Cannabis sativa (PROSEA)

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Plant Resources of South-East Asia
Introduction
List of species


Cannabis sativa L.


Protologue: Sp. pl. 2: 1027 (1753).
Family: Cannabaceae
Chromosome number: 2n= 20, 40, 80

Vernacular names

  • Hemp, Indian hemp, marihuana (En)
  • Chanvre (Fr)
  • Indonesia: ganja (general), ginje jawa (Javanese)
  • Malaysia: ganja
  • Thailand: kancha, kancha cheen (general), paang (Shan-Mae Hong Son)
  • Vietnam: gai mèo, lanh mán, cần xa.

Origin and geographic distribution

C. sativa is the only species in Cannabis. It 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 is one of the oldest of cultivated plants. Hemp was valued by the Chinese 8500 years ago, and it may be one of the oldest non-food crops. It was introduced into western Asia and Egypt, and subsequently Europe during the period 2000-1000 B.C., and cultivation in Europe became widespread from about 500 AD onwards. From Central Asia it spread eastward to China, Indo-China, Thailand and the Malesian region. It might have occurred in Java already 1000 years ago, in Malaysia more than 300 years ago, and in the Philippines more than 200 years ago. It was introduced into South America in 1545 and into North America in 1606. Nowadays, it is cultivated in many parts of the temperate, subtropical and tropical regions. In many countries including those in South-East Asia, however, its cultivation is prohibited by law.

Uses

Hemp provides different products: therapeutics and narcotics (flowers and leaves), fibre (stems), oil (seeds) and food for humans and animals (seeds). It was probably first used as a source of fibre: the oldest remains of cloth made from hemp date back 6000 years. The use for seed oil is more recent, but began at least 3000 years ago, and the earliest reference to narcotic use appears to date from 5000 years ago in China. The earliest recorded medicinal use of hemp is found in a Chinese pharmacopoeia of 4700 years old.

All parts of the plant are used in Chinese medicine. The seeds are considered useful as a tonic, alterative, emmenagogue, laxative, demulcent, diuretic, anthelmintic, narcotic and anodyne. They are prescribed in fluxes, post-partum difficulties, obstinate vomiting, and used externally on eruptions, ulcers, wounds and favus. The specially prepared seeds are prescribed for uterine prolapse and to aid parturition, and as a febrifuge. Hemp is a sedative of the stomach, used to treat dyspepsia with painful symptoms, cancers and ulcers. It is also used to treat migraine, neuralgia, tetanus and rheumatism.

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

The inconsistency of its therapeutic activity, the poor keepability of its preparations, the difficulty in deciding optimal doses and the emergence of synthetic analgesics and hypnotics led to the use of hemp being gradually abandoned in the first half of the 20th Century, and there is very limited authorized medicinal use today. In recent years, hemp drugs have been advocated as very useful to treat spasm in patients suffering from multiple sclerosis, to treat increased pressure within the eyeball and to treat loss of appetite in AIDS patients. The best known application, however, is its use as a sedative in cancer patients, and to treat side-effects of cancer chemotherapy e.g. nausea, vomiting and convulsions. One of the cannabinoids, Δ9-tetrahydrocannabinol, the best known active compound of hemp, is marketed as an anti-emetic (sometimes also called dronabinol) e.g. in the United States. Other potential applications of isolated cannabinoids include the use as antiglaucoma, anti-asthmatic, anticonvulsant, spasmolytic and analgesic.

The narcotic use varies between cultures, and many descriptive terms exist. In India, where the use of Cannabis as a drug became more important in the last millennium than anywhere else in the world, three types of preparations are distinguished: "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 "charas" (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 drug types contain a resin from the glandular hairs on leaves, stems and inflorescences.

The bast fibres of C. sativa are traditionally used to make yarns, twines, ropes, nets and paper, while the wooden core of the stems is normally used as animal bedding or fuel. Hemp yarns are mainly made by wet spinning 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 machines. However, the hemp fibres and yarns currently on the market do not meet the requirements of the textile industry with respect to fibre fineness, homogeneity, flexibility and distribution of fibre length. In Malesia, hemp is not important as a fibre crop, but in Thailand its fibres are used to make ropes and textile, especially in the northern part of the country.

Carton can be made from pulp of the different fibre types of hemp. High quality paper (monetary bills) and specialty applications (diapers, bandage) may be envisaged using pulp from bast or core.

