Cannabis sativa (PROTA)

Plant Resources of Tropical Africa
Introduction


General importance
Geographic coverage Africa
Geographic coverage World
Stimulant
Medicinal
Forage / feed
Fibre

Cannabis sativa L.

Protologue: Sp. pl. 2: 1027 (1753).

Family: Cannabaceae (Cannabinaceae)

Chromosome number: 2n = 20, 40, 80

Synonyms

Cannabis indica Lam. (1785).

Vernacular names

Cannabis, hemp, marihuana, hashish (En).
Chanvre, chanvre indien, cannabis (Fr).
Cânhamo (Po).
Mbangi (Sw).

Origin and geographic distribution

Cannabis is a native of the temperate parts of Asia: near the Caspian Sea, in Iran, the Kirghiz steppe, southern Siberia and probably also the Himalayas and northern India. It escapes cultivation and naturalizes easily. Cannabis is one of the oldest non-food crops. It was already valued by the Chinese 8500 years ago. It was introduced into western Asia and Africa, and subsequently Europe in 2000–1000 BC, and cultivation in Europe became widespread from about 500 AD onwards. From Central Asia it spread eastward to China and South-East Asia. It was introduced into the Americas in the 16^th century. At present it is cultivated all over the world, although in most countries, including those in Africa, cultivation as a psychotropic drug is prohibited by law.

Uses

Cannabis provides several useful products: therapeutics and narcotics (flowers and leaves), fibre (stems), oil (fruit, colloquially often called seed) and food for humans and animals (fruit). It was probably first used as a source of fibre: the oldest remains of cloth made from cannabis date back 6000 years. Its cultivation for oil is more recent, but began at least 3000 years ago, and the earliest reference to narcotic use dates from 5000 years ago in China. The earliest recorded medicinal use of cannabis is found in a Chinese pharmacopoeia of 4700 years old. All parts of the plant are used in Chinese medicine and in traditional medicine in Africa. The fruits are considered useful as a tonic, alterative, emmenagogue, laxative, demulcent, diuretic, anthelmintic, narcotic and anodyne. They are prescribed for dysentery, post-partum difficulties, obstinate vomiting, and used externally on eruptions, ulcers, wounds and favus. The specially prepared fruits are prescribed for uterine prolapse, to aid parturition, and as a febrifuge. Cannabis products are used as sedative of the stomach, to treat dyspepsia with painful symptoms, cancers and ulcers. They are also used to treat migraine, neuralgia, tetanus and rheumatism.

In western medicine, preparations rich in cannabinoids were extensively used between the middle of the 19^th century and the Second World War as an anticonvulsant, analgesic, sedative and soporific, and also to treat tetanus, neuralgia, uterine haemorrhage, rheumatism, epilepsy, migraine, convulsions, spasms and pain. It was considered a milder and less dangerous analgesic than opium.

The inconsistency of its therapeutic activity, the poor shelf-life of its preparations, the difficulty in deciding optimal doses and the emergence of synthetic analgesics and hypnotics led to the use of cannabis being gradually abandoned in the first half of the 20^th century, and authorized medicinal use is very limited today. In recent years, cannabis drugs have been advocated as useful for treating spasms in patients suffering from multiple sclerosis, lower intra-ocular pressure in developing glaucoma, as an analgesic in patients with rheumatism and for treating loss of appetite in AIDS patients. The best known application, however, is its use in cancer patients as a sedative and for treating side-effects of chemotherapy e.g. nausea, vomiting and convulsions, for which it is licensed in e.g. the United States and the United Kingdom. The main active compound of cannabis, Δ⁹-tetrahydrocannabinol (= Δ⁹-THC, dronabinol), is marketed as a medicine for these puposes.

The narcotic use varies between cultures. In Africa cannabis resin (locally known as ‘dagga’) has extensive ritual uses in many countries. In India 3 types of preparations exist: ‘bhang’ (dried, powdered plant, made into a drink with milk or water), ‘ganja’ (dried flowering tips of female plants, usually smoked, sometimes eaten or drunk) and ‘hashish’ (crude resin scraped from the plant, which is smoked, sometimes eaten). In the western world, ‘marihuana’ usually refers to a preparation comprising crumbled leaves, small twigs and flowering parts of female plants, whereas ‘hashish’ is a stronger preparation, with more resin and little recognizable plant material. All these drugs contain resin from the glandular hairs on leaves, stems and inflorescences.

