Is UV Light Beneficial for Your Plants? A Scientific Investigation

We’ve talked briefly about the effects of ultraviolet (UV) radiation on cannabis plants in a previous blog post. In the growing community, the jury is still out on whether or not UV Light is actually good for your plants or not. Some people swear by it, and many new LED grow lights come with UV-emitting LEDs. Other people think it’s a waste of energy for the plant, with no beneficial effect on the plant’s health or THC production. In this blog post, we’re going to investigate some peer-reviewed scientific literature regarding the topic and try and find a conclusive answer to the UV debacle.


The best peer-reviewed articles available to us providing the theory that UV light is beneficial for plants and increases THC production are from 1983 and 1987. Pate (1983) first suggested that UV exposure increased THC production in cannabis. In 1987, Lydon et al. also found that THC concentration was slightly increased in leaf and flower tissues of a sativa in reaction to UVB exposure. However, these results have yet to be reproduced, so the only primary scientific evidence of UV exposure and THC increases are from these 30-year old studies. But, in those studies, no other effects of UV light were measured, so what else could UV light be doing to your plants?

Results of the Lydon et al. 1987 study, visualized by Zhang & Bjorn (2009)

Ultraviolet light is emitted by the sun, but you probably already know that. UV light is the main cause of sunburn in humans, and can also cause certain types of skin cancers (Brash et al. 1991). UV light is harmful thanks to its ability to directly cause mutations in the DNA of cells. But it’s not just humans that are susceptible to the harmful effects of UV light. Microbes such as bacteria, fungi and viruses are all harmed by UV light, but also, your precious, precious plants.


It has been well-described that high quantities of UV-B radiation (280-315nm) can cause permanent damage to the macromolecules of plants (Jordan et al. 1996), prompt the production of excess reactive oxygen species – which induce damage to plant cell tissues (Brosche et al. 2003), and can affect the structural integrity and reproductive viability of cells (Frohnmeyer and Staiger 2003). It’s maybe unlikely that UV light produced from grow lights such as those in LEDs can produce enough UV to harm your plant to the point of killing it. But blasting it with high energy, harmful UV radiation could simply cause the plant to invest more energy in defense and repair mechanisms, taking away from the energy that could be invested in growth and psychoactive compound production. The plant is likely ‘wasting’ energy producing antioxidants to negate the effects of reactive oxygen species (Sharma et al. 2012), and other enzymes to repair cell components and DNA (Manova & Gruszka, 2015) rather than investing in growth and THC production.

Now, it’s important to note that right about now, a pro-UV grower is ready to argue everything I just explained above. UV proponents claim that trichomes, the cute little resin glands present on cannabis leaves that contain THC and other cannabinoids and terpenes, are produced in greater quantities when exposed to UV light as a defense mechanism. Think like some sort of cannabis sunscreen. But if that is the case, then why wouldn’t cannabis plants produce trichomes throughout their lifecycle, and not exclusively during the flowering life stage? And, as Reddit user /u/LEDwizard theorizes, why wouldn’t hemp plants produce trichomes, while sativa and indica do, despite them being so genetically similar. In addition, it seems that cannabidiol (CBD) and cannabichromene (CBC) would be more efficient for a plant to produce as an antioxidant to deal with UV light, and are both more effective at absorbing UV light. Remember from earlier, Pate (1983) claimed that THC increased and CBD decreased as a result of UV exposure, so the theory of THC being produced as a defense mechanism doesn’t sit quite right.

uv light cannabis

However, there is still potential for some benefits of UV light when growing. As mentioned earlier, UV light can also have harmful effects on bacteria, viruses and fungi (Ozcelik, 2007). Because UV can directly induce mutations in DNA, and cause other damaging effects to cells, applying UV light to viruses, bacteria and fungi is a common sterilization technique in many laboratory, commercial and industrial settings. This is an intriguing implication for cannabis growers. If the UV light is sterilizing the air around the plants of any air-borne microbes, as well as the surfaces of leaves, perhaps the plant does not need to produce as many secondary compounds to combat pathogenic microbes. Perhaps the slight increase in THC production documented by Lydon et al. (1987), and that others claim to have observed, is a secondary effect: since the plants invest less in defense mechanisms, they can invest more energy into growth and THC production. It’s also possible that these beneficial microbicidal effects are quickly negated by the harmful effects of UV on the plant itself. But since plant tissues are generally more robust than that of most microbes due to their thick cell walls and high antioxidant concentration, perhaps there is a sweet-spot amount of UV to apply. Unfortunately there isn’t much primary literature to back up this theory in our cannabis-growing specific context.

