Light

 

The proper quality and quantity of light is very important for the vigorous growth of Cannabis. Light must be made up of wavelengths necessary both for photosynthesis and for the induction or inhibition of flowering. Duration of light must also be correct to allow a vegetative and floral phase in the life cycle. Cannabis normally grows in a temperate climate from the time of the last frost in the spring until the mid-autumn. As a result, the first phase of its life cycle is vegetative during days of increasing length, while the second phase is floral during days of decreasing length. If grown out of season, in autumn or winter, Cannabis will produce intersexual flowers and hermaphrodite individuals.

 

Temperature

 

Much of the effect of temperature on Cannabis is connected with the transpiration rate of the plant. Cannabis has a high transpiration rate and in hot climates is very susceptible to wilting. The growth of glandular trichomes and secretion of resin during hot weather guards against desiccation of the tissues, lowering the transpiration rate through the epidermal surfaces of the plant by lowering leaf temperature. Cannabis is well adapted to heat, but is not particularly tolerant of low temperatures. It will endure light frosts near 0° C (32°F), but a hard frost or a light frost of any duration will nearly always result in death. Low temperatures inhibit photosynthesis and slow the metabolic rate of plants; extended cool weather usually stunts the growth of Cannabis. The cool ground temperatures of autumn and winter in a temperate climate are largely responsible for inhibiting the germination of summer seed until the warm days of spring.

 

Temperature differentials between air and soil apparently have an observable effect on the phenotype of Cannabis. Nelson (1944) performed experiments under four temperature conditions with results as follows:

 

H/H - Shoot and Root: 30°C

Maximum elongation, earliest maturation Maximum nodes, many staminate flowers Minimum leaf area, maximum leaf abscission

Maximum water consumption

 

H/L - Shoot: 30°C; Root: 15°C

Minimum weight, many staminate flowers

Maximum stem weight

 

L/H - Shoot: 15° C; Root: 30°C

Pistillate to staminate reversal

Maximum individual leaf size

Maximum stem diameter

Maximum weight

 

L/L - Shoot and Root: 15°C

Pistillate to staminate reversal

Maximum leaf area

Minimum water consumption

Maximum root water content

Latest blooming, many pistillate flowers

 

This study emphasizes the importance of edaphic (soil) as well as air temperatures to the structural development of Cannabis, and hints at how they interact in determining phenotype.

 

Moisture

 

Cannabis flourishes in a well-drained soil with an adequate supply of water; it attains large size in an irrigated habitat but is stunted by aridity. Standing water is quite detrimental to Cannabis since the roots suffocate easily. Therefore, porous organic soil with high sand content and moderate slope seems best suited for proper growth provided water is readily available.

 

Varying moisture conditions influence the structural development and morphology of Cannabis, depending on the water requirements of the plant at different phases in its life cycle. During germination, the seed must be in continuous contact with moist soil for at least four days and seeds will readily germinate in standing water. Drying of the soil during germination almost always kills the embryo. At the seedling stage, excess moisture results in the rapid elongation of hypocotyl and epicotyl. In high humidity this rapid elongation will continue, producing plants with long inter-nodes while plants in arid conditions have short internodes. This rapid elongation also produces very flaccid primary fibers, causing many young seedlings to fall over soon after the cotyledons open, If humid conditions continue, the secondary fibers will also be soft. In arid conditions both primary and secondary fibers are shorter and very brittle by comparison. In this case, the stem often buckles in the wind instead of bending. In arid conditions more glandular trichomes are produced on the surface of the calyxes, leaves, and stems than in humid conditions; leaves tend to be narrower, thicker, and more highly serrated than the broad, thin leaves of humid habitats.

 

At anthesis (flowering) a marked increase in water uptake occurs in both staminate and pistillate individuals. Water needs are high during flowering and lack of moisture will surely inhibit floral formation. Dehiscence (dispersal of pollen) is aided by arid weather which also triggers pistillate plants to form glandular trichomes on the calyxes and adjacent leaflets. This aids in the control of transpiration and lowers water requirements. The most resin is therefore produced in a warm, arid environment with adequate light cycles. Resin production slows, however, when pollination occurs and the calyx starts to dry slowly as the seed forms. Arid conditions promote the dispersal of seeds since they are more easily freed from the calyx by agitation when the calyx is dried.