The bast fibres of cannabis (hemp) are traditionally used to make yarn, twine, rope, nets and paper, while the wooden core of the stems is used as animal bedding or fuel. Hemp yarn is mainly made by wet spinning of the long fibres. Improved yarn quality can be obtained by ‘cottonization’ of hemp, involving the chemical or mechanical rearrangement of bast fibres. This makes it possible to process the fibres on cotton spinning machines. However, hemp fibres and yarns currently on the market rarely meet the requirements of the textile industry with respect to fibre fineness, homogeneity, flexibility and length distribution. In Africa cannabis is not important as a fibre crop.

Cardboard can be made from hemp fibre. In the building industry, fibre and particle boards containing hemp fibre are used indoors for insulation. Hemp fibres are also used in inorganic matrix composite (IMC) technology to strengthen plaster boards, tiles, concrete, mortars and plasters. The oil in the fruits is used in cosmetics and as surfactants in detergents and can be used as a substitute for linseed oil in paints and soap. Cannabis fruits are edible, and used on a limited scale as human food, as bird and poultry feed, and as fish-bait. The press cake remaining after oil extraction is used to feed livestock.

Production and international trade

Few statistics on production of cannabis for narcotic purposes exist as the cultivation, possession, preservation, transportation and trade are illegal in most countries. Cannabis drugs (herbal and resin) are by far the most commonly consumed illegal drug in the world and worldwide an estimated 161 million people used cannabis in 2003, of which 37 million in Africa. Average annual consumption is estimated at 200 g of the herbal drug or 150 g resin per user. In 2003 worldwide cannabis herb production was estimated to be more than 40,000 t, of which 30,000 t reached end users, valued at retail level at about US$ 113,000 million. Africa was a large producer (28%) with as major exporters Nigeria, South Africa, Malawi, Lesotho, Swaziland and Tanzania. The major importer is Europe, but the largest market is North America. Worldwide cannabis resin production is estimated at 7400 t, of which 6000 t reached end users, valued at retail level at about US$ 28,000 million, with northern Africa (Morocco 40%), Turkey and the Middle East (30%) as major suppliers. Major importers are Europe (78%), followed by Africa (9%) and Asia (8%). In India cannabis is cultivated for ‘ganja’ by a few licensed growers, and the drug is a monopoly of the Indian Government.

In 2004 world production of hemp fibre and tow was 66,000 t from 50,000 ha, of which 40,000 t was produced in Asia, mainly in China and Korea. World production of cannabis fruits in 2004 was about 30,000 t, of which 24,000 t in China.

Properties

Cannabis is classified as a narcotic belonging to the psychotropic disrupters. It alters perceptions and emotions. More than 460 chemical constituents have been isolated from cannabis, and the amount of literature on their chemistry and biological activity is overwhelming. The most interesting compounds for medicinal purposes are the cannabinoids, which are mainly present in the leaves and flowering tops of female plants, and accumulate in the bracts and resin, but are absent in the fruits and stems. Cannabinoids are terpenophenolics, classified into several groups. At present, about 60 of these compounds are known, and the main representatives of each of these groups are: cannabigerol (CBG), cannabidiol (CBD), cannabichromene (CBC), cannabicyclol (CBL), cannabielsoin (CBE), cannabinol (CBN), cannabinodiol (CBDL), cannabitriol (CBTL), Δ⁸-trans-tetrahydrocannabinol (Δ⁸-THC) and Δ⁹-trans-tetrahydrocannabinol (Δ⁹-THC, sometimes also called Δ¹-THC in a different numbering system). In each group, the cannabinoids can be present either as neutral phenylmethylethers or as acidic analogues, differing only by the presence of a carboxyl group. Δ⁹-THC has two acidic analogues: Δ⁹-tetrahydrocannabinolic acids A and B. Acidic cannabinoids are regarded as the genuine compounds; the carboxylic group, however, is very unstable and decarboxylation readily occurs. Δ⁹-THC itself is not stable either: on prolonged storage it is converted into CBN. CBD and cannabidiolic acid are the main components of the glandular hairs (up to 15%); the remaining cannabinoids occur in smaller amounts.

