While the focus for using artificial light in controlled environment agriculture has been primarily on red and blue light, growers may be missing out on the benefits that far-red light has to offer.
When it comes to using artificial light, especially with LEDs, in controlled environment production, growers are primarily using a combination of red and blue light or white light.
“Plants under red and blue light have a decent photosynthetic rate,” said Dr. Shuyang Zhen, who is a postdoctoral fellow in the Plants, Soil and Climate Department at Utah State University. “Adding far-red light, which are photons with wavelengths from 700-750 nanometers (nm), can increase the photosynthetic rate as plants now utilize light more efficiently to produce carbohydrates.
“However, with most LEDs there is no far-red light at all. If growers are using a broader spectrum white LED like cool white or warm white, they have a small fraction of far-red light, but it is not enough. We tested white LEDs that contain 2-8 percent far-red light and found there was an increase in the photosynthetic rate and efficiency compared to red/blue LEDs, which do not contain any far-red. But the amount of far-red light in white LEDs is not enough to maximize the photosynthetic rate and efficiency. These LEDs can be made more efficient by including additional far-red light.”
Photos courtesy of Dr. Shuyang Zhen, Utah St. Univ.
Impact of far-red light on photosynthesis
Zhen said many growers are familiar with how far-red light can affect plant morphology.
“Far-red light can cause stems to elongate and leaves to expand,” she said. “Far-red light also has some effect on flower regulation.”
Zhen has focused her research on the effects of far-red light on photosynthesis.
“We have looked in detail at how photosynthesis works,” she said. “There are two photosystems that are connected to carry out the light reaction of photosynthesis. Far-red light only stimulates one of those photosystems. The other photosystem is not really stimulated.
“Overall, there really isn’t much photosynthetic activity occurring by far-red light alone. There is a big decrease in photosynthetic activity when the light goes above 700 nanometers, which is the far-red light region. That is the reason that those light wavelengths have been ignored. But the photosynthetic rate is boosted when red, blue and far-red light are combined. Far-red, blue and red light have a synergistic effect.”
Impact of far-red light on plant growth
Zhen and her colleagues trialed the impact of far-red light on canopy photosynthesis of over a dozen plant species, including greenhouse leafy greens, cucumbers and tomatoes and field crops, including potatoes, rice, wheat and corn. Sunlight has almost 20 percent far-red light.
“When plants are exposed to a cool white LED, which contains about 2 percent far-red, by adding up to 40 percent far-red light the photosynthetic rate is increased,” she said. “All of the species we trialed benefited from the addition of far-red light in terms of increasing photosynthesis.”
Zhen said the effects of far-red light during long-term plant cultivation varied depending on the plant species.
“Photosynthesis for all of the species benefitted from far-red light, but there were differences in the morphological responses of the plants,” she said. “Lettuce exposed to far-red light had expanded leaves and an increased leaf area. This is good thing because lettuce can capture radiation more efficiently so they capture more light and grow faster.”
Zhen grew green-leaf lettuce varieties with red and blue LEDs and cool white LEDs, which are commonly used by commercial growers.
“We designed the experiment so the total number of photons (400-750 nm) for all of the light treatments was the same,” she said. “The plants were placed under LEDs with and without far-red light.
“The morphological response for lettuce grown under far-red light was leaves that expanded faster resulting in better radiation capture. Plants produced 30 percent more biomass. Long term there is this benefit with lettuce.”
The study with lettuce was stopped before the plants were ready to harvest. However, based on the results, Zhen said it could be concluded that lettuce grown with far-red light could shorten the production time.
“During the four weeks that the plants were exposed to far-red light they grew bigger and faster,” she said. “It is reasonable to say that the plants could have reached salable size sooner compared to the treatments with no far-red light. For the production of green lettuce I would recommend incorporating far-red light.
“For other species far-red light may not be as beneficial. The increase in biomass might be in the stem and cause the plants to stretch. Cucumber was one of the species that adding far-red light long term doesn’t have much benefit.”
Based on the results of her trials Zhen said there is compelling evidence that increasing the amount of far-red light increases the photosynthetic rate.
“Further research needs to be done to determine the effects of far-red light on long term crops like cucumber and tomato,” she said. “Does exposure to far-red light and the accumulation of biomass speed up flowering? That part is not as well characterized. I haven’t done much in that area of research. There is research going on at other universities that characterize the long term effects of far-red light.”
The effects of UV light
Zhen is also interested in studying the effects of ultraviolet light on the photosynthetic rate.
“UV-B light wavelengths from 280 to 320 nm tend to trigger secondary metabolite production like the flavoring compounds in plants,” she said. “An example is field-grown tomatoes vs. greenhouse-grown tomatoes. Greenhouse glazing blocks UV light so plants often don’t produce as much of the flavoring compounds. These compounds are important for crops like herbs including basil. UV light may also trigger some stress responses causing plant damage.
“I am interested in UVA wavelengths from 320-400 nm. We started with violet photons that peak around 400-408 nm. These wavelengths are different from UV light. We are looking at the photosynthetic efficiency of these violet wavelengths, which have the potential to be utilized by growers. A typical white LED doesn’t contain any wavelengths below 400 nm.”
Zhen is using violet LEDs to study the impact on photosynthesis and the long term growth of cucumber and lettuce.
“For both of these species, violet wavelengths were as efficient for photosynthesis as commonly used blue LED wavelengths,” she said. “There wasn’t much difference in the photosynthetic rate at plant canopy level. But for cucumber there was 15 percent more biomass production under violet wavelengths than under blue wavelengths, mainly due to leaf expansion. In the case of lettuce, violet light actually caused bleaching or yellowing of the leaves. We are trying to determine the effects of violet light on photosynthesis and plant growth before looking at the effects of shorter-wavelength UV photons.
“LED technology is moving so fast. Growers have the ability to change the colors and the intensity, but they can’t really take full advantage of that amazing capability because we still don’t fully understand how a particular color and intensity impact short- and long-term plant growth and development. We also need to learn more about how species and cultivars respond to the wavelengths. An increase in the short-term photosynthetic rate may not correlate with a long-term response or an increase in growth.”
For more: Shuyang Zhen, Utah State University, Plants, Soil and Climate Department, Logan, UT 84322; shuyang.zhen@usu.edu.
David Kuack is a freelance technical writer in Fort Worth, Texas; dkuack@gmail.com.