 

Edaphic Conditions

 

Physical properties, acidity-alkalinity (pH) and nutrient level are the most important edaphic (soil) conditions affecting the growth of Cannabis. Important physical properties of the soil are drainage, tilth, and organic content. Soil must drain well for the proper growth of Cannabis, since the roots are easily attacked by fungi and do not tolerate standing water. Alluvial-sandy soils and loamy-sandy soil's are well suited as long as proper root growth can take place. Cannabis is a tall plant of open environments, and a widely dispersed, fibrous root system is necessary to support its mass during wind and rain. High organic content aids root growth, loosening and lightening the soil as well as retaining moisture. However, too high an organic content may raise the acid level of the soil beyond a tolerable limit for the growth of Cannabis.

 

The pH of the soil is crucial to proper Cannabis growth. A range of 6.5 to 7.5 (7.0 is neutral) is best. In this range Cannabis can properly absorb nutrients and carry on its life functions. Also, in a more acidic soil, nutrients are locked up in acid salts and cannot be utilized by the growing plant.

 

Symptoms caused by improper acidity may cause plants to be runted with curled foliage and few fruits or flowers. Because nutrients are bound in acid soil, the plant may show several nutrient deficiencies simultaneously. Highly acidic conditions will also limit the growth of beneficial soil organisms, while highly alkaline conditions may cause salts to accumulate in the soil, possibly limiting water uptake by the roots.

 

Both macro- and micro-nutrients are important to the growth of Cannabis. The requirement for each nutrient, its utilization by the plant, symptoms of its absence, and its effect on productivity are different for each of the nutrients. These must be discussed separately, along with variations in the requirements and responses of staminate and pistillate individuals.

 

Nitrogen is the first of the macro-nutrients and is largely responsible for stem and leaf growth, overall size and vigor. Nitrogen is vital in the production of chlorophyll, and therefore the entire photosynthetic metabolism of the plant may be upset by a deficiency of nitrogen. The result is slow growth and stunted foliage. Cannabis has a very high nitrogen requirement and tends to strip nitrogen from the soil. Nitrogen deficiency is characterized by chlorosis (loss of chlorophyll) of the older leaves followed by a gradual chlorosis of the entire plant with only the meristem remaining green to the end. An overabundance of nitrogen causes wilting of the plant and, shortly thereafter, a total change in all tissues from green to copper brown. The proper nitrogen level results in uniformly green plants with large leaves and long stems.

 

Production of fiber may be increased by the addition of nitrogen to the soil. Experiments by Black and Vessel (1944) showed that an increase in yield of 1.74 ton per acre resulted from the addition of nitrogen at the rate of fifty pounds per acre. This study also states that nitrogen is beneficial later in the plant's life rather than earlier. This raises an important point regarding utilization of nutrients, especially as it relates to sexual expression. Talley (1934) and Tibeau (1936) both investigated the utilization of nitrogen by Cannabis. Talley observed the carbohydrate-to-nitrogen ratios in staminate and pistillate plants and found that staminate plants have a higher carbohydrateto-nitrogen ratio than pistillate plants, although total carbohydrate content is very diverse. Pistillate plants, however, show a higher percentage composition of nitrogen than staminate plants and very consistent values for carbohydrate-to-nitrogen ratio at the time of flowering. He attributes this difference to the varying growth habits of the staminate and pistillate plants. The staminate plant enters senescence just after flowering, unable to return to a vigorous state following the initial dehiscence of pollen, while the pistillate plant goes on flowering for up to three months. The staminate plant has no need to maintain its nitrogen level, but the pistillate plant must continue to utilize nitrogen to form the foliage associated with its flowering organs. Another explanation comes from the work of Tibeau, who showed that an overabundance of nitrogen at the time of floral differentiation resulted in almost all pistillate plants while an absence of nitrogen resulted in nearly all staminate plants. It may be that the nitrogen level of the soil, through some metabolic pathway, influences floral differentiation. A soil over-rich in nitrogen throughout the plant's life produced dark-green leafy plants which did not survive to flower.

 

Black (1945), however, denies the effect of any macro-nutrient on sexual expression in Cannabis; his results showed little change in sex ratio with various nutrient treatments.

