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Taking LEDs to the next level

McGill University bioresource engineer Mark Lefsrud said it’s time to take plant research and production to a higher level with LEDs.

Much of the research that has been done with LEDs for the last 50 years has been conducted with low light levels.
“The biggest research area right now is with LEDs at high intensities of light,” said Mark Lefsrud, associate professor in the Bioresource Engineering Department at McGill University in Quebec, Canada. “We have to get into high intensity lighting studies to truly understand what is happening in plants. NASA has done some preliminary work with high intensity LEDs, but we have taken the research further.”

Mark Lefsrud, McGill University
Mark Lefsrud at McGill University is starting to do high intensity lighting studies to understand what is happening in plants under light levels of 5,000 micromoles and higher.
Photos courtesy of Mark Lefsrud, McGill Univ.

Lefsrud said most of the earlier LED studies that he and other researchers have been doing have looked at light levels of 150 micromoles of light (micromoles per square meter per second or μmol·m-2·s-1) and lower, and how the plants responded to those light levels.

“No one grows plants at 150 micromoles,” he said. “Plants are usually grown at 300 micromoles and higher. We have gone well above 1,500 micromoles and in some of our LED tests, we have gone as high as 5,000 micromoles. The plants start responding to what I consider to be more normal field type conditions.”

 

Incorrect assumptions

Lefsrud said the reactions of plants to higher light levels are not what the assumptions have been up to this point for all the earlier research.

“One assumption is that shade plants can’t handle light levels as high as sun plants, which is actually backwards,” he said. “We have found that it is the complete opposite. We are finding that shade plants are able to handle higher light intensities better than sun plants.

“Lettuce is considered a shade plant and tomato is considered a sun plant. When we shine 5,000 micromoles of light on these two plants, the tomato plant suffers and we can kill it quite easily. We don’t see the death of lettuce at higher light levels. Lettuce carries on like nothing has happened to it.”

Lefsrud said that one assumption is sun plants tend to be more succulent with thicker leaves so they can handle higher light levels.

“Another assumption is that shade plants, even though they’re called shade plants, aren’t true shade plants,” he said. “They’re sun spot plants. They can handle the bright light beams that come through the plant canopy and then the light beam disappears and they prepare for the next beam. The sun spot plants have more adaption for fluctuations in light levels as opposed to sun plants which have to receive more continuous high light.”

 

Higher light level LEDs

Lefsrud said it’s possible that growers haven’t tried producing at higher light levels because they were unaware that the plants can tolerate these levels.

“Another reason is that we have never been able to achieve higher light levels of 5,000 micromoles from these lamps,” he said. “And would it be cost effective to even try to produce these light levels? Now that we know that we can achieve higher light levels, let’s see what we can do with them. As research scientists we have to get up to higher light intensities. Currently we are trying to do things up around 10,000 micromoles of light.

“We’ve had to make the high light lamps ourselves or had them custom made to reach these light levels. That is one of the challenges—being able to manufacture lights that can reach these high light levels.”

Lefsrud said nearly half the energy that is going into an LED comes out as light, the other half is heat that has to be dealt with.

“The lights that give off these higher levels generate a lot more heat,” he said. “We have to use a water cooled jacket to cool the LEDs that produce 5,000 micromoles.

“Most LED bulbs can’t handle that kind of heat and burn out. When we started pushing these bulbs, the manufacturer told us the bulbs can’t handle the heat. We were told the bulbs would only last a few seconds at these high intensities. We cooled the bulbs down to -20ºC (-4ºF) with a water jacket and were able to run the bulbs for two weeks.”

Lefsrud said the technology is coming quickly and he expects that within the next year LEDs will be able to deliver these higher light levels.

“It’s not only the plant reactions at those light levels, but it also changes how the lights can be installed,” he said. “Growers won’t have to shine light only from above any more. The lights will also be able to shine from below or on the side. The lights could be mounted on booms and be moved. There are also more possibilities with interlighting.

“We have assumed that the lamps have to be shined from above like the sun. We have done many research tests that have shone the lights from below the plants and they do just as well as when the light is shined from above.”

 

Maximizing plant growth

Another part of Lefsrud’s LED research deals with maximizing growth with the minimum amount of light possible.

“What are the wavelengths (colors) of light that produce the most amount of plant growth with the least amount of light possible?” he said. “We chose to study lettuce, tomato and petunia looking at the PAR light spectrum that was available. We have been looking at other wavelengths where we see plants growing at twice the normal rate. We are finding that the plants grow faster as we get away from far red light and more into the red.

“We think that we can produce lettuce plants at twice the speed. For a lettuce crop that grows heads weighing 20 grams in the first two weeks, we can speed up the crop development to produce 40 gram heads in the same amount of time. We can theoretically double the growth rate. Lettuce seems to be a more aggressive crop than tomatoes. We are not sure about the growth rate for tomato and petunia as we haven’t completed the research.”

Mark Lefsrud, McGill University tomato
Mark Lefsrud found the best growth rate for tomato is around an 8:1 to 10:1 red to blue ratio, around 640 nanometers for the red and 440 nanometers for the blue.

Lefsrud said the research is narrowing in on a few wavelengths that most researchers haven’t been paying much attention to.

“From a growth standpoint, we don’t think far red is useful at all,” he said. “It’s red and a couple of other wavelengths.

“We found the best growth rate for tomato is around an 8:1 to 10:1 red to blue ratio. Roughly around 640 nanometers for the red and 440 for the blue. The more blue that was used, the more flowers and fruit were produced. High levels of red (19:1) was one of the ratios we trialed, where we saw more vegetative (leaf) growth occurred, but there wasn’t as much flowering and fruit.”

Lefsrud said his research has not gone as far as determining what levels of red and blue light should be given while the plants are vegetative or when they are start to flower.

“We are fairly sure that higher levels of red light increase the vegetative growth and then after that a grower would want to go to a higher level of blue light for tomato,” he said. “For tomato, 40 days after germination, the plants should be given more blue light. There are a number of research papers that have indicated that higher levels of blue light cause increased flowering. We believe that the blue light is critical.”

 

For more: Mark Lefsrud, McGill University, Bioresource Engineering Department, Ste.-Anne-de-Bellevue, QC, Canada; (514) 398-7967; mark.lefsrud@mcgill.ca; http://www.mcgill.ca/biomass-production-lab

 

David Kuack is a freelance technical writer in Fort Worth, Texas; dkuack@gmail.com.