cannabis pathogens uv light
Unfortunately, there still has yet to be a recent study that can recreate similar conditions to Lydon (1987) and Pate (1983) to definitively determine if UV light really does have a significantly beneficial effect on the THC production in cannabis. The best we can conclude from the scientific literature is thus: Although primary studies from almost 30 years ago and anecdotal evidence from the growing community may suggest that UV light is beneficial in producing higher levels of THC and other psychoactive cannabinoids, there is yet to be a peer-reviewed conclusive study to reproduce these results. However, UV light can perhaps be useful to promote healthier growth in plants by sterilizing the air around the plants and the surface of the leaves and buds, killing off any potential pathogenic bacteria, fungi or viruses. Since UV also has a documented detrimental effect on the health of plant cells and tissues, it could also possibly be causing more harm than good. Another though is that since there is UV light in sunlight, and plants evolved under the sun, maybe a little UV light is important for some mysterious chemical signalling pathway we have yet to discover. Whatever your conclusion about the effects of UV, most grow lights only produce a small amount, and you can always cover them up if you’re really worried about it. And hopefully sometime soon, someone will recreate the conditions of Lydon’s 1987 study and have the results published in a peer-reviewed journal!

 

References:

David W. Pate. (1983). Possible Role of Ultraviolet Radiation in Evolution of Cannabis Chemotypes. Economic Botany, 37(4), 396-405. Retrieved from http://www.jstor.org/stable/4254533

Lydon, J., Teramura, A. H. and Coffman, C. B. (1987), UV-B radiation effects on photosynthesis, growth and cannabinoid production of two cannabis sativa chemotypes. Photochemistry and Photobiology, 46: 201–206. doi:10.1111/j.1751-1097.1987.tb04757.x

Jordan, B. R. (1996). The effects of ultraviolet-B radiation on plants: a molecular perspective. Adv. Bot.22, 98–138. doi: 10.1016/s0065-2296(08)60057-9

Brosché, M., and Strid, A. (2003). Molecular events following perception of ultraviolet-B radiation by plants. Physiol. Plant. 117, 1–10. doi: 10.1034/j.1399-3054.2003.1170101.x

Frohnmeyer, H., and Staiger, D. (2003). Ultraviolet-B radiation-mediated responses in plants. Balancing damage and protection. Plant Physiol. 133, 1420–1428. doi: 10.1104/pp.103.030049

Pallavi Sharma, Ambuj Bhushan Jha, Rama Shanker Dubey, and Mohammad Pessarakli, “Reactive Oxygen Species, Oxidative Damage, and Antioxidative Defense Mechanism in Plants under Stressful Conditions,” Journal of Botany, vol. 2012, Article ID 217037, 26 pages, 2012. doi:10.1155/2012/217037

Manova V, Gruszka D. DNA damage and repair in plants—from models to crops. Frontiers in plant science. 2015; 6: 885 doi: 10.3389/fpls.2015.00885

Brash, D., Rudolph, J., Simon, J., Lin, A., McKenna, G., Baden, H., . . . Ponten, J. (1991). A Role for Sunlight in Skin Cancer: UV-Induced p53 Mutations in Squamous Cell Carcinoma. Proceedings of the National Academy of Sciences of the United States of America, 88(22), 10124-10128.

Berrin Ozcelik , 2007. Fungi/Bactericidal and Static Effects of Ultraviolet Light in 254 and 354 nm Wavelengths . Research Journal of Microbiology, 2: 42-49.

Zhang WJ, Bjorn LO, The effect of ultraviolet radiation on the accumulation of medicinal compounds in plants, Fitoterapia (2009), doi: 10.1016/j.fitote.2009.02.006

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