Cannabinoids and environmental stress
Desiccation
THC is a viscous hydrophobic oil (Garrett and Hunt 1974) that resists crystallization (Gaoni and Mechoulam 1971) and is of low volatility (Adams et al. 1941). Since the sticky resins produced and exuded on the surface of the plant are varying combinations of THC, other cannabinoids and a variety of terpenes, they can be seen as analogous to the waxy coatings of the cacti and other succulents that serve as a barrier to water loss in dry environments.
Bouquet (1950) has mentioned that the western side of Lebanon's mountainous Cannabis growing areas is less favorable for resin production because of humid sea winds. De Faubert Maunder (1976) also observed that the copious separable resin needed for hashish production occurs only "in a belt passing from Morocco eastwards, taking in the Mediterranean area, Arabia, the Indian sub-continent and ending in Indo-China." These are mostly areas notable for their sparse rainfall, low humidity and sunny climate. Is it merely coincidence that resin is produced according to this pattern, as well?
Experimental evidence is accumulating that reinforces these notions. Sharma (1975) reported a greater glandular trichome density on leaves of Cannabis growing in xeric circumstances. Paris et al. (1975a) have demonstrated a marked increase in the cannabinoid content of Cannabis pollen with decreased humidity. Murari et al. (1983) grew a range of Cannabis fiber cultivars in three climatic zones of Italy and found higher THC levels in those plants grown in the drier "continental" (versus "maritime") climate. Hakim et al. (1986) report that CBD-rich English Cannabis devoid of THC produced significant amounts of THC and less CBD, when grown in the Sudan. This trend was accentuated in their next generation of plants.
Haney and Kutscheid (1973) have shown significant correlations of plant cannabinoid content with factors affecting soil moisture availability: content of clay or sand, percent slope of plot, and competition from surrounding vegetation. In some cases, this last factor was noted to have induced a stunted plant with "disproportionally smaller roots", which would tend to increase both the frequency and severity of desiccation stress.
In a study of 10 Kansas locations, Latta and Eaton (1975) found wide differences in plant cannabinoid content, observing that "delta-9-THC ranged from 0.012 to 0.49% and generally increased as locations became less favorable for plant growth, suggesting increased plant stress enhanced delta-9-THC production." Mention was also made of a positive correlation between competing vegetation and THC content. Although the sampling area was not considered very moisture deficient, they speculated that "Greater difference among locations might have been observed under drought conditions."
Temperature
Temperature may play a role in determining cannabinoid content, but perhaps only through its association with moisture availability. Boucher et al. (1974) reported an increase in cannabinoid content with temperature (32o C. vs. 22o C.), however, some variables such as increased water loss due to accelerated evaporation and plant transpiration at high temperatures were left unaccounted. In contrast, Bazzaz et al. (1975), using 4 Cannabis ecotypes of both tropical and temperate character, demonstrated a definite decrease in cannabinoid production with increased temperature (32o C. vs. 23o C.). Later studies by Braut-Boucher (1980) on clones of 2 strains from South Africa revealed a more complex pattern of biosynthesis according to strain, gender and chemical homologue produced. Clearly, further study of this parameter is needed.
Soil Nutrients
Mineral balance seems to influence cannabinoid production. Krejci (1970) found increases related to unspecified "poor soil conditions". Haney and Kutcheid (1973) have shown the influence of soil K, P, Ca and N concentrations on Illinois Cannabis. They report a distinctly negative correlation between soil K and plant delta-9-THC content, although K-P interaction, N and Ca were positively correlated with it. These minerals were also shown to affect the production of CBD, delta-8-THC and cannabinol (CBN), although the latter two compounds are now thought to be spontaneous degradation products of delta-9-THC. Kaneshima et al. (1973) have demonstrated the importance of optimal Fe levels for plant synthesis of THC. Latta and Eaton (1975) reported Mg and Fe to be important for THC production, suggesting that these minerals may serve as enzyme co-factors. Coffman and Gentner (1975) also corroborated the importance of soil type and mineral content, and observed a significant negative correlation between plant height at harvest and THC levels. Interestingly, Marshman et al. (1976) report greater amounts of THC in Jamaican plants growing in "organically" enriched (vs. artificially fertilized) soils.