Current fibre applications in building and construction materials are fibre and particle boards, panels and inorganic matrix composites (IMC). Boards and panels are mostly used indoors as a non-structural material for insulation. The applications of IMCs include plaster boards, tiles, concrete, mortars and plasters. Important arguments for using plant fibres in IMCs are asbestos substitution, saving of weight, waste management and the good compatability with the matrix due to the hydrophylic properties of plant fibres.

One technologically innovative application of bast fibres of hemp is in fibre reinforced structural materials called composites, where the fibres replace the glass fibres that are normally used. Combined with the application of biodegradable resins such as cellulose, starch or casein, the use of natural fibres in composites has a clear ecological advantage over traditional materials. Markets for these often specialized and costly products are manufacturers of automotive and aircraft interior parts, of machines, of sports and leisure goods and of biomedical aids, and the construction industry.

The oil in the seeds can be used as a substitute for linseed oil in paints and soap. Other applications of the oil are in cosmetics and as surfactants in detergents.

Hemp seed is edible, and used as human food on a limited scale. It is also used as bird and poultry feed. The press cake remaining after oil extraction is used to feed livestock.

Production and international trade

The production areas of hemp differ, depending on the product required. Hemp for narcotic properties is mainly grown in warmer climates. It is cultivated for "ganja" in India (Bengal, Madras and Mysore) by a few licensed growers, and the drug is a monopoly of the Indian Government. In many countries it is grown for narcotics even though this is illegal.

In 1991-1993, the annual world production of hemp fibre and tow was 120 000 t, of which 80 000-90 000 t was produced in Asia, mainly in India (45 000 t) and China (20 000-25 000 t). The annual world production of hemp seed in 1991-1993 was around 40 000 t, of which 22 000-25 000 t was produced in China.

There are no statistics on production in South-East Asia as the cultivation, possession, preservation, distribution, transportation and trade of hemp are forbidden by law e.g. in Singapore since 1870, in Burma (Myanmar) since 1873 and in Indonesia since 1927.

Properties

Hundreds of different components have been isolated from hemp, 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 seeds and stems.

Cannabinoids are terpenophenolics, classified into several groups on basis of their structures. At present, some 60 of these compounds are known, and the main representatives of each of these groups are: cannabigerol (CBG), cannabidiol (CBD), cannabichromene (CBC), cannabicydol (CBL), cannabielsoin (CBE), cannabinol (CBN), cannabinodiol (CBDL), cannabitriol (CBTL), (-)-Δ8-trans-tetrahydrocannabinol (Δ8-THC) and (-)-Δ9-trans-tetrahydrocannabinol (Δ9-THC). In the latter compound, Δ9-THC refers to the more common dibenzopyran system of numbering; when the less common monoterpenoid system of numbering is used, this compound is called Δ1-THC. In each group, the cannabinoids can be present either as neutral phenolics/phenolmethylethers or as one or more isomeric acidic analogues, differing only in the presence of a carboxyl group (e.g. cannabigerolic acid, cannabidiolic acid, cannabinolic acid). Δ9-THC has two acidic analogues: Δ9-tetrahydrocannabinolic acids A (carboxyl group at position 2) and B (carboxyl group at position 4). Acidic cannabinoids are regarded as the genuine compounds; the carboxylic group, however, is very unstable: decarboxylation readily occurs, e.g. during growth of the plant, storage of plant products or upon analysis.

The biosynthesis of the cannabinoids starts with the condensation of geranylpyrophosphate and olivetolic acid (a polyketo-acid) into cannabigerolic acid. Cannabigerolic acid is an intermediate of major importance in the formation of several types of cannabinoids e.g. CBC and CBD. The latter undergoes a second cyclization to yield Δ9-THC-acid; decarboxylation finally gives Δ9-THC. Δ9-THC itself is not quite stable either: e.g. on prolonged storage the compound is converted into CBN via formation of an additional aromatic structure.

Besides the cannabinoids, the presence of various other components in C. sativa is documented: flavonoids (e.g. canniflavon-1, canniflavon-2), phenolic spiroindanes, dihydrostilbenes, dihydrophenanthrenes and spermidine alkaloids (cannabisativin, anhydrocannabisativin in the leaves, stems and roots). C. sativa 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 C. sativa is mainly based on Δ9-tetrahydrocannabinol; other cannabinoids seem to have less, if any biological activity, although many of them have never been studied well. Of the two major Cannabis products, good quality marihuana contains 0.1-2.7% Δ9-THC and hashish 4-10% Δ9-THC. CBD and cannabidiolic acid are the main components of the glandular hairs (up to 15%); the remaining cannabinoids occur in smaller amounts.

