THC ? UVB !
Cannabis Sativa, constitute an important source of pharmaceuticals and other compounds of economic importance. These compounds are secondary metabolites (cannabinoids) and are species specific. Although the biosynthetic pathway of the major cannabinoids has been established (Mechoulam, 1970; Shoyama et al., 1984), the regulation of cannabinoid production remains unsolved. Some researchers claimed that cannabinoid production is genetically controlled and uninfluenced by environmental factors (Doorenbos et al., 1971; Fetterman et al., 1971) while other studies indicate that although the primary control appears to be genetic, cannabinoid production can be modified by environmental conditions (Haney and Kutscheid, 1973; Latta and Eaton, 1975; Turner et al., 1982).
It has been reported long ago that relationship exists between cannabinoid content and the
altitude at which C. sativa is grown. Mobark et al (1978) suggested that the high-altitude environment was responsible for an increased production of propyl cannabinoids in plants grown at 1300 m. Another report showed the average total cannabinoid content of wild, mature (flowering) Indian C. sativa from different elevations between 250 – 2000 m was decreased when grown at sea level in Mississippi, USA (Turner et al., 1979). A possible explanation is that high elevation means lesser atmosphere between the cannabis plants and the sun, leading to a higher exposure to UV rays.
One factor which appears in both high-altitude and tropical environments is relatively higher ultraviolet radiation. Wavelengths of light that are shorter than the PAR spectrum (<400 nm) radiation, have limited effect on photosynthesis; however, discrete photomorphogenic effects are observed when UV-B (290–315 nm) sensing systems are triggered (Frohnmeyer and Staiger, 2003; Folta and Carvalho, 2015). Measurements made in Utah showed that the biologically effective ultraviolet-B radiation flux UV-BBE (280-315 nm), showed an overall plant response functions greater by 32% at height of 3,350 m compared to 1500 m (Caldwell et al., 1980). Latitudinal variations of solar UV-B radiation are also considerable.
Figure 1. light wavelength spectrum
Although UV-B represents a threat to plant integrity in large quantities (Teramura, 1983), smaller quantities of UV-B have important benefits such as promoting pest resistance, increasing flavonoid accumulation, improving photosynthetic efficiency, and serving as an indicator of direct sunlight and sunflecks (Ballaré et al., 2012; Wargent and Jordan, 2013; Zoratti et al., 2014; Moriconi et al., 2018). Tolerance to UV-B radiation in some plants has been attributed to their ability to produce secondary metabolites, such as flavonoids, which absorb and prevent actinic UV-B radiation from penetrating plant tissues (Caldwell et al., 1983; Flint et al., 1985). Cannabinoids which are found in a fresh plant tissue appear in the form of acids (Doorenbos et al., 1971) and are found to strongly absorb UVB radiation, making these compounds likely candidates as solar screens.
Another support was given by Pate (1983) that found that C. sativa populations originating from high UV-B environments contained little or no cannabidiol (CBD) but high levels of Δ9 tetrahydrocannabinol (Δ9-THC), and the opposite was found for populations from low UV-B environments, and proposed that the two distinct C. sativa chemotypes (drug and fiber) evolved as a result of selective pressures brought about by UV-B radiation. Other research found that in both leaf and floral tissues the concentration of Δ9-THC but not of other cannabinoids increased linearly with UV-B exposure in drug-type Cannabis sativa plants, but not in fiber type plants of the same species. UV-B light reportedly elicits THC accumulation in both leaves and buds (Pate, 1983; Lydon et al., 1987; Potter and Duncombe, 2012).
Figure 2. UV-B lamp emphasizing light appearance
- Recent research shows that plants sense UV-B radiation in at least four different ways:
Photogeneration of reactive oxygen species (ROS) – sensing is performed by the plant different photochemical processes – ROS are metabolic products that regulate plant growth, development and survival under fluctuating conditions (abiotic and biotic stress-related events). (A.-H.-Mackerness and coworkers, 2000).
- Photodamage to DNA (nucleotide dimer formation) – the damage starts a signaling pathway (Beggs et al. 1994) The UV effect is counteracted by visible light which allowed photo repair of DNA.
- UVR8 – specific UV-B receptor with maximum absorption in the 280–290 nm range – requires a low fluence rate for signal transduction (Brown B. A, Jenkins G.I 2008). of at least 0.1 μmol m−2 s−1 for signal transduction
- Another specific UV-B receptor with maximum absorption in the 300–310 nm range – requires an approximately tenfold higher fluence rate than the previous receptor for signal transduction
UV-B responses can also be modulated by a UVR8-independent signal and UV-A radiation, since plants’ responses to UV-B light are regulated by both UVR8-dependent and -independent pathways (Morales et al., 2013; Li et al., 2015; Jenkins, 2017).