 

Phosphorus is required by Cannabis for its general vigor and is especially needed at the time of flowering since it is associated with the metabolism of sugar, an energy source for growth, and the production of resin and seed. This seems contrary to the findings of Black and Vessel (1944) who report that the application of phosphorus to increase production appears most effective early in the season, with decreasing effectiveness as the season progresses. The yields in their experiment, however, were measured by fiber production, eliminating any need for the plant to flower; this might explain the discrepancy.

 

Phosphorus deficiency affects the most mature leaves first, resulting in dark, dull-green leaves with a curled-under edge. The veins on the leaves' abaxial surface may show a purple tint along with the petioles and stem tips. This is due to an overabundance of anthocyanin. However, this condition is found in many individuals not deficient in phosphorus and may be linked to genetic as well as environmental factors.

 

Potassium has the most subtle role of the macro-nutrients in plant nutrition. Although it is needed in conjunction with the other macro-nutrients in all stages of development it is most needed at the time of flowering and is involved in the metabolism of many activators associated with flowering. Signs of potassium deficiency are stunted growth along with yellowing of the older leaves, followed by necrosis characterized by dark spots and curled edges of a copper grey color. The effect of potassium on production in Cannabis is linked to the presence of adequate amounts of both nitrogen and phosphorus. Tibeau (1936) showed that plants with an adequate supply of nitrogen and phosphorus grew very vigorously when given excessive amounts of potassium. She also noted that plants recovered from potassium starvation rapidly but never reached the size of plants with a continuous supply.

 

Micro-nutrients are important to Cannabis, as they are to all plants, and many specific relation- ships between micro-nutrients and the proper growth of Cannabis may be observed. Iron is used by the plant in the synthesis of enzymes that are essential links in photosynthetic and respiratory pathways. A deficiency is characterized by the chlorotic condition of leaves in the meristematic tips of limbs, rather than older leaves. This is because iron is not very soluble and is less easily translocated within plant tissues than are nitrogen compounds. Calcium deficiencies also appear in meristematic regions, causing weak, brittle stems and the death of apical meristems. This effect on meristematic tissue results from interference with the synthesis of calcium pectate needed as a bond in the middle lamellae of multiplying cells. Magnesium is an integral part of chlorophyll and its absence causes, in older leaves, greyish white spots or yellowing of tissues adjacent to veins followed by chlorosis of the entire leaf; young leaves are dark green in color. Sulphur is used by plants to build proteins; a deficiency appears as a general chlorosis of the plant, starting with the younger leaves.

 

Shortages of boron produce a swelling of the basal section of the stem followed by splitting and rotting. A general chlorosis of the leaves followed by a turn to bronze or bronze orange, accompanied by a swelling in the tips of the lateral roots, usually indicates a chlorine deficiency. Zinc deficiencies result in very small curled leaves with yellowed tissue near the veins. Stems are elongated with only the top cluster of leaves possessing viable axial buds. Manganese and molybdenum shortages result in chlorosis of tissue between the major 'veins in leaves near the stalk of the plant, spreading to the stem tips, where leaves often become twisted. Copper is also essential for healthy, vigorous growth in Cannabis; deficiencies may result in brittle, easily-broken stems.

 

Wind

 

Cannabis is a wind-pollinated plant and relies on air currents to ensure completion of the life cycle by dispersing pollen and knocking mature seeds to the ground. Pollen may travel in the wind up to 200 miles (Sack, 1949). The most common way for genetic information to be carried from one Cannabis population to the next is by wind-blown pollen. A fibrous root system and tall, flexible stem allow Cannabis to withstand relatively high winds. Wind tends to increase the transpirational flow by increasing evaporation from the epidermal tissues. Trichomes may aid the plant by cutting the circulation of air adjacent to the epidermis of stem and leaf tissue. Constant exposure to breezes strengthens the fibers in the stem, while plants grown in stagnant air tend to be weak and droop under their own weight.

