Harrow research team finds key to continuous greenhouse lighting

Growing greenhouse crops under continuous lighting has been a dream since the 1930s and moved closer to reality with the advent of LED lighting. 

Now, a team of researchers at Agriculture and Agri-food Canada’s Harrow Research and Development Centre aims to make continuous lighting an actuality. 

Continuous lighting effectively spreads a plant’s total lighting needs across a 24-hour period and pushes half of it into night time when electricity rates are typically lower. That offers significant economic benefits for greenhouse producers and reduces stress on the electrical grid.

Why it matters: Continuous lighting in greenhouse production can help growers use significantly less energy while maintaining plant yields.

Since beginning to explore the concept in 2018, the Harrow centre’s research team has published several papers in scientific journals, the most recent entitled “Continuous lighting can improve yield and reduce energy costs while increasing or maintaining nutritional contents of microgreens.” It appeared in the Sept. 30 edition of Frontiers in Plant Science.

The research outlines a discovery that the team’s plant physiologist, Jason Lanoue, thought was impossible — that a plant’s circadian rhythm can be disrupted by continuously applying lower-intensity light.

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Speaking to Farmtario, Lanoue said the research began when he was a University of Guelph PhD student partnering with AAFC at Harrow through the research affiliate program. The team’s first extended-lighting trial was on tomatoes but it has since also worked with cucumbers, peppers and microgreens.

Prior to LEDs, the most recent technological advancement in lighting relevant to greenhouses was high-pressure sodium (HPS) luminaries, he said. This allowed limited flexibility in light intensity but offered no ability to change the broad-spectrum quality of light.

LEDs, by contrast, can be readily managed for intensity and also to produce light in different wavelengths of the colour spectrum. Given ever-improving smart communications technologies that allow for remote programming, Lanoue said practical possibilities abound.

He refers to what LED lighting provides as “dynamic flexibility.” The programmable features offered by LED manufacturers means “we can implement these strategies without having to add extra fixtures.”

It’s a case of using an existing technology better, he said.

In HPS settings, attempts to extend lighting periods typically led to the yellowing of leaf material known as chlorotic injury. It’s a plant response to stress. In the case of extended lighting, the stress arises from the plant’s inability to maintain the process of transporting sugars — produced in the leaves through photosynthesis — away from the leaves into other parts of the organism. This build-up of sucrose and starch inhibits photosynthesis, leading to a reduction in chlorophyll.

According to an AAFC news release on the research, “since plants typically do not like growing under continuous light, the team developed a unique lighting strategy, called dynamic 24-hour lighting, which uses a shift in light spectrum and/or intensity between the day and night periods. This allowed the plants to continue growing, stress-free, as well as they would under a traditional photoperiod.”

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Is it natural for a plant to require rest?

Lanoue said it’s widely accepted that mammalian brains could accomplish more than they typically do if only other elements of the body didn’t require rest. Likewise with plants, there’s evidence showing the capacity for photosynthesis and growth is greater than what typically occurs in nature but other limiting factors get in the way.

Real-life examples are plants that grow under brief but intense continuous-lighting environments in high sub-polar regions of the north and south. And through the work his team is doing, Lanoue said they are able to unlock this extended photosynthetic potential in a different way.

Some species seem more tolerant of the treatment. Varieties tested have been limited to those commercially available for Ontario growers. Trial results from both the Harrow research station and on-farm trials at Allegro Acres in Kingsville reveal cucumbers and microgreens as the top candidates for continuous lighting regimes.

“While tomatoes and peppers can handle continuous lighting, we have yet to optimize a lighting strategy for them. That’s something we’ll have to look at further to see if we can get a better response,” said Lanoue.

He has been studying intensity and spectrum. The Greenhouse Physiology Team has also begun looking at temperature in the greenhouse as another variable, and Lanoue suggests this may be part of the key to achieving similar yields as conventional lighting regimes, but to increase yields under continuous lighting.

Another step in the research is to determine which genes are responsible for allowing photosynthesis and sugar transfer to continue beyond daytime hours.

Lanoue believes the research, and the possibilities it represents, are attracting attention from commercial growers and lighting companies.

Greenhouse growers could reduce their lighting costs and there’s the added benefit of lessening the daytime load on the electrical grid and reducing the greenhouse sector’s climate change footprint.