Insect predation
Wounding of the plant has been employed as a method to increase resin production (Emboden 1972). This increase may be a response to desiccation above the point of vascular disruption. Under natural circumstances, wounding most often occurs as a result of insect attack. This is a source of environmental stress which the production of terpenes and cannabinoids may be able to minimize. Cannabis is subject to few predators (Smith and Haney 1973, Stannard et al. 1970) and has even been utilized in powdered or extract form as an insecticide (Bouquet 1950) or repellent (Khare et al. 1974). Its apparent defensive mechanisms include a generous covering of non-glandular trichomes, emission of volatile terpenoid substances, and exudation of the sticky cannabinoids. Cannabis is often noted for its aromatic quality and many of the terpenes produced are known to possess insect-repellent properties. Among these are alpha and beta pinene, limonene, terpineol and borneol. Pinenes and limonene comprise over 75% of the volatiles detected in the surrounding atmosphere, but account for only 7% of the essential oil (Hood et al. 1973). Consistent with glandular trichome density and cannabinoid content, more of these terpenes are produced by the inflorescences than the leaves, and their occurrence is also greater in the female plant (Martin et al. 1961).
No insect toxicity studies using isolated cannabinoids have been published to date. Rothschild et al. (1977) found THC-rich Mexican (vs. CBD-rich Turkish) Cannabis fatal to tiger moth (Arctia caja) larvae, but not Nigerian grasshopper (Zonocerus elegans) nymphs. Rothschild and Fairbairn (1980) later found that pure THC (vs. CBD) sprayed on cabbage leaves, does repel the large white cabbage butterfly (Pieris brassicae).
The cannabinoids may also serve as a purely mechanical defense. A tiny creature crossing the leaf surface could rupture the tenuously attached globular resin reservoirs of the glandular trichomes (Ledbetter and Krikorian 1975) and become ensnared in resin. A sizable chewing insect, if able to overcome these defenses, would still have difficulty chewing the gummy resin, along with the cystolithic trichomes and silicified covering trichomes also present on the leaf. The utility of these epidermal features as insect antifeedants is also inferable from their predominant occurrence on the insect-favored abaxial leaf surface. Although the above strategies represent a seemingly sophisticated system, many other plants (Levin 1973) and even arthropods (Eisner 1970) utilize similar defense mechanisms, often employing identical terpenes!
Competition
Terpenes may also help to suppress the growth of surrounding vegetation (Muller and Hauge 1967, Muller et al. 1964). Haney and Bazzaz (1970) speculated that such a mechanism may be operative in Cannabis. They further ventured that since the production of terpenes is not fully developed in very young plants, this may explain their inability to compete successfully with other vegetation until more mature. The observation (Latta and Eaton 1975) of increased THC production by plants in competition with surrounding vegetation "at a time in the growing season when moisture was not limiting", may indicate a stimulus for cannabinoid production beyond that of simple water stress.
Bacteria and fungi
The cannabinoids may serve as a protectant against microorganisms. Cannabis preparations have long served as medicines (apart from their psychoactive properties) and are effective against a wide variety of infectious diseases (Kabelic et al. 1960, Mikuriya 1969). These antibiotic properties have been demonstrated with both Cannabis extracts (Ferenczy et al. 1958, Kabelic et al. 1960, Radosevic et al. 1962) and a variety of isolated cannabinoids (ElSohly et al. 1982, Farkas and Andrassy 1976, Gal and Vajda 1970, Van Klingeren and Ten Ham 1976). CBG has been compared (Mechoulam and Gaoni 1965) in both "structure and antibacterial properties to grifolin, an antibiotic from the basidiomycete Grifolia conflens." Ferency (1956) has demonstrated the antibiotic properties of Cannabis seed, a factor that may aid its survival when overwintering. Adherent resin on the seed surface, as well as a surrounding mulch of spent Cannabis leaves, may serve in this regard.
Some of the many fungal pathogens that affect Cannabis include Alternaria alterata (Haney and Kutsheid 1975), Ascochyta prasadii (Shukla and Pathak 1967), Botryosphaeria marconii (Charles and Jenkins 1914), Cercospora cannabina and C. cannabis (Lentz et al. 1974), Fusarium oxysporum (McCain and Noviello 1985), Phoma sp. (Srivastava and Naithani 1979) and Phomopsis ganjae (McPartland 1984).
While A. alterata attacks Illinois Cannabis and destroys 2.8-45.5% of the seed (Haney and Kutsheid 1975), the balance of these species are leaf spot diseases. McPartland (1984) has demonstrated the inhibitory effects of THC and CBD on Phomopsis ganjae. However, De Meijer et al. (1992), in evaluating a large collection of Cannabis genotypes, did not find a correlation between cannabinoid content and the occurence of Botrytis. Fungal evolution of a mechanism for overcoming the plant's cannabinoid defenses may be responsible for their success as pathogens. Indeed, some have been demonstrated to metabolize THC and other cannabinoids (Binder 1976, Binder and Popp 1980, Robertson et al. 1975).
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