Aloe (PROSEA)

Plant Resources of South-East Asia Introduction

List of species

Aloe L.

Protologue: Sp. pl. 1: 319 (1753); Gen. pl. ed. 5: 150 (1754).

Family: Asphodelaceae

Chromosome number: x= 7; A. ferox, A. vera: 2n= 14

• Aloe vera (L.) Burm.f.

• Aloe (En)
• Indonesia: lidah buaya
• Vietnam: lô hội.

The approximately 330 species of Aloe originate from Africa, Madagascar and Arabia. The centres of diversity are South Africa (Transvaal) and the region of Eritrea, Ethiopia and northern Somalia. Over 100 species are cultivated and there is an overwhelming number of hybrids and cultivars. All Malesian Aloe have been introduced and are popular garden and pot plants. A. vera was formerly produced on Barbados, where it had been introduced early in the 16th Century. Nowadays, it is cultivated commercially in the United States, Mexico, the Caribbean, Israel, Australia, Thailand and South Kalimantan. Commercial plantations of A. ferox have been established in Albertinia in South Africa.

Aloe juice, contained in the pericyclic cells of the vascular bundles in the leaf, is used to make the laxative drug known as "aloe", "aloes" or "bitter aloes". Aloe gel, the mucilage from polyhedral cells in the central part of the leaf, is claimed to have healing properties as well.

The three main types of commercial aloe drug are "Curaçao aloe", from A. vera, "Cape aloe", mainly from A. ferox and "Socotrine aloe", from A. perryi Baker. Aloe was already known to the Greeks as early as the 4th Century B.C. The drug was used by Alexander the Great, Dioscorides, Celsus and Pliny and by later Greek and Arabian physicians. It is used as a laxative, purgative and vermifuge. An ordinary dose takes 15-18 hours to produce an effect. It acts mainly on the large intestine. The effect of aloe can be strong, and nowadays it is used in combination with weaker purgatives or antispasmodic drugs. "Cape aloe" has more powerful purgative properties than the other two; "Socotrine aloe" is the mildest. In Thai traditional medicine, aloe is always included in the drug recipe because traditional doctors believe that patients will recover faster if unwanted material is expelled from the body. Both "Curaçao aloe" and "Cape aloe" are used for veterinary purposes in Europe.

Externally, aloe gel has proven effective in the treatment of skin burns by X-ray radiation. Furthermore, A. vera gel has gained popularity as a folk remedy worldwide, and numerous claims have been made for its medicinal properties. However, experimental results only support its applications for wound healing, treatment of burns, anti-inflammatory and diabetic activities. A. vera gel has been approved by the US Federal Drug Administration only for first aid treatment of burns and cuts. Aloe gel is widely used as a hydrating and skin-protecting agent in creams and liquids such as sun lotion, shaving cream, lip balm and healing ointments. It is also gaining importance as a refreshing and nutritive ingredient in food and drinks in Indonesia and Thailand.

Many other Aloe species are popular pot plants or garden ornamentals in the Malesian region (e.g. A. arborescens Miller, A. saponaria (Aiton) Haw.). Apart from their ornamental value they are often used in folk medicine as hair lotion and to promote wound healing. The sticky leaf sap of A. camperii Schweinf. is reported to be used in Papua New Guinea to treat severe burns. In Vietnam, an Aloe species is applied as a laxative and emmenagogue, whereas the leaves are eaten cooked with sugar or made into a potage.

In 1992 the local Aloe industry based on A. ferox in South Africa was worth about US$ 0.5 million. Though considerable quantities are marketed and used locally, most of the "Cape aloe" produced in South Africa is exported to Europe, especially to Italy, France and Germany.

The international market for Aloe gel is not very open. Most gel is bought by the cosmetic industry which demands high quality. An export permit is compulsory because all Aloe species are listed in the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) Appendix II.