UVR8 – major photoreceptor
The UVR8 UV-B photoreceptor exists as a homodimer (combination of two mirror-image structural proteins) that instantly monomerizes (splits) upon UV-B absorption. The UVR8 monomer than reacts with other molecule (COP1) initiating a molecular signaling pathway that leads to gene expression changes. This signaling output leads to UVR8-dependent responses including UV-B-induced photomorphogenesis and the accumulation of UV-B-absorbing flavanols. Many suggestions on the pathway exist yet to be proved:
Secondary metabolites & Biosynthesis pathway
Secondary plant metabolites such as carotenoids, flavonoids, and anthocyanins accumulate in plant cells and leaves as light-screening compounds to limit damage caused by high light intensity and UV radiation (Takahashi and Badger, 2011; Darko et al., 2014). Secondary metabolites biological synthesis can be triggered by several environmental factors: as stress response to protect the plant from possible attack of herbivory or parasites in a wounded tissue; UV-B radiation as they may have a sun screening effect that protects cells from the radiation. This would be especially likely if the metabolites are concentrated in the epidermis or other superficial tissues. cannabinoids are synthesized in secretory cells inside glandular trichomes, which are highly concentrated in unfertilized female flowers before senescence (Potter, 2004, 2009). Lydon et al. (1987) reported increased THC concentrations in leaves and buds when cannabis plants were grown with supplemental UV-B radiation of 3 h daily, suggesting that cannabinoids may play some role in UV protection.
The biosynthetic pathway of cannabinoid synthesis is shown below. It is not known which enzyme or enzymes for Δ9-tetrahydrocannabinol biosynthesis are induced or stimulated by UV-B radiation, but one can speculate. The gene for polyketide synthase catalyzing the synthesis of olivetolic acid possesses strong sequence homology with chalcone synthase (CHS) and may have evolved from this. Chalcone synthase (CHS) is one of the classic UV-B-regulated enzymes.
Figure 3. Biosynthetic pathway of cannabinoids
Many other suggestions on the pathway exist yet to be proved:
- UV stress stimulates cannabis’ production of malonyl-CoA and phenylalanine via the phenylpropanoid pathway. Cannabis uses malonyl-CoA to make Olivtol, which is then used to produce THC.
- When UVR8 protein photoreceptor splits apart. It is attracted to the gene plastids in the leucoplasts of the glandular trichomes, there it stimulates production via the MEP pathway for terpene synthesis and the polyketide pathway, specifically malonyl-CoA. MEP pathway is used the synthesis of geranyl diphosphate (GPP) and malonyl-CoA in the polyketide pathway to make olivetolic acid (OLA). This reaction is catalyzed by polyketide synthase (PKS) enzyme and an olivetolic acid cyclase (OAC). The geranylpyrophosphate-olivetolate geranyltransferase catalyzes the alkylation of OLA with GPP leading to the formation of CBGA, the central precursor of various cannabinoids.
- Other claims are made stating UVB is the genetic activating frequency of light that stimulates the production of these compounds in the colorless, non-photosynthesis leucoplasts of the disc cells at the base of the trichome reservoir. This triggering create the chemical pathway for enzymatic conversion of acid forms into active forms (THCA to THC).
Figure 4. Cannabinoids complete pathway
Some claims exist that the increase in THC levels after exposure to UV-B is contributed by the additional trichomes produced. The trichomes contain THC so increasing these structures will obviously lead indirectly to higher THC levels. Other claims the trichomes serve as physical barrier and can block harmful UV-B rays from reaching the sensitive photosynthetic tissues of the leaves. Trichomes are thought to function as “sunblock” for plants.
Figure 5. Cannabis glandular trichomes closeup
The glandular trichomes type produce and store large amounts of cannabis resin. Female plants are particularly rich in glandular trichomes. Within the glandular trichomes three types can be described:
- Bulbous trichomes – 15-30 µm, no production of cannabinoids or terpenes.
- Capitate-sessile trichomes – 25-100 µm, globular-shaped head, occur on stems, leaves, and bracts, produce relatively lower levels of cannabinoids throughout the plant’s life cycle.
- Capitate-stalked trichomes – 150-500 µm, tiny mushroom shape, primary source of the cannabinoids, terpenes, and other plants oils, develop only after flower formation, occur especially on the bracts subtending a flower and seed.
Some perspective: human hair is roughly 75 µm and red blood cell is approximately 5 µm.
High-THC strains also have larger glandular trichomes compared to low-THC strains (Small, E. & Naraine, S.G.U, 2016). The heads of glandular trichomes in high-THC strains are four times larger in diameter than that of low-THC strains (Small, E. & Naraine, S.G.U, 2016). In other plants (tomato) it was found that increase of the light intensity (Nihoul, P, 1993) and/or day length (Gianfagna, T. J. et al 1992) (mint) increases trichome head diameter and trichome density respectively. In other plants (olive, mustard), high levels of UV-B light were strongly correlated with trichome density and terpene content (Liakoura, V. et al. 1997; Yan, A. et al 2012).
We in Fotonica™ acknowledge and agree with the high importance of the complete range of light spectrum even those wavelengths which exceeds the PAR range, the above review is supporting the essence of providing the UV-B wavelength. The DLED™ system (Dynamic Light Emitting Diode) in our EVA3™ gives the decision makers the possibility to involve timely exposure to UV-B for the increase of THC levels with avoidance of the harmful effects of overexposure.
Unleashing plant’s potential means taking the productivity efficiency to its maximum cost-effective point, increasing the cultivator profitability through control and confidence. This can only be achieved through superior lighting technologies such as the EVA3™.
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