How Manipulating Light Treatments Affects Cannabis Plant Expression

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What the latest research shows about the relationship between plant growth and light.

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Photo by cendeced | iStockPhoto; Photo illustration by Tiana Kropko

Plants convert light into chemical energy for photosynthesis, but there is more to light than just nutrients for growth. Light treatments also influence plant development and can influence photomorphogenesis (the physical effect of light on plant qualities like shape, appearance, color, taste and secondary metabolite production).

In our most recent guest column, published in the January 2021 issue of Cannabis Business Times (bit.ly/CBT-RII-light) we reviewed how growers can evaluate manufacturer claims about horticultural light fixtures and described the characteristics of LEDs that are third-party verified to ensure longevity, safety, and quality—along with energy efficiency. In this column, we share the latest research on how cultivators can better determine which lighting systems will help them optimize specific biological results so cultivators remain up to (light) speed.

From Research Labs to Test Gardens to Commercial Scale

The Resource Innovation Institute works to bridge the knowledge gap between academics studying how to optimize light for plant growth and cultivators dialing in their environments to achieve business goals by summarizing the findings of leading researchers and practitioners. With aggregate scientific and peer-reviewed guidance, cultivators can successfully adopt technology to drive resource efficiency and cost reductions. The insights in this article are derived from publicly available published studies by and ongoing interactions with researchers like Dr. Bruce Bugbee (Utah State University Crop Physiology Lab), Dr. Mark Lefsrud (McGill University's Biomass Production Laboratory), Leora Radetsky (DesignLights Consortium, formerly of the Lighting Research Center), and Dr. Erik Runkle (Michigan State University’s Controlled-Environment Lighting Laboratory). These experts are performing research in test chambers at universities and national laboratories and interacting with producers in the field growing various crops in diverse environments.

RII also compiles scientific literature and has benefited from service on our Technical Advisory Council by private consultants like Morgan Pattison, who are connecting dots between the R&D divisions of lighting manufacturers and federal agencies like the U.S. Department of Energy (DOE) and the U.S. Department of Agriculture (USDA). In addition, RII routinely speaks with former academic researchers and researchers at leading manufacturers conducting independent research and participating in RII Technical Advisory Council dialogues on cultivation efficiency and productivity. Our intent is to ensure cultivators can benefit from aggregated, validated insights with the market.

It is our hope that cultivators performing their own tests within their canopies benefit from what we have learned so far about the emerging connections between studies across growing environments.

Get On Your Plants’ Wavelength

Light is electromagnetic (EM) radiation, a stream of photons containing different amounts of energy. Although heat energy and light are different things, thermal energy produces radiation in the EM spectrum via infrared light waves, a flow of photons with low energies, longer wavelengths, and lower frequencies. Visible light has more energy and includes the Photosynthetically Active Radiation (PAR) range, wavelengths of light between 400 and 700 nanometers (nm) that drive photosynthesis. Ultraviolet (UV) light is even more energetic with even shorter wavelengths and higher frequencies (see chart below).

Light recipe describes a polychromatic spectrum made up of different ratios of spectra such as red:blue and red:far red, which can only be accomplished with light-emitting diodes with different proportions employed in LED light fixtures. Spectral quantum distribution (SQD) plots illustrate the distribution of photon flux per photon wavelength over the photosynthetic and far-red range wavelengths. The SQD plots below show a high-pressure sodium (HPS) light fixture in pink and an LED light fixture shaded in color.

LEDs can provide a wide range of customizable spectral quantum distributions, with the distribution shown below being just one example of a light recipe. LED light fixture manufacturers can provide unique light recipes targeting specific plant responses, including far-red and UV spectra that high-intensity discharge (HID) options cannot offer, as those fixtures cannot produce customizable spectra. The spectrum of double-ended HPS lights can vary by manufacturer, but is not customizable.