The aloe drug contains 15-40% aloe-emodin-anthrone 10-C-glucosides (hydroxy-anthraquinone derivatives) such as aloin, hydroxyaloins and (in A. ferox) aloinoside. Aloin, sometimes referred to as barbaloin, is a mixture of aloin A and aloin B, which interconvert through the anthranol form. Furthermore, the juice contains a pyrone derivative (aloenin) and resins: free and 8-C-glucosylated-2-acetonyl-7-hydroxy-5-methylchromones (e.g. aloesone, furoaloesone, aloeresin A, aloeresin B (aloesin) and aloeresin C). A. ferox also contains free or glycosylated feroxidin (a tetralin) and feradolide (a dihydroisocoumarin). "Cape aloe" drugs must contain at least 18% hydroxy-anthraquinone derivatives, "Curaçao aloe" at least 28%. "Curaçao aloe" is almost entirely soluble in 60% alcohol and does not contain more than 30% of substances insoluble in water. It should not contain more than 12% moisture and 3% ash. "Cape aloe" should not contain more than 12% moisture and 2% ash, and should have a water-soluble fraction of at least 45%.

The compound responsible for the laxative properties is aloin. Experiments with rats have shown that aloin itself is inactive as a laxative, but that it is activated to aloe-emodin anthrone, a purgative component, by Eubacterium sp. In the diarrhoea induced by aloin, the increase in water content might be a more important factor than the stimulation of peristalsis. Daily and prolonged use of laxatives of this type may lead to serious problems, such as dependence and "cathartic colon". Anthraquinone laxatives should not be used longer than 8-10 days, or by children younger than age 12. Contra-indications for aloe drugs include pregnancy, breast-feeding, intestinal inflammations and haemorrhoids. Preferably, aloin should be administered together with an antispasmodic, to moderate its griping action. Possible side-effects of aloin include congestion and irritation of the pelvic organs. Anthranoid-containing laxatives such as aloe may play a role in colorectal cancer. There are some published data on the genotoxic potential of anthranoids, and there is evidence of a tumorigenic potential in rodents.

A. vera gel has earned a reputation as a miracle drug. It is very rich in water, but does not appear to contain very specific compounds. The major constituents are polysaccharides (pectins, hemicelluloses), plus amino acids, lipids, sterols and enzymes. It has been proven to be effective for burn treatment, because of its anti-inflammatory and wound-healing properties. The active constituents are aloctin A, aloctin B, bradykininase and magnesium lactate. Its healing properties may also be due to hydrating, insulating and protective activities resulting from the high water content. Furthermore, it has been claimed that acemannan, the major carbohydrate fraction of A. vera gel, has various therapeutic properties, including acceleration of wound healing, immune stimulation, anti-cancer and antiviral effects, and that acemannan may partly function through macrophage activation. In experiments with rats both topical and oral treatments with A. vera had a positive influence on the synthesis of glycosamino glycans, thereby beneficially modulating wound healing. The healing of dermal wounds in diabetic rats is also positively influenced by oral and topical application of A. vera. The antidiabetic effects of aloe gel probably have an influence on phases such as inflammation, fibroplasia, collagensynthesis and maturation and wound contraction. These antidiabetic effects have been confirmed in animal experiments. Subsequently, clinical trials in Thailand gave satisfactory results. Active constituents identified in this research were polysaccharides and glycoprotein. Besides the activities mentioned above, aloe gel also exhibited antiviral effects. a placebo-controlled, double-blind study showed that a topically applied A. vera extract of 0.5% in a hydrophylic cream is beneficial for patients suffering from psoriasis. It did not show toxic or any other objective side-effect. A clinical test with patients with advanced solid tumours, for whom no standard effective therapy is available, suggested that A. vera extract in combination with the immunomodulating neurohormone melatonin may produce some therapeutic benefits, at least in terms of stabilization of disease and survival.

Psyllium (Plantago spp.), which is a natural bulk laxative, is one of the substitutes for anthraquinone-containing laxative drugs such as aloe, that may cause dependency and/or cathartic colon. However, in Thailand, anthraquinone-containing preparations from Senna alata (L.) Roxb. and other Senna and Cassia species are also sometimes recommended as substitutes for aloe. Centella asiatica (L.) Urb. is recommended as a substitute for aloe gels in wound treatments. Its triterpenes have exhibited both wound healing and antibacterial activities.

