Prota 11(1): Medicinal plants/Plantes médicinales 1
Journ. Linn. Soc., Bot. 30: 410 (1895).
2n = 14
Origin and geographic distribution
Aloe rabaiensis occurs in Somalia, Kenya and Tanzania.
In Kenya and Tanzania a leaf decoction of Aloe rabaiensis is taken to cure an enlarged spleen. It causes vomiting and diarrhoea. Together with Acokanthera schimperi (A.DC.) Schweinf., the exudate is part of an arrow poison made by the Giriama people of the coast of Kenya. The Digo people of Tanzania apply the heated and pounded leaves to swellings. Small portions of the root are taken as a purgative. The exudate is harvested as it can be concentrated easily into a solid material suitable for trading, called ‘bitters’.
Production and international trade
Aloe rabaiensis is one of the Aloe species in Kenya harvested illegally from the wild. It appears that there is a substantial international trade in concentrated exudate, but the exported product is probably a mixture of exudate from several Aloe species.
The distinctive constituents in Aloe leaves are phenolic compounds, including chromone, anthraquinone or anthrone derivatives. Some of the compounds are found in many species, whereas others occur in only a few. The widespread anthrone glucoside aloin A (barbaloin) is present in the exudate of Aloe rabaiensis. The exudate also contains aloeresin-D. Aloin is the active ingredient in the ‘bitters’ and has purgative properties. Other compounds of interest for their medicinal activity in this and other Aloe species are the lectins and the polysaccharides contained in the gel.
Succulent perennial shrub up to 2 m tall; stem branching from base, erect or sprawling, often supported by surrounding shrubs. Leaves in a lax rosette, persistent; stipules absent; petiole absent; blade lanceolate, 30–45 cm × 3–8 cm, apex long-acuminate, margin with firm, brown-tipped teeth 2–3 mm long, 1–1.5 cm apart, blade fleshy, greyish green often tinged reddish with few scattered whitish spots on leaves of young shoots; exudate yellow. Inflorescence consisting of head-like racemes up to 8 cm × 8 cm, densely flowered; peduncle up to 60 cm long, with 5–9 branches, lower branch sometimes rebranched; bracts lanceolate, 10–12 mm × 3 mm. Flowers bisexual, regular, 3-merous; pedicel 1–1.5(–2) cm long; perianth tubular, 2–2.5 cm long, slightly inflated around the ovary, lobes 6, 1–1.5 cm long, orange-red, yellow at the mouth, sometimes entirely yellow; stamens 6, exserted; ovary superior, 3-celled, style filiform, stigma head-shaped, exserted. Fruit unknown.
Aloe comprises about 450 species in Africa and Arabia, of which c. 315 occur in mainland Africa, c. 100 are endemic to Madagascar or the Indian Ocean islands (including the former Lomatophyllum) and c. 50 occur in Arabia. The taxonomy is complicated by the occurrence of interspecific hybrids both in the wild and in cultivation. Aloe rabaiensis belongs to a group of species with obvious stems that grow into shrubs of over 2 m tall. Several other species in this group have medicinal uses. In Rwanda the leaf extract of Aloe dawei A.Berger is drunk to cure malaria and leaf sap is used as drops to treat inflammation of the ear. In the highlands of Kenya the leaves of Aloe kedongensis Reynolds are used to treat colds, fever, diarrhoea and malaria. Veterinary use comprises treatment of poultry diseases and East Coast fever in cattle. Aloe kedongensis is planted to form live fences and is used as a dye. The roots are added to honey beer to promote fermentation. Aloe ngongensis Christian occurs in the highlands of Kenya and Tanzania and has long been considered as conspecific with Aloe rabaiensis. It has the same medicinal uses as Aloe rabaiensis, and is also harvested from the wild for exudate extraction. The leaf sap of Aloe nyeriensis Christian from the Central Province of Kenya is locally used by women to clear up pimples and blemishes on the face. The leaf exudate contains aloin A (barbaloin) and homonataloin. Natural hybrids of Aloe rabaiensis with some other species have been reported.
Aloe rabaiensis grows in sandy soil in open bushland up to 500 m altitude.
Aloe rabaiensis can be propagated by cuttings or by seed. It is possible to use tissue culture techniques to produce large numbers of plants for establishing plantations.
Although harvesting from wild plants is illegal, exudate is collected from many Aloe species in Kenya. In order to harvest the exudate from Aloe rabaiensis, a hole is dug in the ground and lined with a container. Cut leaves are arranged around the edge of the hole at an angle allowing the exudate to drain into the container. The exudate is transferred to bottles or jerry cans for sale to a dealer. The exudate is placed in a large drum and boiled down so that it becomes reddish black and very viscous. It is then transferred from the drum to sacks, in which it is left to cool and harden. After a day of cooling the material is solid and black, called ‘bitters’, and is ready for sale to a middle man, who will sell it on to an exporter. Because of the illegality in Kenya the whole marketing chain is shrouded in secrecy. Harvesting from wild plants is generally destructive. Development of properly managed Aloe plantations will lead to conservation of the wild plants and substantial yields from the plantations.
Aloe species grown in Kenyan gardens for decorative purposes are susceptible to fungal attack, scale insect and mealy bug infestation. Newly established plantations will need to be closely monitored to watch for signs of diseases and pests.
Genetic resources and breeding
Aloe rabaiensis is collected indiscriminately from the wild, with no selection factors involved. The few plantations now being established are stocked using plants collected from the wild and probably from several species.
Once the legislation is in place and enforced, and once plantations are established, Aloe rabaiensis should be a valuable crop for marginal areas where low rainfall makes it difficult to gain good yields from conventional crops. Borrowing technology from better-studied Aloe species such as Aloe vera (L.) Burm.f. will make progress in management fast and could lead to the production of leaf gel in addition to ‘bitters’. The Kenya Aloes Working Group intends to look for ways of improving the processing and quality control of the Aloe products.
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Correct citation of this article:
Newton, L.E., 2006. Aloe rabaiensis Rendle. In: Schmelzer, G.H. & Gurib-Fakim, A. (Editors). Prota 11(1): Medicinal plants/Plantes médicinales 1. [CD-Rom]. PROTA, Wageningen, Netherlands.