Broad spectrum LEDs provide light treatments delivering photons from many or all of the wavelengths across the PAR range, allowing for a greater proportion of blue, red, and far red photons to influence plant growth and development in beneficial ways. The impacts of changes to ratios between blue:red and red:far red are crucial for growers to understand to maximize the benefits of LED solutions.

This chart illustrates electromagnetic (EM) spectrum frequencies and where PAR is found on the EM radiation spectrum.
Chart courtesy of RII

Light Quality Impacts Plant Qualities

While cannabis secondary metabolites are complex, they can be broken down into two basic families of compounds: 1) cannabinoids, of which THC and CBD are just two of more than a hundred; and 2) terpenes, a large family of compounds associated with aroma and taste. GPP (Geranyl pyrophosphate) is a building block in the biosynthesis of cannabinoids and monoterpenes; studies by lighting manufacturers have shown that GPP can be manipulated with light treatments, and those companies are researching the relationship between secondary metabolite expression and light spectra with great interest. Some growers working with them on confidential field studies have found that light quality can influence these branching elements that affect THC and CBD content as well as terpene expression. Research from studies at the Crop Physiology Laboratory at Utah State University has also found spectral effects on cannabinoid synthesis, but these effects are extremely small compared to genetic differences.

Ranging from low to high wavelengths, different quantities of photons in the different portions of the light spectrum affect other aspects of plant expression in both positive and negative ways. Years of study by researchers and in cultivation facilities around the world have established some key findings that cannabis growers can use to create the optimal light treatment for their plants.

Ultraviolet (UVC (100 - 280 nm), UVB (280 - 320 nm), UVA (320 - 400 nm))

Light from the UV spectrum can suppress or inoculate plants against pathogens, according to multiple studies. The Lighting Research Center at Rensselaer Polytechnic Institute has performed experiments with UV lighting on cucumbers, squash, strawberries, and grape cultivars to investigate downy mildew, and UVC was noted to suppress powdery mildew.

Pigments are both aesthetic and chemical. Some cannabis growers treating flowering canopy with UV light have observed increased purpling, induced by greater presence of anthocyanin pigment. While genetics and plant temperature are contributors to pigmentation, purpling can also be influenced by light treatments, with UV and low-wavelength blue light treatments producing darker leaves and flowers. Studies conducted by Michigan State University have found increased UV exposure produced redder varieties of red-leafed lettuce, as anthocyanin is used by plants to alleviate the stress induced by wavelengths, like UV, damaging their DNA.

According to some manufacturer researchers, there is a popular perception that UV can increase cannabinoid content like THC, but researchers working with Texas Original Compassionate Cultivation showed UV both reduced yield and “did not affect cannabinoid or terpene concentration.”

Spectral quantum distribution (SQD) plots illustrate the distribution of photon flux per photon wavelength over the photosynthetic and far-red range wavelengths.
Chart courtesy of Fluence by OSRAM

Blue (450 - 485 nm)

Blue is the single biggest factor affecting plant quality according to many published studies and researchers interviewed by RII. Michigan State University researchers have published studies showing how blue light induces a stress response in plants (like UV) which can influence pigmentation and chemical content of shoots, leaves, and flowers.

More blue photons can also result in enhanced secondary metabolite expression, but at the expense of lower yields due to the stress response caused by higher-energy wavelengths like blue and UV; research on lettuce by Michigan State University showed that increasing the blue proportion of light for lettuce decreased growth by up to 63%.

Blue wavelengths also can suppress plant stretch and can counter elongation effects of other light spectra, producing more compact plants, shorter stems, and smaller leaves. A more compact plant can be good, but broader plants can get more airflow through the canopy.

When using light treatments with more blue wavelengths, growers report lowering light intensity to counteract stretch suppression and recommend evaluating red:blue ratio of light fixtures, which impacts both plant yield and photomorphogenesis as shown in studies on tomatoes and hydroponic lettuce by McGill University. For a data point that could be connected to cannabis, the highest biomass production (excluding fruit) occurred with the 19:1 red:blue ratio in the tomato study.

aleksle | Adobe stock

Red (625 - 700 nm)

Red drives photosynthesis and impacts plant yield, and can be seen as the counterpoint to blue light. As wavelengths approach 700 nm, plants respond by creating larger leaves, which results in higher yields because bigger leaves are bigger collectors of photons; those extra collected photons can be used to generate more biomass, which results in more production of flowers and bigger yields. A greater proportion of red photons can improve yields but can adversely impact quality.