• More or less succulent shrubby perennials, often with very short stem and fleshy fibrous roots.
• Leaves arranged spirally in a rosette, sometimes distichous, linear to lanceolate or triangular, very thick and fleshy, sheathing at the base, margins usually sinuate-dentate, often spiny apically, sometimes entire, surface sometimes spiny, containing colourless, yellow, brown or purple sap.
• Inflorescence a pseudo-lateral, simple or branched, long-cylindrical raceme.
• Flowers bisexual, protandrous, pedicellate; tepals 6, usually connate into a tube, sometimes outer 3 free, fleshy, apices sub-acute to obtuse; stamens 6, in 2 rows of 3; ovary superior, 3-locular, style filiform, longer than stamens, stigma small.
• Fruit a loculicidal capsule, many-seeded.
• Seeds elongate and ovoid, grey or black, arillate.

Aloe species follow the Crassulacean acid metabolism (CAM) pathway. CAM plants are able to fix CO₂at night and to photosynthesize with closed stomata during the day, thus minimizing water loss under arid conditions. This, plus their succulent leaves, stems and the presence of a thick cuticle, makes them well adapted to dry conditions.

The flower morphology of A. ferox suggests that the flowers are pollinated by birds. However, in Africa honey bees also play a role in the pollination. A. ferox is self-incompatible and only a few flowers per raceme flower simultaneously, apparently to promote cross-pollination. The stamens are exserted in the morning, then wither and withdraw in the afternoon, whereas the style is exserted on the second day of anthesis. In Africa, A. vera flowers and fruits normally. In India and other areas outside its natural range, however, fruit formation is rare, and any fruits that do form have seeds which do not germinate. This failure to set fruit is presumed to be caused by pollen sterility and self-incompatibility.

Aloe was formerly (and sometimes still is) included in the family Liliaceae s.l. Nowadays, it is usually placed in Asphodelaceae and sometimes in a separate family Aloeaceae. Aloe and related genera like Haworthia, Gasteria and Astroloba can be distinguished by their succulent leaves, vascular bundles in a ring around ground parenchyma, a cap of aloin cells at the phloem pole, chemical properties of the often coloured and/or pungent sap, and the homogeneity of the chromosome composition. The numbers of species described in Aloe differs considerably and synonymy and names for subspecific rank are overwhelming. The names A. vera and A. barbadensis are still contentious among specialists.

Aloe grows in a wide range of climatic conditions. Though the root system is shallow, it can be grown under dry conditions. Waterlogging should be avoided and Aloe thrives best on well-drained, rich soils.

A. ferox is one of the dominant species in the "succulent bushland" vegetation in South Africa. It is especially abundant on arid rocky hillsides up to about 1000 m elevation, where mean temperatures range from 27-31°C and annual rainfall from 50-300 mm. Severe drought is said to stop juice production.

Aloe can be propagated vegetatively, through suckers, offsets or cuttings, or by seed.

In A. vera cultivation, vegetative propagation is usually preferred above propagation by seed, because of the poor seedling emergence and the faster initial growth of suckers. Water deficiency may lead to decreased sucker formation. Suckers can be cut from the mother plant when they are 15-20 cm long. They may be grown in a nursery during the first year. After transplanting, the distances between and within rows in large-scale Aloe plantations are usually at least 0.5 m.

In vitro reproduction of Aloe is also possible, but has received little attention. With A. ferox, only plant regeneration from root and embryo tissue has succeeded. In the case of A. vera, micropropagation through in vitro culture of vegetative meristems, as well as in vitro regeneration of leaf base explants appears to be possible.

Aloin, aloesin and aloeresin have been found in A. ferox plantlets regenerated in vitro from root and embryo tissue, but could not be detected in the callus.

Aloe species need to be left 2-3 years after transplanting before they can be used to obtain juice. However, for the production of gel in Thailand the first cut is made 6-12 months after transplanting. Weeding may be carried out mechanically or chemically; glyphosate is reported not to cause damage to A. vera. As the formation of too many suckers may retard growth and reduce yields of the mother plant, early removal of suckers is recommended, to obtain larger leaves. Water requirement for gel production is much higher than that for juice production, as gel yields are directly related to the amount of water available for the crop. Therefore, gel production may be increased through irrigation. The application of nitrogen may result in higher gel yields, because of faster leaf formation and higher yields per leaf. However, applying large amounts of water and nitrogen may reduce gel quality. Other cultural practices that may favour leaf production are mulching, shading and furrow cultivation.