Various preparations of Cannabis or Δ9-THC have traditionally been used for their psychological manifestations. The predominant central-nervous-system (CNS) response to Δ9-THC in humans include analgesia and anti-emesis, as well as a "psychological high" state with alterations in cognition and memory, and a decrement in psychomotor performance. The acute toxicity of Δ9-THC is reported to be very low (e.g. 128 mg/kg intravenous in the monkey); there are no documented cases of human death caused by this component or hemp. The initial effects caused by a common "dose" of inhaling one cigarette with 2% Δ9-THC, or by an oral application of 20 mg of the purified compound are described as a feeling of well-being, euphoria and relaxation, and effects on the sensorium, sense of time, short-term memory and motivation. Higher doses may induce anxiety which may become panic, dysphoria and hallucinations. Tolerance to many of these effects has been found in test animals: use in humans also led to rapid development of tolerance. Chronic use of hemp leads to a weak physical dependence, but psychological dependence is substantial and dependent on the user's history. 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 rapidly in 3-4 days. Since the 1970s, many studies have been carried out to determine the impact of the use of hemp on health, but the long-term effects are still not well known.

Studies on the relations between the structure and activity of cannabinoids have shown that prerequisites for the psychotropic activity are the pyran structure, a stereochemical (-)-configuration, a trans configuration of the Δ9-bond and a free phenolic group. This might explain why both Δ9-THC and its metabolites formed by hydroxylation (e.g. 11-hydroxy-Δ9-THC) are active, whereas CBG, CBD and CBC are inactive. At present, the exact action of cannabinoid drugs in the brain is still poorly understood. Very little is known yet about the neuroanatomical location of the cells responsible, or the cellular mechanisms involved. Cannabinoid drugs have been found to inhibit adenylate cyclase activity in a model neuronal system; this ability was furthermore related to the ability of these compounds to produce effects on the central nervous system. These results led to the identification of the presence of a specialized cannabinoid receptor in brain homoginates of the rat.

Besides the psychological effects, a vast array of other effects is known from the cannabinoids affecting e.g. the immune system, the hormonal system, cell growth and cell structures.

The effects on the immune system have been studied in mice, using sheep red blood cells (SRBC) as the antigen. Animals treated with Δ9-THC (10 and 15 mg/kg) during the primary immunization period exhibited a suppression of the primary humoral immune response. Mice treated with Δ9-THC during the secondary immunization period showed no measurable suppression of the secondary humoral immune response to SRBC. However, when mice were given Δ9-THC (10 and 15 mg/kg) during primary immunization, the secondary humoral was suppressed (existence of a memory aspect). In all experiments CBD or CBN were inactive (10 and 25 mg/kg).

When effects of cannabinoids on the hypothalamic-pituitary axis were studied in vivo in the rat, Δ9-THC and CBN both produced an acute suppression of plasma luteinizing hormone (LH), plasma testosterone and hypothalamic noradrenaline (norepinefrine) metabolism. There were no effects on plasma follicle-stimulating hormone or hypothalamic LH-releasing hormone (LHRH). These results therefore suggest that decrease of LH secretion is due to reductions in noradrenaline stimulated LHRH release (hypothalamic level), rather than to changes in LHRH synthesis or pituitary LHRH response.

Cannabinoids have been found to affect the growth, proliferation and division of a variety of cell types. Reduction of cell growth and division has been observed in protozoans. Other cell systems sensitive to cannabinoid-induced effects on cell growth include HeLa cervical carcinoma cells (Δ9-THC, Δ8-THC, 11-hydroxy-Δ9-THC and CBN), Lewis lung carcinoma cells (Δ9-THC, Δ8-THC and CBN; but CBD appeared to stimulate growth) and B103 neuroblastoma cells (Δ9-THC).

Cannabinoids furthermore are highly lipophilic molecules. This property can lead e.g. to cannabinoids partitioning into the lipid phase of biological membranes. Interaction has been shown with subcellular structures such as mitochondria, lysosomes and the mitotic apparatus. A reorganization of microtubules, microfilaments and neurofilaments was reported in B103 neuroblastoma cells, following treatment with Δ9-THC (1-100 μM). On B103 cells, the change in cytoskeleton corresponded with changes to the overall morphology of the cells.

