Why You Should Include UV-A In LED Grow Lights

Many growers misunderstand ultraviolet light.  They think that it will always hurt their plants.  Or, they think that only UV-B is helpful in increasing the medicinal quality of plants.  Both those statements are untrue.  In this article, we’d like to tell you four reasons you should be growing with UV-A.  Before that, though, a reminder on what UV is:

What is UV?

UV is electromagnetic radiation just beyond visible light in energy but lower in energy than x-rays.  They range from 400 nm to 10 nm (which is a greater wavelength diversity than visible light, ranging from 400 to 700 nm).  For plants, there are two relevant types of UV: UV-A and UV-B.  UV-A is the lowest energy UV and ranges from 400 nm to 315 nm.  UV-B is higher energy than UV-A and ranges from 315 nm to 280 nm.  At sea level near the equator, 6% of solar radiation is UV – 5.7% is UV-A and 0.3% is UV-B.  Depending on latitude, elevation, and time of year, plants receive 10 to 100 times more UV-A than UV-B.  Higher energy UV light, such as UV-C, is filtered out by our atmosphere and doesn’t reach Earth’s surface. (And thank goodness!  UV-C is quite dangerous to living organisms.)

Reason 1 to use UV-A: It Can Increase Plant Yields

The effects of UV light on plants is a highly investigated topic with research spanning decades.  Why is there so much research on this topic?  Greenhouses primarily.  Agriculture within greenhouses is a multi-billion dollar industry and most greenhouses are UV deficient because the most common material that covers greenhouses block UV, such as glass or polycarbonate.

Much of the initial studies on the effects of UV confused the subject.  These studies only used parts of plants – chloroplasts or thylakoids – and not whole leaves or by tracking a plants growth over time.  These incomplete studies wrongly gave UV a reputation that it didn’t deserve and underestimated the ingenuity of plants that have become highly adapted to UV.  Below is a sampling of research that tells the complex, multifaceted, and often species dependent responses to UV.

Experimental Effects of UV-A Showing Increased Photosynthesis and Growth
Crop	Results	Citation
Microgreens of basil, beets, and pak choi	“Almost all supplemental UV-A irradiation treatments resulted in increased leaf area and fresh weight.”	Brazaitytė, A., et al. “Effect of supplemental UV-A irradiation in solid-state lighting on the growth and phytochemical content of microgreens.” International Agrophysics 29.1 (2015): 13-22.
Poa annua, Sorghum balepense, andNerium oldeander	340 nm UV-A enhanced photosynethesis by 8-10% with non-saturating background PAR at 500 umol m-2s-1.	Mantha, Sailaja V., Gregory A. Johnson, and Thomas A. Day. “Evidence from Action and Fluorescence Spectra that UV‐Induced Violet–Blue–Green Fluorescence Enhances Leaf Photosynthesis.” Photochemistry and photobiology 73.3 (2001): 249-256.
Pimelea ligustrina	Increased photosynthesis by 12%	Turnbull, Tarryn L., Alexandra M. Barlow, and Mark A. Adams. “Photosynthetic benefits of ultraviolet-A to Pimelea ligustrina, a woody shrub of sub-alpine Australia.” Oecologia 173.2 (2013): 375-385.
Leaf lettuce (Lactuca sativa var. crispa)	Increased leaf size and dry weight of plant.	Chang, Chung-Liang, and Kuang-Pi Chang. “The growth response of leaf lettuce at different stages to multiple wavelength-band light-emitting diode lighting.” Scientia Horticulturae 179 (2014): 78-84.
Cucumber (Cucumis sativus)	Cucumber plants grown under UVA light were found to have higher photosynthetic potential and increased transcription of the genes required for carbon fixation compared to plants grown under red, green, or yellow light	Wang, G., M. Gu, J. Cui, K. Shi, Y. Zhou, and J. Yu. 2009. Effects of light quality on CO2 assimilation, chlorophyllfluorescence quenching, expression of Calvin cycle genes and carbohydrate accumulation in Cucumis sativus. J. Photochem. Photobiol., B 96:30–37.

Reason 2 to use UV-A: It Can Increase How Nutritious Your Plants Are

Analogously to how little bit of UV can be good for humans since it helps us produce Vitamin D, so plants also respond to low dose UV by producing antioxidant compounds like flavonoids and phenolic compounds (incidentally, these compounds give fruit and vegetables their vibrant purple, red, and blue colors).   Luckily for us, it just so happens that many of these compounds are powerful antioxidants and are very healthy to eat.  Flavonoids have been highly correlated with living longer, weighing less, having a healthier heart, lowering cancer rates, and avoiding neurodegenerative disease.  Other phenolic compounds play important roles in cancer prevention and treatment.

Experimental Effects of UV-A on Plant Nutritional Compounds
Crop	Results	Citation
Microgreens of basil, beets, and pak choi	“Almost all supplemental UV-A irradiation treatments resulted in…a trend to increase DPPH free-radical scavenging activity, total phenols, anthocyanins, and α-tocopherol.”	Brazaitytė, A., et al. “Effect of supplemental UV-A irradiation in solid-state lighting on the growth and phytochemical content of microgreens.” International Agrophysics 29.1 (2015): 13-22.
Peppermint  (Mentha piperita)	“…increased leaf area, total phenols, and terpenoid productivity when applied to peppermint plants.”	Maffei, M., et al. “UV-A effects on photomorphogenesis and essential-oil composition in Mentha piperita.” Journal of Photochemistry and Photobiology B: Biology 52.1 (1999): 105-110.