No serious diseases are known for A. vera. In India, Alternaria alternata and Fusarium solani are causes of leaf spot disease. In Aruba, leaf rot caused by Erwinia chrysanthemi occurs occasionally.

Aloe juice is often collected by cutting off the leaves transversely close to the stem and positioning them in such a way that the juice drains into pots, tubs, vessels or even a simple canvas placed over a depression. The juice may also be obtained by squeezing the leaves or by warm or cold water retting. In South Africa, A. ferox is preferably tapped during the rainy season, because then the juice is more abundant, but tapping is also carried out in other periods of the year, except for the driest months. The leaves are usually cut in the morning and it takes 4-5 hours for the juice to drain from a pile of leaves. Only older leaves are cut; younger ones and growing tips are spared. In A. ferox in South Africa, the aloin content of the leaf juice was found to differ markedly between provenances. Large variations in aloin content have been found in A. vera too, with the highest concentration in exudates from younger mature leaves.

Aloe gel is obtained after eliminating the outer tissues of the leaf. In Aruba, gel is obtained by cutting open the leaves lengthwise and scraping the gel from the leaf blade. The youngest leaves (< 25 cm) are not suitable because of the small amount of gel, but the leaves should not be too old, because gel quantity and quality may decline. Individual A. vera leaves in Aruba reach their maximum fresh weight after about 40 weeks of growth. In a system where only selected leaves are cut, the possibilities for mechanical harvesting are limited.

In Aruba, A. vera plants can produce 16-20 leaves per year under optimal moisture supply and sufficient nitrogen fertilization. With a plant density of 50 000 plants/ha and an average fresh leaf weight of 0.2 kg, this implies a gel yield of 160-200 t (560-840 kg dry matter) per ha.

Collected Aloe juice is usually concentrated by boiling and then cooling. "Curaçao aloe" may also be vacuum evaporated and then concentrated. On cooling, a solid, amorphous extract forms, which constitutes the drug. Its appearance varies with the concentration process used and the species. If the juice has been concentrated slowly, for instance in the sun or over a low fire, the cooled extract is opaque and waxy ("hepatic aloe") and aloin crystals are visible under the microscope. If, on the other hand, the juice has been concentrated rapidly, for instance over a fierce fire, the cooled extract is semi-transparent ("glassy aloe") and no aloin crystals are visible under the microscope. Aloe from A. vera is usually "hepatic", aloe from A. ferox "glassy". Collected gel in Aruba is purified by centrifugal removal of cell wall material, and the liquid pure gel is stabilized by adding chemicals.

As a result of the continuous vegetative propagation, the genetic variability within A. vera in cultivation seems to be rather small. Nevertheless, high-yielding plants of A. ferox and A. vera may be selected and propagated for commercial cultivation. Many Aloe species hybridize in the wild if their area of distribution and period of flowering overlap, and it is easy to produce hybrids in cultivation. Because of their popularity as a greenhouse plant various Aloe species are potentially at risk of extinction. No germplasm collections and breeding programmes are known to exist.

A. vera and A. ferox might be potential crops in arid regions. A. vera is of particular interest, since its gel can be used in burn and wound care. It still remains beneficial as a household remedy: fresh gel can easily be prepared and applied to the wound. In its use as a laxative, however, the drug is tending to be replaced by other laxatives such as purified anthraquinones from Cassia and Senna species and bulk laxatives as from Plantago spp.

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• Grieve, M., 1994. A modern herbal. Tiger Books International, London, United Kingdom. pp. 26-29.
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• Reynolds, G.W, 1966. The aloes of tropical Africa and Madagascar. The Trustees the Aloe Book Fund, Swaziland, South Africa. 537 pp.
• Reynolds, G.W., 1982. The aloes of South Africa. 4th Edition. A.A. Balkema, Rotterdam, the Netherlands. 538 pp.
• van Schaik, A.H., 1994. Growing Aloe vera for gel production: report on four years of agricultural research on Aruba. Department of Agronomy, Wageningen Agricultural University, the Netherlands, & Department of Agriculture, Husbandry and Fisheries, Aruba, Dutch Caribbean. 58 pp.

• Aloe ferox
• Aloe vera

• N.O. Aguilar & M. Brink