitudinal range, up to 2000 m altitude.
Management Any part of the stem of Alocasia , as well as suckers, can be used as planting material. A. macrorrhizos is usually planted at the beginning of the rainy season, in either full sun or partial shade. When cultivated for the starch, plants are allowed to grow for 10 months to several years before the stems are harvested.
Genetic resources
Several Alocasia species are only known from a few collections and/or localities; this applies particularly to approximately 8 species endemic to Borneo. Several species with ornamental value are on the one hand potentially threatened by unsustainable collecting from the wild, and on the other hand are open to ex situ conservation through ornamental horticulture, sometimes even sustained by tissue culture. A. longiloba is widely distributed and locally common and seems not threatened, although some forms of this variable species with ornamental value fall in the group indicated above. A. macrorrhizos seems to be distributed directly by human activity and is perhaps merely a cultigen.
Germplasm collections of Alocasia exist in several parts of the world, the most important being located in Bangi (Selangor, Malaysia; National University) with 53 accessions, Hanoi (Vietnam; National Genebank) with 33 accessions, and Apia (Samoa; IRETA University) with 108 accessions.
Prospects
The trypsin and chymotrypsin inhibitor found in A. macrorrhizos might be of interest for conferring insect resistance in transgenic plants. Some other compounds also deserve more attention, e.g. the lymphocyte-stimulating lectins, as well as the toxic principles, which are occasionally responsible for poisoning in humans.
Literature
245, 325, 445, 566.
Selection of species
Authors
Hadi Sutarno