Reason 3 to use UV-A: It Can Increase How Tasty Your Plants Are/ UV-A Can Boost Terpene Levels

Those same “sunscreen” compounds mentioned above are also the source of much of the flavor within plants.    

Reason 4 to use UV-A: It Can Make Your Plants More Resistant to Fungal Infections

Exposure to UV can increase the thickness of the leaf “skin” or epidermis, thus increasing its resistance to fungal infections.  (Victório, Cristiane Pimentel, et al. “Effects of Supplemental UV‐A on the Development, Anatomy and Metabolite Production of Phyllanthus tenellus Cultured In Vitro.” Photochemistry and photobiology 87.3 (2011): 685-689.)

How Does UV Increase Photosynthesis and Growth?

You may be wondering, “How can UV-A increase plant growth when it isn’t very photosynthetically active?”  The magic of UV-A isn’t in how much it is photosynthetically active (though to a respectable degree it is for much of the UV-A spectrum and is well absorbed by chlorophyll a and carotenoids). What’s most important is what it triggers in your plants.

UV Tells Your Plants to Change Growth Pattern, Chemistry, and Transpiration

Light isn’t just energy for plants; it’s also information.  Plants have evolved quite incredible ways of “seeing” what is around them so they can adjust their growth to optimize energy capture.  The number one thing that plants need to “see” is other plants.  If another plant is above or to the side of them, they can adjust the number, size, and distribution of leaves; the chemistry of its leaves; and where new growth should occur.  All this allows it to capture the greatest amount of light despite this competitor.

We are talking about more than just determining what direction has the brightest light; it’s also about determining what wavelengths of light are present and where.  When light passes through or off of a plant, UV, blue, and red light are heavily filtered and green and infrared are left behind.  Therefore, a plant knows they are in direct or bright sunlight when there is high levels of UV, blue, and red.  The reverse is also true – if there is low levels of UV/blue/red and high levels of green and infrared then the plant will think it is being shaded.  The most common reaction to a plant thinking it is being shaded is to extend its stems significantly and stretch out.  If this reaction takes place and the plant isn’t being blocked (like in a grow room with high green and infrared) this wastes energy and reduces yield.

UV-A along with blue triggers a number of photoreceptors (molecules that detect light and send signals to the plant).  Those currently identified include crytochrome, phototropin, ZTL/FKF1/LKP2, and to a lesser extent phytochrome.  These photoreceptors trigger a number of changes, including increasing chlorophyll production, creating larger leaves that capture more light, and opening the stomata on leaves letting in more carbon dioxide.

Experimental Effects of UV-A on Plant Growth
Crop	Results	Citation
Microgreens of basil, beets, and pak choi	“Almost all supplemental UV-A irradiation treatments resulted in increased leaf area and fresh weight.”	Brazaitytė, A., et al. “Effect of supplemental UV-A irradiation in solid-state lighting on the growth and phytochemical content of microgreens.” International Agrophysics 29.1 (2015): 13-22.
1-year-old seedlings of woody Mediterranean species (P. lentiscus, D. gnidium and P. angustifolia, R. sempervirens, L. nobilis and I. aquifolium)	UV-A increased root mass under drought conditions.	M. Bernal, L. Llorens, J. Badosa, D. Verdaguer, Interactive effects of UV radiation and water availability on seedlings of six woody Mediterranean species, Physiol. Plant. 147 (2013) 234–247.
Soy bean	UV-A caused more branching, less stretching.	Zhang, Lingxiao, et al. “Solar ultraviolet radiation exclusion increases soybean internode lengths and plant height.” Agricultural and Forest Meteorology 184 (2014): 170-178.
Arabidopsis	UV-A caused significantly larger leaves.	Biswas, Dilip K., and Marcel AK Jansen. “Natural variation in UV-B protection amongst Arabidopsis thaliana accessions.” Emirates Journal of Food and Agriculture 24.6 (2012): 621.
Leaf lettuce (Lactuca sativa var. crispa)	Increased leaf size and dry weight of plant.	Chang, Chung-Liang, and Kuang-Pi Chang. “The growth response of leaf lettuce at different stages to multiple wavelength-band light-emitting diode lighting.” Scientia Horticulturae 179 (2014): 78-84.
Peppermint  (Mentha piperita)	Increase leaf area.	Maffei, M., et al. “UV-A effects on photomorphogenesis and essential oil composition in Mentha piperita.” Journal of Photochemistry and Photobiology B: Biology 52.1 (1999): 105-110.

UV Makes Your Plants Glow (Not a Joke)

UV can make your plants glow and it’s that glow that can increase the efficiency of photosynthesis.  Just like under a black light (which, of course, emits UV) phenolic compounds in the leaf fluoresce (albeit imperceptibly to the human eye).  This blue-green emitted light then subsequently drives photosynthesis elsewhere in the leaf.  How cool is that!?  (Mantha, Sailaja V., Gregory A. Johnson, and Thomas A. Day. “Evidence from Action and Fluorescence Spectra that UV-Induced Violet–Blue–Green Fluorescence Enhances Leaf Photosynthesis¶.” Photochemistry and photobiology 73.3 (2001): 249-256.)