Numerous investigations are also available on the application of Cannabis or cannabinoids as drugs. The licensed medicinal use of Δ9-THC (dronabinol) in e.g. the United States and the United Kingdom is for prevention of nausea and vomiting in patients undergoing cancer chemotherapy. The effects of Cannabis on the gastro-intestinal propulsion and motility have been studied in detail in rodents. In mice and rats, intravenous injection of Δ9-THC slowed the rate of gastric emptying and small intestine transit. In the rat, the substance inhibited gastric emptying and small intestinal transit more than large bowel transit, indicating a selectivity for the more proximal sections of the gut. A decrease in frequency of both gastric and intestinal contractions without altering the intraluminal pressure was also found. Such changes probably reflect a decrease in propulsive activity, without changes in basal tone. CBD had no effect on gastric emptying or intestinal transit. In patients, cannabinoids as anti-emetics are as effective as the well known phenothiazines. Side-effects are relatively common. In some studies a third of patients experienced dysphoria, and up to 80% had somnolence. It was also found that Δ9-THC is more rapidly and reliably absorbed from the lungs than from the gut, and patients taking the drug by smoking can thus titrate their own dose.

Cannabis has 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. There have been 3 trials with oral Δ9-THC in patients with multiple sclerosis. In a placebo controlled study of 9 patients, doses of 5 or 10 mg Δ9-THC improved spasticity compared with the placebo. Four patients exhibited an objectively measured benefit, described as substantial, 2 patients also claimed subjective improvement of symptoms. One further patient claimed subjective improvement, but this was not confirmed objectively. In a second study 2 out of 8 patients receiving doses of 5-15 mg Δ9-THC experienced both subjective and objective improvement in tremor. A further 5 claimed mild subjective improvement in tremor and general well-being, but this was not confirmed objectively. The third study included 13 patients with multiple sclerosis spasticity that proved untreatable with standard muscle relaxants. Using doses of 2.5-15 mg Δ9-THC in a double blind, placebo controlled trial, patients considered that spasticity had improved; however, neurologists blinded to the treatments could not differentiate between Δ9-THC and placebo. Furthermore, doses over 7.5 mg were relatively poorly tolerated, with symptoms of weakness or psychoactive effects.

In one experiment CBD was given orally to 5 patients with dystonic movement disorders. A dose-related improvement in dystonia, ranging from 20-50% was observed in all patients. In 2 patients, however, the higher doses worsened co-existing parkinsonism. CBD also appeared promising as an anticonvulsant in epilepsy. In a controlled study, adding this cannabinoid to the prescribed anticonvulsants produced improvement in 7 patients with grand mal; 3 of them showing substantial improvement.

Cannabis has been reported to cause bronchodilation, so Cannabis derivatives have therefore 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. One interesting finding for future research is that cannabinoids may affect the bronchi by a mechanism differing from that of the more familiar anti-asthmatic drugs.

Cannabinoids might be used to treat wide-angle glaucoma, which is a major cause of blindness. In a number of patients, Cannabis caused a dose-related, clinically significant decrease in intra-ocular pressure, lasting several hours. Though it does not cure the disease, Cannabis can slow down the progressive loss of sight when conventional medicines fail and surgery is too risky. CBN and CBG have been administered to cats, topically or chronically. Whereas CBN had a modest effect on intra-ocular pressure after a single dose, it caused a more significant reduction in ocular tension during chronic administration. CBG had similar effects, but its chronic administration induced a larger response. CBN caused ocular toxicity (conjunctival erythema and hyperaemia). CBG, however, lacked these toxicities; its ocular hypotensive effect, therefore, is most interesting.

Finally, Cannabis and/or Δ9-THC may help to increase food intake and slow weight loss in e.g. cancer and AIDS patients. Preliminary clinical trials on this are showing promising results.

Aqueous extracts of hemp seeds have furthermore showed strong nematicidal activity on the larvae, eggs and cysts of the nematode Heterodera schachtii . An aqueous extract from the flowers had less activity, and extracts from leaves, stems and roots showed no activity. The nematicidal activity is probably due to the compounds 7-methyl-1,2,3,4-tetrahydroxy-phenazine and 3-acetyl-4-hydroxy-6-methyl-2-pyridone.