In addition to cannabinoids, Cannabis sativa contains various other compounds: flavonoids, phenolic spiroindanes, dihydrostilbenes, dihydrophenanthrenes and spermidine alkaloids. It also contains an essential oil whose main components are β-caryophyllene, humulene, α-pinene, β-pinene, limonene, myrcene and cis-β-ocimene.

The pharmacological activity of cannabis is mainly based on Δ⁹-THC; other cannabinoids seem to have less, if any, biological activity, although many of them have never been studied well. Of the two major cannabis drug-products, good quality marihuana in the 1960–1970s contained 0.1–2.7% Δ⁹-THC and hashish 4–10%. In the 2000s cannabis can be much higher in THC, ranging 5–12(–31)%, but the average user seems to prefer the lower concentrations. The predominant effects of Δ⁹-THC on the central nervous system in humans include analgesia and anti-emesis, as well as a ‘psychological high’ state with alterations in cognition and memory, and a decrease in psychomotor performance. The acute toxicity of Δ⁹-THC is very low; there are no documented cases of human death caused by this component or cannabis. The initial effects caused by a common dose of inhaling one cigarette with 2% Δ⁹-THC, or by oral intake of 20 mg of the purified compound e.g. in biscuits, cakes or herbal teas, are described as a feeling of well-being, euphoria and relaxation, and effects on the sensorium, sense of time, short-term memory and motivation. Δ⁹-THC levels in the blood plasma are highest within 15 minutes after inhalation and maximum effects are felt 30–40 minutes later, gradually diminishing until at most 3 hours after inhalation. Bio-availability of Δ⁹-THC is slower and weaker when the drug is ingested orally, but then the effects last longer. Higher doses may induce anxiety, even panic, dysphoria and hallucinations. Prolonged use in humans and in test animals leads to rapid development of tolerance. Chronic use of cannabis leads to a weak physical dependence, but psychological dependence is substantial and dependent on the user’s history and personality. Chronic use may also lead to paranoid psychosis. An overdose is mainly marked by a psychotic state (anxiety, suicidal tendencies, deep mental confusion), which may last for a week. Interruption of drug intake in chronic users may cause withdrawal syndrome, which subsides in 3–4 days. Since the 1970s, many studies have been carried out to determine the impact of the use of cannabis on health, but the long-term effects are still not well known.

Cannabinoids act on the body through their action on the endogenous cannabinoid system, consisting of neurochemical substances (endogenous ligands) and specific receptors. In humans two types of endogenous cannabinoid receptors are known: CB1 and CB2. CB1 is mainly expressed in the central and peripheral nervous system, CB2 is expressed essentially in the cells of the immune system. So, CB1 is mainly involved in psychotropic effects, CB2 in immunomodulatory effects. Besides the psychological effects, cannabinoids have a vast array of other effects on, e.g. the hormonal system, cell growth and cell structures.

Although current studies suggest that cannabinoids may be useful in treating a number of human inflammatory disorders, a thorough evaluation of their immunomodulatory effects still needs to be done. Cannabinoid administration in animal models demonstrated acute alteration of multiple hormonal systems, including the suppression of sex hormones, growth hormone, prolactin hormone and thyroid hormone. The effects in humans have been inconsistent though, which can possibly partly be ascribed to the development of tolerance.

