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Exploring a spectrum of possibilities

In addition to changing light levels, researchers are now using selective colours of light to achieve the desired effects on indoor crops. It’s a whole new playground.

December 3, 2019  By Q&A with Dr. Melanie Yelton

Dr. Melanie Yelton shares her insights on using light spectra.

From promoting root growth in ornamentals to increasing fresh weight in lettuce, applying a selective spectrum of supplemental light seems to be a gamechanger.

In In part two of our interview with Dr. Melanie Yelton, the former VP of research at LumiGrow shares her thoughts on emerging spectral research and what’s to come.

Why do plants respond differently to different colours of light?
Light is the only real way for plants to know whether winter is coming or what’s going on in their environment. As you might imagine, they are very attuned to different wavelengths. One that is very well-studied is far-red and its association with flowering.


As a general rule of thumb, blue light will stress a plant a little bit – it’s almost like us getting a suntan. The plants are asking themselves to make protective compounds to protect their tissue, which translates into a denser, more compact plant with thicker cuticles and leaves. A secondary benefit is this more robust leaf can offer a little protection against fungal infections. I’ve walked one particular cucumber house where they’ve had a couple of HPS acres on the left (low blue, high red light) and LED (high blue) environment to the right. Powdery mildew was all over the HPS side, but not very much at all under the LED side. More and more scientists are studying the underlying mechanism.

Red light is for getting that leaf to expand and the plant to stretch. This could be very important in tomato propagation if you want that long stem to give you a nice space to graft. You can push flowering more with a high-red environment as well. More and more, we’re starting to look into other wavelengths, too. I think there’s a lot of work still to be done on understanding where spectrum plays a role and how it can direct plant growth.

That’s true! We see a lot of literature on red and blue light, but less so about the wavelengths in between.
A lot of the work has been limited by the availability of different diodes. As we push the science further and further, it’s going to become more specific to different cultivars and really push the envelope. This is one of the reasons why LumiGrow decided to offer a broad white spectrum and a targeted spectrum, which can be controlled with the red and blue to target different plants and environments that require different strategies of growth.

A large part of my research this year will be contrasting those two and understanding when you want one or the other one. We’re working with Dave Llewellyn and Youbin Zheng at the University of Guelph on a cannabis propagation study to look at the impact of spectrum on root development. One size can do a good job, but I want to keep pushing the envelope to do the best. Understanding the impact of wavelength and how it affects growth – we’re just at the beginning of that science.

Would achieving the desired spectrum require feedback control?
We’re currently using the LumiGrow light sensor to adjust our fixtures in response to the light level in the greenhouse. The nice thing about the sky is that the spectrum’s not perfect, but it’s pretty consistent on a sunny day. There’s a bit of variation at sunrise/sunset and on a cloudy day. We know the percentage of red and blue light coming from the sun, and that never changes. As we become more sophisticated with using spectrum, spectrum-specific feedback control will come into play.

One of the things we did last year was use spectrum at night. Get all the light you can during the day, then we can use spectrum to direct the plant’s growth at night. If we can get the DLI we need during the day, we can give them a high blue environment at night to create that short thick plant, or give them a high red environment for elongation.

With all these new developments in lighting research, what are you most excited for?
The main ones I’m excited for are the integration of technology for all the different inputs for growing plants, and eventually knowing how to really tweak and use them. I feel like I’ve seen this beautiful symphony of what’s about to happen.

In growing tomatoes you want a plant that’s pretty stressed all the time to make it flower. You don’t want a happy plant, you want a stressed plant that will stay in reproductive mode. Growers have become very adept at alternating between fertigation and dry back – it really is like a dance. I think we can do that with light and heat and temperature drops to push the photosynthates into the fruit.

20 years ago, people weren’t thinking about how temperature drops affect fruit quality. Now we can, and we do them. It’s about integration, and light wasn’t even on the table for a long time. All you could do was turn it on and off. We can use temperature and fertigation as tools, but light wasn’t considered to have enough variability to use. Now light has entered center stage – we can control the spectrum, we can control the intensity. We can really use light in a way that we could never before. The fact that we can grow more nutritious plants, plants that taste better, and cannabis that has greater medicinal properties – that’s just thrilling to me. I’m excited for the integration that’s going to happen.

Editor’s note: Responses were edited for length. See here for part 1 of our interview with Melanie.

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