Aloe belongs to the Liliaceae family. The name is derived from the Arabic “alloeh”, which is the name used by the natives of eastern Africa, where the plant was first cultivated. The genus comprises about 300 perennial species, native to both eastern and southern Africa. The juice from the leaves of certain species yields a medical substance, the “aloe” drug. The drug was found cited in Ebers’ papyrus and in the works of Dioscorides. Various species were introduced into and cultivated in the dry area of Africa, in the mountains of tropical Africa, in the Antilles, in India, and in the Mediterranean basin.
Aloe barbadensis: Botany, Distribution, and Importance
Aloe plants may be either stemless, bearing a rosette of large, thick, succulent leaves, or have a stem (up to 1.5 m in length) along with or at the end of which the leaves are borne. Fleshy leaves are usually lance-shaped, up to 50 cm in length, with a sharp apex and a spiny margin, differently colored, from grey to bright green, sometimes striped or mottled; the upper leaf surface is flat or lightly concave, the lower surface strongly convex. The inflorescence (a raceme), is borne on a simple or branched scape, originating from the rosette; flowers are small, tubular, red, yellow, or white. The fruit is a capsule.
The majority of the species belonging to the genus Aloe are diploids with the chromosome number 2n = 14, and with a bimodal karyotype complement including eight long and six short acrocentric chromosomes.
Examination of Aloe leaf sections reveals the presence of three types of cells at the phloem pole of the vascular bundles: aloin cells, outer bundle sheath cells, and fibers. The majority of the species have aloin cells of various sizes which produce a copious exudate containing the active principles of the aloe drug. The theory that aloin cells are storage cells rather than secretory cells remains untested. It was suggested that the aloin cells act as storage tissue and that compounds are synthesized in the surrounding layer of cells of smaller diameter, many of which can be seen to contain globules of unknown constitution. In many species, cells in this region form a more or less distinct layer, usually one cell in thickness.
The most frequently employed species of the genus are Aloe barbadensis, A. ferox, and A. perryi, in addition to the hybrids A. ferox x africana and A. ferox x spicata (); other medicinal aloes are A. chinensis and A. ciliaris (). Aloes are generally much cultivated in the warm areas of the world as ornamentals, especially in public gardens, owing to their stiff habit.
Aloe ferox Mill. (“Cape Aloe”) is commonly cultivated in South Africa for medicinal and cosmetic uses; its cultivation gives the highest production of the drug named Cape aloe. The second-ranking species in drug production is A. barbadensis Mill. (= A. vera L., “Bitter Aloe”), which originated in northern Africa and was then introduced in the Antilles (Barbados, Jamaica, Curacao islands) and Central America (mainly Mexico and Venezuela), where it is commonly cultivated (); the drug is called either Barbados- or Curacao aloe and shows a higher content of active principles (more than double) than Cape aloe. Finally, a lesser quantity of the drug (the so-called Socotrine aloe) is produced in eastern Africa, where the cultivated species is A. perryi (= A. succotrina, “Socotrina Aloe”).
The last available data on world drug production date back many years: Fassina reported a production of 500 tons per year for Cape aloe and 350 tons per year for Barbados aloe. Drug production should have remained practically the same since then.
The drug shows very different morphological characteristics (color, brightness, consistency) according to the species from which it is extracted and preparation methods followed; it has a purgative, eupeptic, and cholagogue effect and is also largely used in the food industry as a bittering substance.
Aloe barbadensis: Conventional Propagation and Production of the Drug
Aloe barbadensis plants are usually vegetatively propagated. New suckers arise from the base of the stem and may be easily cultivated as cuttings, the adventitious rooting of cuttings being rapid.
Vegetative propagation is, in many cases, a forced practice owing to the widespread male sterility affecting this species. Sapre described irregularities during pollen meiosis in A. barbadensis, which gave a substantial clue to the high percentage of pollen sterility (58.3%); nevertheless, even when pollen grains seemed normal, failure in fruit set was observed.
As a general rule, even when seed production would be possible, vegetative propagation gives more rapid results and is preferred. This method of reproduction has been instrumental in determining a strong reduction in genetic variability in cultivated Aloe varieties, which are expected to face weak environmental modifications, such as particular climatic conditions and/or pests and diseases.
Aloe barbadensis plants are cultivated in spaced rows, planted out immediately at the end of the rainy season, and outer leaves are harvested from the second year.
Aloe juice is collected from August to October, by cutting leaves from which the juice flows out.
At present, even though derived from scanty and conventional culture practices, Aloe barbadensis production seems to be enough to meet the demand of the market. In the near future, if the demand for Barbados aloe increases, which may be assumed from the increasing utilization of natural medicinal products, better planning of the cultures and breeding strategy will become necessary, and the utilization of in vitro culture might play a central role. In particular, the clonal micropropagation method set up for Aloe barbadensis by Castorena Sanchez et al. is very efficient in terms of both the number of propagated shoots and the rapidity with which new rooted shoots are achieved, even in comparison with normal vegetative propagation. Micropropagation might be used to achieve a rapid multiplication of very favorable genotypes, i.e., showing higher content in active principles and/or resistance to diseases and climatic changes.
The content of active principles is regulated genetically: in fact, a great interspecific variability was found in the exudates from the leaves of 68 species of Aloe (). These authors suggested cultivating different African species, namely A. classenii or A. turkanensis, which contain very high levels of barbaloin. Problems might arise, however, connected with possible difficulties of these species in adapting to new environments.
On the other hand, owing to continuous vegetative propagation, genetic variability within Aloe barbadensis seems to be very limited; plant regeneration from callus might be useful to introduce new genetic variability and to select plant variants. Indeed, it has been shown that callus formation may allow the regeneration of somaclonal variants. Unfortunately, plant regeneration via callus formation in Aloe barbadensis occurs at low frequency and further studies will be necessary before using this technique. An alternative pathway might be the use of physical mutagenesis followed by multiplication of the favorable mutants through in vitro micropropagation.
In vitro culture techniques will be useful also to study many secondary constituents of the aloe drug and the biosynthetic pathways of many of its components; these aspects are far from being fully known (); this is necessary for better and larger utilization of aloe in medicinal and food industry. Studies on these aspects have been undertaken in callus from root tissues of a kindred species, Aloe saponaria (), and in calli and regenerated plants of Aloe ferox ().
Medicinal and Aromatic Plants III (1991)