Cannabinoids have been found to affect the proliferation and growth of a variety of cell types. Reduction of cell growth and division has been observed in protozoans. Cell proliferation and growth were also reduced in HeLa cervical carcinoma cells, Lewis lung carcinoma cells and B103 neuroblastoma cells. The effects of cannabis on the gastro-intestinal propulsion and motility have been studied in detail in mice and rats. An intravenous injection of Δ⁹-THC slowed the rate of gastric emptying and small intestine transit, but the effect on large bowel transit was less. A decrease in frequency of both gastric and intestinal contractions without altering the intraluminal pressure was also found. In humans, the anti-emetic effects of cannabinoids are equal to those of commonly used phenothiazines. Side-effects are common. In some studies a third of patients experienced dysphoria, and up to 80% had somnolence. Cannabis products have been reported to reduce muscle spasm and tremors in patients suffering from cerebral palsy or multiple sclerosis (MS). On the other hand, it has also been found to impair posture and balance in patients with spastic MS. Preliminary clinical trials have shown that marihuana and Δ⁹-THC may help to increase food intake and slow down weight loss in cancer and AIDS patients. Cannabis has been reported to cause bronchodilation, and cannabis derivatives have been tested as anti-asthma drugs. There has been preliminary research on Δ⁹-THC in the form of an aerosol spray, but other cannabinoids may also be of interest. Cannabinoids may affect the bronchi by a mechanism different from that of other anti-asthma drugs.

Cannabinoids can be used to slow down the development of wide-angle glaucoma, which is a major cause of blindness and is associated with high intra-ocular pressure. In a number of patients, cannabis significantly decreased this pressure for several hours. Aqueous extracts of cannabis fruits have shown strong nematicidal activity on the eggs, larvae and cysts of the nematode Heterodera schachtii.

The bast of the plant cultivated for fibre contains primary and secondary bast fibres rich in cellulose but low in hemicellulose and lignin. Cultivars are available containing 67% cellulose, 13% hemicellulose and 4% lignin. Primary bast fibres are 5–40 mm long and heterogenous, secondary bast fibres are smaller and more uniform with an average length of 2 mm. The woody core contains parenchyma, vessels and thick-walled short fibres with an average length of 0.55 mm. The core fibres consist of 40% cellulose, 20% hemicellulose and 20% lignin.

The fruit of cannabis contains 29–35% oil, 20–24% protein and 20–30% carbohydrates. Compounds with narcotic potential do not accumulate in the fruit but contamination may occur due to contact with the glands of flower bracts and leaves. Washing is sufficient to remove contamination. The fatty acid composition of cannabis oil is: palmitic acid 6–7%, stearic acid 3%, arachidic acid 1%, oleic acid 10–15%, linoleic acid 54–57%, omega-3-linolenic acid 15–21%, omega-6-linolenic acid 2–4% and eicosadienoic acid 0–1%.

Adulterations and substitutes

Because cannabinoids have not been found in other plant species, there are no natural adulterants and substitutes. Synthetic analogues of Δ⁹-THC have been developed, notably nabilone and levonantradol. They have undergone successful clinical trials, but have considerable side-effects. In 2006 the synthetic cannabinoid rimonabant was released as a medicine for a number of complaints, including obesity and drug dependency.

Description

Annual erect herb up to 3(–4.5) m tall, usually branched, dioecious or sometimes monoecious, rather densely appressed-hairy when young. Leaves opposite near base of stem, arranged spirally higher up, palmately compound; stipules free, filiform or narrowly subulate, c. 5 mm long; petiole long; leaflets (3–)5–7(–11), upper leaves often with only 1 leaflet, sessile, lanceolate, 6–14 cm × 0.5–1.5 cm, base narrowed, apex long-acuminate, coarsely toothed, on the upper surface rough with short stiff hairs, on the lower surface appressed hairy, and with many sessile glands. Flowers unisexual, 5-merous, regular; male flowers in short, dense cymes, united into leafy terminal panicles, almost sessile; tepals oblong, free, c. 5 mm long, greenish white, stamens with erect linear filaments and comparatively large anthers 3–4 mm long; female flowers in spike-like fascicles, enveloped by bracts, sessile, perianth absent or present and then undivided, ovary superior, 1-celled, style up to 7.5 mm long, filiform, caducous, with 2 stigmas. Fruit a broadly ovoid, compressed achene 4–5 mm long, faintly keeled on the lateral margins, smooth, shiny, yellowish or brown, closely enveloped by bracts, splitting into 2 halves, 1-seeded. Seedling with epigeal germination.