When using red light, some growers report using 660 nm red light to dose plants on a daily basis to “wake them up” at the start of photoperiods.

Far Red (700 - 800 nm)

Far red influences photomorphogenesis. Growers recommend using far red with care, as it can make leaves bigger if dosed during the day, but can also stretch and elongate plants, which is not usually desirable. Red:far red ratio is important as well; Michigan State University studies of chrysanthemum, dahlia, and marigold show moderate to high red:far red (0.66 or greater) is most effective at interrupting night cycles and found that far red light alone does not regulate flowering.

Far red is a fan favorite of some cannabis growers because of its ability to speed up the cannabis growth cycle. Some far red (730 nm) doses for as little as 10 minutes a day at low light levels can be used to put plants to sleep and artificially extend cannabis night cycles by helping the cultivar get through its critical dark period faster. A longtime medical cultivator in New England raves about using red and far red spectral treatments to change growth cycles completely, creating more yield in less time by running shorter days and nights while keeping daily light integral (DLI) consistent. Growers can run sub-24 hour days by using high-PPFD LED lighting systems to provide enough DLI to achieve 10-hour “days” and using far red spectra to create 10-hour “nights.”

Bright Light, Grow Right

Spectral quality matters, but is less important for plant growth and development than light intensity, as plants detect and react to changes in light intensity more finely than changes in spectral distribution, as has been shown in studies of petunias by Michigan State University, which concluded that light treatments had generally similar effects on seedling growth at the same PPFD. When growing cannabis with LEDs for flowering growth stages, independent manufacturer studies comparing different types of lights (HID versus LED) show promising results suggesting that plants can respond similarly in yield if given the same light intensity.

Light intensity also can be the solution to adverse morphological impacts of spectral quality. Blue stretch suppression can be mitigated in young plants by lowering light levels to limit PPFD. To keep plants short while using far red light treatments, increased light levels can give plants more energy to limit elongation.

LED light fixtures are capable of producing more light (more photons to achieve higher light intensity) than HID or fluorescent light fixtures. While HPS lights may be higher in wattage, they are lower in photosynthetic photon efficacy (PPE), which means less power is turned into photons for plant growth. Higher PPEs allow plants grown with LED to receive higher photosynthetic photon flux density (PPFD) levels, as some LED fixtures may have higher PPFD than the best performing double-ended HPS fixtures and can be mounted closer to flowering canopies. This means cannabis can be grown in high-PPFD environments unattainable with other lighting technologies.

Yarygin | Adobe Stock

Research & Demonstration Establish Best Practices for Optimal Yields and Efficiency

Research is a critical ingredient to understanding the effects of light recipes so that growers can successfully adopt emerging and efficient technologies that can offer customizable spectral treatments and can produce more light to achieve new highs for quality and yield.

Cultivators have multiple goals in today's competitive market, including expanding production, reducing costs, complying with regulations, and reporting on environmental commitments. With light treatments and other techniques, more biomass with specific desired properties can be produced with less energy use on a per gram basis, leading to higher yields, better quality, lower costs, and reduced environmental impacts.

We hope that cultivators who are curious about these findings are encouraged to conduct field studies with their trusted project partners to implement new light treatments and control lighting systems to provide a fundamental feedback loop that will strengthen research going forward.

Editor’s Note: This article is the first in a series of guest columns written for Cannabis Business Times by Resource Innovation Institute (RII) in 2021.

Gretchen Schimelpfenig, PE, is the Resource Innovation Institute’s (RII) Technical Director and manages the organization’s Technical Advisory Council. She is a licensed Civil Professional Engineer in California and Vermont.