The bast of the plant cultivated for fibre contains primary and secondary bast fibres rich in cellulose but low in hemicellulose and lignin. Today, lines are available containing 67% of cellulose, 13% of hemicellulose and 4% of 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 libriform short fibres with an average length of 0.55 mm. The chemical composition of the core fibres resembles that of hardboard with typical values of 40% cellulose, 20% hemicellulose and 20% lignin.

The seeds of hemp contain 29-35% oil, 20-24% protein and 20-30% carbohydrates. The dry seed contains up to 15% fibre, 4-6% minerals and about 6% water. No narcotically active compounds accumulate in the seed, but contamination may occur due to contact with the glands of flower bracts and leaves containing cannabinoids. Washing is sufficient to remove contamination. The most abundant fatty acid in the seed is omega 6 linoleic acid; it comprises 54-70% of total fatty acids present.

Adulterations and substitutes

Because the most important compounds, the cannabinoids, have not been found in plant genera other than Cannabis , there are no natural adulterations and substitutes. Synthetic analogues of Δ9-tetrahydrocannabinol have been developed, notably nabilone and levonantradol. They have undergone successful clinical trials, but still have considerable side-effects.

Description

  • An annual, tall (generally 1-1.5 m tall, sometimes much taller) erect herb, usually branched, dioecious or sometimes monoecious, rather densely appressed-pubescent when young.
  • Leaves opposite near base of stem, arranged spirally higher up, palmately compound, long-petioled; stipules free, filiform or narrowly subulate, about 0.5 cm long; leaflets (3-)5-7(-11), upper leaves often with only 1 leaflet, lanceolate from a narrowed base, 6-14 cm × 0.3-1.5 cm, sessile, long-acuminate, coarsely serrate, on the upper surface very scabrid with short stiff hairs, on the lower surface appressed-pubescent, rather densely beset with sessile glands.
  • Flowers unisexual; male flowers in short, dense cymes, united into foliate, terminal panicles, very shortly pedicelled, with 5 free tepals, oblong, about 5 mm long, membranous, imbricate, finely appressed-pubescent, greenish-white with pellucid white margins, stamens 5, epitepalous, with erect linear filaments and comparatively large 3-4 mm long, basifixed, 2-celled, yellow anthers; female flowers solitary in the axil of a small, primary, membranous, entire bract enveloping the ovary, each enveloped by a spathaceous, conspicuous, acuminate secondary bract, perianth absent, ovary sessile, 1-celled with a solitary pendulous ovule, style central, stigmas 2, up to 7.5 mm long, filiform, caducous.
  • Fruit a broadly oval, much compressed achene, 4-5 mm long, with a concave rimmed base, faintly keeled on the lateral margins, smooth, shiny, yellowish or brown, closely enveloped by the secondary bract; pericarp hard, crustaceous, easily splitting into 2 halves.
  • Seed with unilateral, scanty and fleshy albumen; embryo large, horseshoe-shaped, cotyledons large, radicle long.
  • Seedling with epigeal germination.

Growth and development

Hemp 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 are generally 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 to 5 weeks longer than male plants, until the seed is ripe. The flowers are wind-pollinated. The total growth duration of hemp strongly depends on photoperiod and temperature. The sensitivity to photoperiod starts after formation of a few pairs of leaves and after a certain amount of heat has been received. In practice this means that the optimal production of fibre hemp is limited to regions with relatively long days, in which hemp is able to extend its vegetative growth phase sufficiently to produce long stems. The length of the flowering phase of hemp also depends on photoperiod and temperature. Between plants and in individual plants, flowering is more synchronized at shorter daylength and higher temperatures. The relatively expensive metabolites like cannabinoids and oil formed during and after flowering, may lead to a reduction of the amount of dry mass formed per unit of intercepted radiation. The degree of branching depends on propagation methods and conditions of cultivation.