Other botanical information

Cannabis sativa is the only species in Cannabis, although Cannabis indica L. is sometimes recognized as separate species representing the morphologically and chemically distinct drug types from Afghanistan and Pakistan. Great variation exists in Cannabis as a result of selection for fibre, oilseed or resin. This variation is further enhanced by the ease of crossing between these plant types making all subspecific classifications inexact. A geographical classification is in use for cultivated cannabis, in which North European, Central Russian, Mediterranean and Asiatic types are distinguished. The North European cannabis is characterized by a short stem (< 1.5 m) and early flowering. Fibre and fruit yields are generally low. Central Russian cannabis is cultivated in Europe and Asia between 50–60° latitude. Total growth duration is 90–110 days, with stems reaching 1.3–3 m in height. Fibre yields of these types are average, but high fruit yields may be obtained. Mediterranean cannabis is mainly cultivated south of 50° latitude in Europe. The total growth duration is 130–150 days, with stems reaching 2.5–4.5 m in height. Fibre production can be high and the fibre of good quality. Fruit yields are average. Asiatic type cannabis plants form branched stems of 2.5–3 m with short internodes. Growth duration is 150–170 days. For practical purposes, three types can be distinguished, based on the concentrations of Δ⁹-THC and cannabidiol: the drug (resin) type, with high Δ⁹-THC concentration (> 1%) and low cannabidiol content; the hemp (fibre) and seed types with very low Δ⁹-THC content (< 0.3%) and high cannabidiol concentration; and the intermediate type, with moderately high concentrations of both compounds. However, concentrations may change during the growing season.

Growth and development

Cannabis is normally dioecious, but monoecious cultivars have been bred; the two sexes are normally indistinguishable before flowering. In a dioecious crop, male and female plants tend to be present in similar numbers, but, depending on cultivar and growing conditions, there may be up to 50% more female than male plants. Male plants die soon after anthesis, whereas female plants live 3–5 weeks longer, until the seed is ripe. Flowers are wind-pollinated. The duration of vegetative growth of cannabis strongly depends on photoperiod and temperature. Cannabis is a quantitative short-day plant. The sensitivity to photoperiod starts after formation of a few pairs of leaves and after a certain amount of heat has been received. As a result, optimal production of fibre hemp is limited to regions with relatively long days, in which the vegetative growth phase is sufficiently long to produce long stems. The length of the flowering phase of cannabis also depends on photoperiod and temperature. At shorter daylengths and higher temperatures flowering is earlier and also more synchronized between plants and in individual plants. The degree of branching depends on propagation methods and conditions of cultivation. Cannabis has horizontal leaves, resulting in a high degree of light interception by the top of the canopy. This leads to intense shading. Although this has the advantage that weeds are suppressed, it reduces overall photosynthesis of the crop.

Ecology

Cannabis requires humid tropical conditions to produce narcotic resin. For the production of fibre, temperate climates with temperatures of 15–27°C during the growing season are optimal. Cannabis thrives on moderately to very fertile soils provided there is enough water. It is suitable for alluvial soils along streams and for loamy soils.

Propagation and planting

Cannabis is usually raised from seed. Seeds weigh 1.5–2.5 g/100 seeds. The seed germinates at low temperatures, but not below 1°C. It germinates after 3–7 days and early vegetative growth is often slow. Soil temperatures of 10–12°C are required for optimal crop establishment. Emergence is seriously hampered by soil compaction and water logging. Healthy seed should give 90% germination, and if properly stored it will remain viable for up to 2 years. Most modern indoor growers produce vegetatively propagated crops. Rooted cuttings from female plants develop into a uniform crop of nearly identical seedless females. As long as the material remains free of pathogens, vegetative propagation can be continued. Vegetative clones tend to have higher concentrations of Δ⁹-THC than plants raised from seed, and better developed lateral branches.

Seed rate and spacing depend on the product desired. For ‘ganja’ production in India, seed is sown in rows 1.2 m apart at a rate of 3–5 kg/ha, followed by a thinning when the plants are 20 cm tall. For fibre production, seed is sown densely at a rate of 30–40 kg/ha, either broadcast or in drills. For fruit production row distances of more than 1 m and inter-plant distances of 15–50 cm are used. In China seed plants are sometimes sown in clusters.