 

Biotic Controls

 

Various herbivorous (plant eating) animals prey on Cannabis. Small rodents and birds eat the seeds and sprouts, while rabbits and such grazing animals as deer eat larger seedlings. Sucking and chewing insects, such as bugs, leafhoppers, grasshoppers and aphids, feed on Cannabis. After the pistillate plants start to secrete resin, insects seem to prey only on larger leaves and not on the flowering tops. Perhaps the resin is unpalatable or makes the sucking of juices difficult, suggesting a value of resin secretion. Spider mites and white flies are common occupants of Cannabis plants and are quite harmful. High humidity sometimes fosters fungus infections of the root, stem, and leaf, although resins seem to contain antibiotic compounds that inhibit fungus growth, especially in the flowering clusters. Plants are readily attacked by insects when they approach senescence and resin secretion ceases.

 

Dispersal

 

Natural agents in the dispersal of Cannabis seed include water, wind, and animals. Water affects the dispersal of seed in many ways. As moisture within the plant, it determines how fast the calyx will dry to release the seed. Rain helps to physically remove the seed from the calyx, and rivers wash seeds to new areas where they may come to rest on rich alluvial sands suitable for their germination and growth. Wind acts by knocking seeds to the ground and carrying them for short distances as well as blowing pollen for long distances. Animals most commonly aid in the dispersal of Cannabis seed by ingesting the seed in one location and excreting the still-viable seed in another area. Endozoic (internal) seed transfer by birds is related to the range of the bird, the retention time in the body and the resistance of the seed to digestion. Some seeds benefit in passing through an animal by a decrease in the germination time and by the nutritional content of the excrement containing the germinating seed. Most seeds do not make it through the digestive tract in viable condition, suffering from cracking and digestion. Darwin (1881) reports that Cannabis seeds have germinated in from 12 to 21 hours after passing through the stomachs of various birds, and Ridley (1930) has observed Cannabis seeds in the stomach contents of the European magpie. Seeds may also adhere to an animal in some way, perhaps between its toes or in an ear, but this is less likely. Man also aids in dispersal by carrying seeds on his travels, and by providing nitrogenous wastes suitable for the growth of Cannabis. All these factors may be agents leading to the migration of Cannabis from one area to another.

 

Race

 

As discussed previously, Cannabis is considered a monotypic genus by some investigators who believe the anatomical variation in Cannabis is not a basis for distinguishing various species of genus Cannabis, but rather, varieties of one species: Cannabis saliva L. The polymorphous types of Cannabis evolve from a balance between: a) the phenotypic response of a population of Cannabis to biotic and abiotic factors in the habitat that surrounds it and, b) the genotypic response based on adaptation to its environment of origin. In India, the only differentiation between varieties of Cannabis is between "wild" and "cultivated" types. Both produce fiber and resin, but since the plant is grown there mainly for drug use, the "cultivated" types have a fairly high THC content. Chopra and Chopra (1957) write that "even the plant growing under different climatic conditions in the vast Indo-Pakistan subcontinent shows remarkable variations in appearance; those variations at first may give the impression of separate species." Many researchers have noted the plasticity of Cannabis with examples of both Indian and European plants. Indian plants, when planted in England and France, were indistinguishable from the native European variety after several generations. Conversely, European fiber varieties, planted in Egypt to supply cordage, soon appeared quite similar to the local variety, and the drug content of the resin increased.

 

Many natural abiotic factors such as sunlight, temperature, moisture, and edaphic conditions affect both the phenotypic seasonal morphology of Cannabis and its evolution. The evolutionary response influences the genotypic nature of the population and thus the morphology of generations to come. For example, compare varieties from Colombia (0° to 10° north latitude) with varieties from Mexico (15° to 21° north latitude). Since growing seasons are shorter farther from the equator, a Mexican variety, in order to flower and reproduce before the cooler days of autumn or winter, needs to complete its life cycle in five to six months, while a Colombian variety can take up to seven or eight months. This is primarily an adaptation to differing light cycles.

 

Community

 

As a member of plant communities, Cannabis exerts pressures on itself and on surrounding plant species. The major effect comes from its very high nutrient requirements; little of these nutrients are recycled to other species and it is said to strip the soil. It does provide shade and shelter for smaller plants. Many plants produce herbicides in their leaves, but it is unknown whether terpenes produced by Cannabis are poisonous to other plants and whether this is used to competitive advantage. Insects often prefer other vegetation to Cannabis, possibly because of the unpalatability of the resins.

Cannabis exhibits great plasticity, flourishing nearly everywhere in the world, and this plasticity could likely keep it one step ahead of other plant species in the evolutionary scheme.