Other botanical information

Great variation exists in hemp as a result of selection for fibre, oilseed and/or resin. This variation is further enhanced by the ease of crossing between these plant types. The widespread intergrading between different types makes all classifications inexact. A geographical classification is in use for cultivated hemp, in which North European, Central Russian, Mediterranean and Asiatic types are distinguished. The North European hemp is characterized by a short stem (< 1.5 m) and a premature flowering. Fibre and seed yields are generally low. Central-Russian hemp is cultivated in Europe and Asia between 50-60°latitude. Total growth duration is 90-110 days, with stems reaching 1.3-3 m height. Fibre yields of these types are average, but high seed yields may be obtained. Mediterranean hemp is normally cultivated south of 50°latitude in Europe, although very high fibre production may be obtained by growing these types further north. The total growth duration is 130-150 days, with stems reaching 2.5-4.5 m. Hence, fibre production can be high and of good quality. Seed yields are average because of their relatively long vegetative growth phase. Asiatic type hemp plants form branched stems of 2.5-3 m with short internodes. Growth duration is 150-170 days, but may vary considerably between populations. For practical purposes, three types can be distinguished, based on the concentrations of Δ9-tetrahydrocannabinol and cannabidiol: the drug (resin) type, with high Δ9-tetrahydrocannabinol concentration (> 1%) and no cannabidiol; the hemp (fibre) type with very low Δ9-tetrahydrocannabinol content (< 0.3%) and high cannabidiol concentration; and the intermediate type, with high concentrations of both compounds. However, concentrations may change during the growing season, and other components may also play a role.

Ecology

Hemp can be grown over a great range of altitudes, climates and soils. It requires humid tropical climates to produce narcotic resin. For the production of fibre, climates of the temperate regions with temperatures of 15-27°C during the growing season are optimal. Hemp thrives on moderately to very fertile soils provided there is enough water. It is suitable for alluvial soils along streams or loamy soils with rather high rainfall. The only reliable ecological data available is for European hemp cultivated for fibre. For these cultivars, the temperature requirement to reach the onset of photoperiod-sensitive phase has been quantified at 482 C°d from sowing on, and critical daylengths of 14-14.5 h have been established. Daylengths longer than the critical one extend the length of the photoperiod-sensitive phase, prolonging the vegetative growth phase. For the lower latitudes in the northern hemisphere, this sensitivity to short days limits the potential yield of hemp, while at higher latitudes low temperatures in spring are the main constraint to yield. Hemp has relatively horizontally oriented 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. When oil and resins are also being formed during the reproductive phase, radiation use efficiency may drop significantly. The optimal soils for hemp are sandy loams high in organic matter, with a pH around 6.

Propagation and planting

Hemp is usually raised from seed. The seed germinates at low temperatures, but not below 1°C. Soil temperatures of 10-12°C are required for optimal crop establishment. Emergence is seriously hampered by unfavourable conditions such as soil compaction and waterlogging. Healthy seed should give 90% germination, and if properly stored it will remain viable for up to 2 years. Vegetative propagation using cuttings has been successful. However, there are morphological and biochemical differences between plants derived from seed and vegetative propagules. Vegetative propagules have higher concentrations of Δ9-tetrahydrocannabinol than plants raised from seed, and better developed lateral branches.

The agronomic methods depend on the product desired. For "ganja" production in India, seed is sown in rows 1.2 m apart at a seed 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.

Optimal plant densities of 90-120 plants/m2with row distances of 12-20 cm seem optimal, but row distances of more than 1 m and inter-plant distances between 15-50 cm are used for seed production. In China seed plants are sometimes sown in clusters.

Husbandry

Weeding is rarely necessary because the dense canopy shades out 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 an annual fertilization of 15-20 kg N, 4-5 kg P2O5and 15-20 kg K2O is required. The need for nitrogen is highest during the vegetative growth phase in which green leaf material is produced. Requirements for phosphorus and potassium increase gradually during vegetative growth, peaking around flowering before slowly declining again. In an increasing order and within certain limits, fibre quality is improved by the nitrogen, phosphorus and potassium contents of the hemp plant. Hemp is a suitable crop for rotation with almost any crop, though some problems may occur in rotations with beets, because of nematode infestations. Hemp 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. The male plants produce the best fibre and are sometimes harvested first; the 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-tetrahydrocannabinol, but to prevent seed production in female plants, which would reduce resin production.