Management

Weeding is rarely necessary because the dense canopy shades out most weeds. However, weeds may be a problem in the establishing crop and in gaps. Practical experience with fibre hemp on optimal soils has shown that for the production of 1000 kg stem material a fertilizer application of 15–20 kg N, 4–5 kg P₂O₅ and 15–20 kg K₂O is required. The need for nitrogen is highest during the vegetative growth phase; requirements for P and K increase gradually during vegetative growth, peaking around flowering before slowly declining again. Fibre quality is improved by increasing N, P and K contents of the plant (within certain limts), the effect being strongest for K, then P, and finally N. Cannabis is a suitable crop for rotation with almost any crop, though some problems may occur in rotations with beets, because of nematode infestations. Cannabis suppresses weeds and loosens the soil for the following crops. Moreover, root and leaf material may be left in the field to serve as organic material for the next crop. Male plants produce the best fibre and are sometimes harvested first; female plants are sometimes allowed to stand to set seed for oil production. The Asian practice of removing male plants is not because of their lower content of Δ⁹-THC, but to prevent seed production in female plants, which would reduce resin production.

Diseases and pests

Many diseases and pests have been described in cannabis, but few of them cause economic damage. Damping-off fungi are important because they kill germinating seeds and young seedlings. The most common leaf disease is yellow leaf spot, caused by Septoria cannabis and Sclerotinia sclerotiorum, both attacking the stems and upper leaves. Botryotinia fuckeliana (Botrytis cinerea) causes damage in high humidity conditions. In dense stands of fibre crops it attacks the stalks, whereas in hemp seed and drug crops the fungus tends to attack the female flowers because they retain moisture. Nectria haematococca (anamorph Fusarium solani) and Thanatephorus cucumeris (anamorph Rhizoctonia solani) cause root rot and may lead to losses in France and India, respectively. Cannabis may also suffer from nematodes such as Meloidogyne hapla in northern Europe and the stem nematode Ditylenchus dipsaci in southern Africa, northern America, Australia and Asia. Cannabis may be infected by the parasitic Orobanche ramosa L.

In cannabis crops, the most serious pests are the lepidopterous stem borers Ostrinia nubilalis and Grapholitha delineana, and several beetle larvae boring into stems and roots, e.g. Psylliodes attenuata, Ceutorhynchus rapae, Rhinocus pericarpius, Thyestes gebleri and several Mordellistena spp. The most important leaf-eating pests are larvae of Autographa gamma, Melanchra persicariae, Spodoptera exigua and Mamestra configurata. Flowering tops are commonly infested by budworms, such as Helicoverpa armigera, Helicoverpa zea and Heliothis viriplaca. Various aphids also infest cannabis and may act as a vector for viruses. The role played by terpenoid substances and exudates of cannabinoids in repelling insects has been neglected and deserves investigation. Another natural defence mechanism of cannabis, i.e. its cover of non-glandular trichomes, may serve as a mechanical defence against predators.

Harvesting

How cannabis is harvested depends on the product. For ‘ganja’ production, male plants are pulled out as soon as they are recognized and before they shed pollen. Unfertilized female plants are left, and harvested when flower stalks begin to turn yellow, about 5 months after sowing. For the production of ‘hashish’, the resin is collected by men who run through the plantation in leather garments. The resin sticks to the garments and is then scraped off. Another method is to collect the resin by squeezing plant tops between the palms of the hand. Plants are harvested for fibre or seed by cutting the stems manually or by machinery. In China branched plants on the edges of the field are left for seed production.

Yield

The average yield of ‘ganja’ in India is about 280 kg/ha. Elsewhere, average yields of 750 kg/ha on rain-fed land and 1270 kg/ha on irrigated land are mentioned. The extraction rate for resin is about 2.8%, averaging 10–45 kg/ha. Stem yields (yielding approximately 25–35% fibre) are usually between 3–8 t/ha, with a potential of 20 t/ha. When cannabis is grown solely for seed, yields of 1300–1700 kg/ha may be obtained.

