The effect of ultraviolet (UV) light on cannabinoid production has been a point of contention among cannabis growers for several years. According to some, exposure to this short-wavelength radiation causes plants to ramp up their THC levels in an attempt to protect themselves from becoming damaged. However, a new study that is yet to be peer-reviewed or published suggests that this is not the case after all. UV light actually causes a decrease in overall biomass without boosting cannabinoid yields[i].
Ultraviolet Light And Cannabinoids
Ultraviolet light has a wavelength of between 100 and 400 nanometres, shorter than that of visible light. As such, UV is invisible to the human eye, although its effects certainly don’t go unnoticed, as it is responsible for the tanning and burning of our skin by the sun. As a natural defence, our bodies produce a UV-blocking compound called melanin, and it has been theorised that THC performs this same function in cannabis plants.
As such, many growers believe that exposing their plants to ultraviolet light causes them to increase their cannabinoid levels, and a small amount of research conducted way back in the 1980s would appear to confirm this. For example, one study found that cannabis plants grown at higher altitudes – where UV light is more prevalent – tend to have a higher ratio of THC to CBD[ii]. Another concluded that exposing plants to UV light in a controlled environment causes their THC concentrations to rise by up to 28 percent[iii].
However, cannabinoid concentrations in commercially available cultivars are much higher these days than they were 30 to 40 years ago, so we can’t assume that ultraviolet light would have the same effect on modern strains. In the second of the aforementioned studies, the absolute increase in THC was only 7 milligrams per gram of plant material, which may have been substantial back then but is pretty inconsequential in the context of today’s cannabis plants, which can contain more than 200 milligrams of THC per gram.
Therefore, more research is needed to determine whether or not ultraviolet light impacts cannabinoid yields in a meaningful way, which is why the authors of the new study sought to repeat the experiments conducted in the 80s using modern cannabis cultivars.
Ultraviolet Light Doesn’t Boost Cannabinoids In Modern Plants
The team grew two cannabis cultivars called ‘Low Tide’ and ‘Breaking Wave’ to conduct their research. During flowering, plants were kept on a 12-hour photoperiod, with supplemental ultraviolet light supplied for 3.5 hours each day.
Reporting their findings, the authors note that ultraviolet light generated no increases in cannabinoids such as THC, CBD or CBG in either of the strains used in the study. However, some other effects were seen, the majority of which were detrimental.
For instance, the overall flower mass-produced by both cultivars was found to decrease in response to UV exposure, while whole-plant and leaf sizes also dropped. In addition, leaf malformations became more common, with many curling at the edges after being dosed with ultraviolet light.
Flowers also degraded considerably faster when blasted with UV, with stigmas turning brown shortly after appearing.
Based on these findings, the authors conclude that ultraviolet light doesn’t cause plants to increase their cannabinoid production, and in fact, has many detrimental effects on leaves and flowers. However, while UV may not boost THC levels in individual plants, the researchers point out that “the upregulation of Δ9 -THC under UV stress may be an adaptive response (i.e., over generations) rather than an acclimation response (i.e., during a single production cycle).”
In other words, ultraviolet light may drive cultivars to adapt by increasing their cannabinoid production over numerous generations. Naturally, though, more research is needed To determine whether or not this truly is the case.
[i] Rodriguez-Morrison V, Llewellyn D, Zheng Y. Cannabis inflorescence yield and cannabinoid concentration are not improved with long-term exposure to short-wavelength ultraviolet-B radiation. – https://www.preprints.org/manuscript/202106.0317/v1
[iii] Lydon, J., Teramura, A.H., Coffman, C.B. (1987). UV-B radiation effects on photosynthesis, growth and cannabinoid production of two Cannabis sativa chemotypes. Photochem. Photobiol. 46:201–206. doi: 10.1111/j.1751-1097.1987.tb04757.x. – https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1751-1097.1987.tb04757.x