By Qingwu (William) Meng
You’re considering new LEDs for your vertical farms. What colors should you get? Would you be better off with classic red and blue light or broad-spectrum, white light? It mainly comes down to whether green light is useful to plants, how much it costs, and how we perceive it.
To answer this question, we need to better understand light and how plants use it.
Just like what we see in a rainbow, light comes in a spectrum of colors depending on its wavelength. A rainbow has light ranging from violet to red (400 to 700 nm). Our visual sensitivity peaks in green light (500 to 600 nm) and tapers off in blue (400 to 500 nm) and red light (600 to 700 nm). Plants also have “eyes” for light, but their “eyes” perceive light differently from ours. They use not only the light we can see, but also ultraviolet (UV, 100 to 400 nm) and far-red light (700 to 800 nm) beyond the visible wavelengths.
Light provides energy for photosynthesis. The most important plant pigments in photosynthesis are chlorophylls, which absorb red and blue light while reflecting green light back in our eyes. The fact that plants absorb light selectively explains why most leaves are green. Red and blue light are considered the most efficient parts of the light spectrum for photosynthesis based on a classic photosynthesis curve created by Dr. Keith McCree in the 1970s (McCree, 1971). Although red light seems even more efficient than blue light, LED manufacturers still add a little blue light, attempting to activate chlorophylls and balance plant shape.
Does this mean green light is useless to plants? To a group of researchers at Utah State University (Snowden et al., 2016) using light to improve plant productivity, it’s a resounding no. When a light spectrum has up to 30% green light, it turns out to be generally as good as red and blue light for plant biomass gain. While the upper leaves of a plant absorb most red and blue light, they transmit more green light to lower leaves for photosynthesis. Meanwhile, other pigments, such as phycoerythrin, can absorb green light well to drive photosynthesis. Green light can also promote stem elongation and thus increases light capture for plants.
What’s more, the McCree curve has its own limitations. It’s based on instant measurements on single leaves in low light. Because plants constantly adapt to their environment and make the best of it, the McCree curve doesn’t predict how plants grow in the long term. So, this makes it only useful in certain situations. Also, the lowest dip in the McCree curve is in fact in the upper blue region, not green.
A more practical reason for excluding green LEDs is dealing with what we call the “green gap”. The most essential material in green LEDs is gallium nitride (GaN). There, positively charged holes and negatively charged electrons combine to generate light. However, an electric field in GaN stands in between them to reduce light output. As a result, green LEDs are far less energy efficient than red and blue LEDs, giving off less light using the same amount of energy.
While engineers and physicists are tackling the “green gap” through innovative material manufacturing, there’s another way to get green light. If you put a yellow phosphor coating on blue LEDs, it’ll convert some blue light to green and red light. These three colors of light together produce white light. Although the phosphor conversion makes white LEDs less efficient than red and blue ones, there’s an upside. White light makes it easier to detect pests, diseases, and nutrient deficiencies.
Since plants can use green light well, whether to include it depends on other goals. To create a pleasant work environment, white LEDs are the way to go. To use less energy for the same amount of light, red and blue LEDs are winners. Regardless, how green light affects plant growth generally shouldn’t be the barrier in your decision-making. However, green light responses can vary among different plant species, so performing your own trial is important.
It may be just fine to include some green light while maintaining yield, but it’s still unclear how green light may affect plant quality characteristics such as color, taste, and texture. The new Controlled-Environment Lighting Laboratory (CELL) at Michigan State University is on a mission to address these aspects.
Some LED manufacturers have realized how complex yet useful green light can be for indoor farming. Osram has recently developed adjustable LEDs with multiple color channels, including green, primarily for research applications. Fluence Bioengineering is a fervent advocate for white LEDs. Current research is even pushing the envelope of plant lighting beyond red, green, and blue light because far-red and UV light also have unique roles in plant growth. Our fast-growing knowledge on how plants respond to light will identify useful light combinations and ultimately, help growers improve the yield and quality of their produce.
To learn more about green light, check out this article on Greenhouse Product News.
McCree, K.J., 1971. The action spectrum, absorptance and quantum yield of photosynthesis in crop plants. Agricultural Meteorology, 9, pp.191-216.
Snowden, M.C., Cope, K.R. and Bugbee, B., 2016. Sensitivity of seven diverse species to blue and green light: Interactions with photon flux. PloS one, 11(10), p.e0163121.
Qingwu (William) Meng is a Ph.D. student in Horticulture at Michigan State University and the creator of LightHort, a science blog that communicates horticultural lighting research to the public. He is using light to improve yield and quality of food crops grown in controlled environments. Previously, he earned his Bachelor’s in Agricultural and Environmental Engineering at China Agricultural University and his Master’s in Horticulture at Michigan State University.