Handling after harvest

For the production of ‘ganja’, harvested inflorescences are trodden and pressed into flat cakes. For fibre production the cut stems are graded by pulling out the longest and medium stems, respectively. Short and twisted stems are not used for fibre production. Leaves are stripped off with a knife. The stems are then dried in the field for 2–4 days. About 200 stems are bundled and immersed in water for a 3-day retting. The retted stems are dried and, if necessary, retted and dried a second time. Fibre bundles are subsequently stripped off by hand from the partly wet stems and dried on drying-lines before marketing. Another method to obtain the fibres is to dry the stems completely before breaking and combing them. The yield of fibre with the dry method is slightly higher than with the wet method; 10% versus 6% of the dry stem mass. In Europe hemp is harvested mechanically by cutting, drying in swathes on the field for a few weeks, and subsequently pressing and baling in one operation. The yield of fibre is on average 15%. The fineness of the fibre and cleanness or degree of retting are decisive for processing to ropes and textiles. Sometimes, male and female plants are therefore separated and processed differently. On modern spinning machines, in which production speeds are the determining factor, fibre length may be a limitation to the use of bast fibre. Parallel processing of the fibrous raw material is required, to prevent entanglement; this adds substantially to the costs of the fabrics. Hemp fibres may be used for paper production in which the chemical content of the fibres mainly determines the quality of the pulp. The drainability and bleachability of the pulp and sheet forming are among the many factors that determine the applicability of the pulp. For the application of fibre in composite materials the following properties are of interest: high tensile strength, rigidity, impact resistance, small volume shrinkage during curing, resistance to corrosion, low density, non-toxicity, recyclability, ease of disposal and economic price.

Genetic resources

Although the N.I. Vavilov All-Russian Scientific Research Institute of Plant Industry, St. Petersburg, Russian Federation holds 450 accessions and the Institute of Crop Science (CAAS), Beijing, China 200 accessions, there has been limited preservation of germplasm of Cannabis sativa in gene banks because of its bad image as a narcotic. Together with the declining interest in breeding and maintaining cultivars, this has led to an impoverishment of germplasm resources. However, a large reservoir of natural variation is maintained by wild types.

Breeding

Genetic variability is large. Production of large amounts of pollen and wind pollination tend to lead to extensive genetic exchange between different domesticated types and between domesticated and wild plants. Breeding has mostly focused on the creation of monoecious cultivars. In Europe breeding and selection work is directed at obtaining hemp types with a bast fibre content higher than 30% and Δ⁹-THC levels below 0.3%.

Prospects

Although the resin present in cannabis has been recognized to have therapeutic value, the use of cannabis as a medicinal plant is limited, mainly because it is prohibited in most countries. However, research is increasing on cannabis drugs for relieving patients suffering from diseases such as multiple sclerosis, cancer and AIDS and for the prevention of glaucoma. Since many young people smoke ‘ganja’ or ‘marihuana’, illegal cultivation of cannabis in remote areas is common and increasing. If cannabis were to become an accepted legal medicine worldwide, prospects for large-scale cultivation in tropical Africa would be good.

Hemp grown for fibre has a long history and still has good prospects, but not yet for tropical Africa where ecologically adapted cultivars are lacking. The plant fibre products have major advantages in various branches of industry. They are biocompatible and biodegradable, thereby reducing the environmental burden of waste materials. The excellent nutritional qualities of seed oil and seed protein and the ease with which it can be fitted into crop rotations may lead to an expansion of cannabis production in western countries. The research priorities include: developing ecologically adapted cultivars, determining optimal production techniques, developing field fibre extraction techniques, quantifying the relationships between production conditions, processing and fibre quality, and optimizing the management of production chains to the best use of all components of cannabis.

Major references

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Author(s)

P.C.M. Jansen, PROTA Network Office Europe, Wageningen University, P.O. Box 341, 6700 AH Wageningen, Netherlands

Correct citation of this article

Jansen, P.C.M., 2006. Cannabis sativa L. In: Schmelzer, G.H. & Gurib-Fakim, A. (Editors). PROTA (Plant Resources of Tropical Africa / Ressources végétales de l’Afrique tropicale), Wageningen, Netherlands. Accessed 22 December 2024.

See the Prota4U database.