Diseases and pests

Diseases and pests in hemp may be plant specific or general. Reported damage by hemp-specific organisms are from Grapholitha delineana , Melaspora cannabina , Phorodon cannabis , Psylliodes attenuata and Septoria cannabis . Seeds of hemp may be infected by the parasitic Orobanche racemosa L. or hemp killer. The most important non-specific diseases and pests in hemp are: Botrytis cinerea , Ostrinia nubilalis and Sclerotinia sclerotiorum . Yield may also be depressed by Cuscuta europaea L., Fusarium spp., Ditylenchus dipsaci , Tetranychus urticae , some insects of the Noctuidae and larvae of Agriotes lineata , Melolontha melolontha and Tipula paludosa . Hemp may also suffer from nematodes such as Meloidogyne hapla in northern Europe and, in northern India Neottolenchus clarus and Quinsulcius similis . The role played by essential terpenoid substances in repelling insects and exudates of cannabinoids (as antibiotics) has been neglected and deserves investigation. Another of hemp's natural defence mechanisms - its covering of non-glandular trichomes - might serve as a mechanical defence against predators.

Harvesting

How hemp is harvested depends on the product. For "ganja" production, male plants are pulled out as soon as they are recognized and before the pollen is shed. Unfertilized female plants are left, and harvested when flower stalks begin to turn yellow, at about 5 months after sowing. For the production of "charas", the resin is collected by men who run through the plantings clad 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 manually or by machinery. In China, manual harvest is by cutting the stems; the branched plants are left on the edges of the field and their seed is harvested for sowing.

Yield

The average yield of "ganja" in India is about 280 kg/ha. Stem yields (yielding approximately 25-35% fibre) are usually between 3-8 t/ha, with a potential of 20 t/ha. When hemp is grown solely for seed production, yields of 1300-1700 kg seed per 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. The short and twisted stems are for local use. Leaves are stripped off with a knife. The stems are then dried on the field for 2-4 days. About 200 stems are then bundled and immersed in water for a 3-day retting. After retting, another 2-3 days are used for field drying. Depending on the degree of retting, a second retting is carried out or stems are allowed to dry further in the field, this time in bundles. Fibre bundles are subsequently stripped off by hand from the partly wet stems and dried on lines before marketing. Another method to obtain the fibres is to dry the stems completely before breaking and to comb them. The yield of ribbon with this dry method is slightly higher than with the semi-wet method; 10% versus 6% relative to 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 ribbon is on average 15%. The centres for commercial production of sowing seed for fibre hemp are in France, Hungary and Poland. The fineness and coarseness of the fibre and cleanness or degree of retting are decisive for traditional processing of ropes and textiles. Sometimes, male and female plants are therefore separated and processed differently. The stems are also graded during harvest. 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 easily 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 material the following properties are of interest: high tensile strength, rigidity, impact resistance, small volume shrinkage during curing, resistance to corrosion, low density, non-toxic, recyclability, ease of disposal and economic price. These quality characteristics are partly inherent to the natural fibre, but those like tensile strength and rigidity are determined by cultivar choice, growing conditions, environment, mechanization and processing.

Genetic resources and breeding

There has been limited preservation of germplasm of C. sativa in gene banks because of its bad image as a narcotic. Together with the declining interest in breeding and maintaining of cultivars, this has led to an impoverishment of germplasm resources. However, a large reservoir of natural variation is maintained by wild forms.

The difference in timing of anthesis of male and female plants promotes outbreeding. Production of large amounts of pollen and wind pollination tend to lead to extensive genetic exchange between different domesticated forms and between domesticated and wild plants. Breeding has mostly focused on the creation of monoecious varieties. In Europe, breeding and selection work is directed at obtaining hemp types with a bast fibre content higher than 30% and Δ9-tetrahydrocannabinol levels below 0.3%.

Prospects

Although the resin present in hemp has been recognized to have therapeutic value, the use of hemp as a medicinal plant is limited. This is mainly because the cultivation, possession, preservation, transportation and trade of hemp is prohibited in most countries. However, research is increasingly being initiated on hemp drugs for relieving patients suffering from diseases such as multiple sclerosis, cancer, AIDS and glaucoma. Since many young people in big cities smoke "ganja" or "marihuana", the drug is probably being smuggled into South-East Asia, and illegal cultivation of hemp in remote areas may still be found.

Hemp grown for fibre has a long history and still has very good prospects. The plant fibre products have major advantages in various branches of industry. They are biocompatable and biodegradable, thereby reducing the environmental burden caused by consumer goods and disposables, building and construction material and civil engineering. The research priorities include: determining optimal primary production techniques, developing field fibre extraction techniques, quantifying the relationships between primary production conditions, processing and fibre quality, and optimizing the management of production chains to best use all components of hemp.

Literature

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Authors

N. Wulijarni-Soetjipto, Anas Subarnas, S.F.A.J. Horsten & N.C. Stutterheim