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Redefining the McCree Curve at Utah State University

Redefining the McCree Curve at Utah State University

Evaluating Photosynthetically Active Radiation with Fluence LED Systems for Increased Photosynthesis

“LEDs are causing paradigm shifts in our understanding of photobiology” – Dr. Bruce Bugbee, Ph.D. Department of Plants, Soils and Science; Utah State University

For far too long, outdated research has limited horticulturalists’ understanding of photosynthetically active radiation. Otherwise referred to as PAR, these are the critical wavelengths of light between 400 and 700nm that drive plant growth.

Seminal experiments by Dr. Keith McCree in the 1970s defined range of PAR; however, many plant scientists (including McCree himself) have known that wavelengths outside PAR can effectively drive photosynthesis. This has led to a fragmented understanding of how plants respond to wavelengths of light. Because of this knowledge gap, crop productivity in commercial horticulture has been less than optimal.

And so, for forty years, the McCree Curve has been PAR gospel.

Until now.

“The McCree Curve is misleading,” says Dr. Bruce Bugbee of Utah State University. “It’s a little early to say the definition of photosynthetically active radiation should be modified, but that’s what we’re finding.”

As a preeminent researcher and author/co-author of nearly 200 peer-reviewed journal articles, Bugbee finds fundamental importance in the synergy of different wavelengths. While McCree’s research relied on single-wavelength treatments in single-leaves, Bugbee’s current experiments take a holistic approach to evaluating plant responses to spectral composition. In particular, he’s investigating whole-plant response to wavelengths outside of PAR that have been considered to be unused by plants—and finding that they substantially impact photosynthesis and growth. His soon-to-be published research will prove pivotal to controlled environment agriculture and, in the coming years, commercial cultivators can expect better yields per kilowatt hour from their LED fixtures.

Using combinations of “white” and single-wavelength LED lights from Fluence Bioengineering, Bugbee is precisely manipulating full-spectrum light in ways McCree could not. McCree was limited to the legacy technologies of prisms and light filters to create isolated spectra to measure single leaf (not whole plant) photosynthesis at low light, which prove misleading in high-intensity light and real-world growing conditions.

“With fixtures like we have from Fluence, we can test physiologically relevant light levels. At less than 100 µmol/m2/s [as used by McCree], plants are starving for light, and their response to a given wavelength can be different than it would be in full sunlight,” says Bugbee.

McCree’s original experiments used only a single leaf to assess CO2 uptake (photosynthesis) at each wavelength. Bugbee and his team are using a whole-plant gas-exchange chambers —and a longer test length of light exposure—to provide more realistic measurements. Additionally, Bugbee is assessing modulating the test wavelengths in combination with a full-spectrum light rather than in isolation like McCree. And while McCree had to obtain single-wavelength light through filters, the new research employs precise control of spectrum and light levels.

With Fluence LEDs, Bugbee is achieving better design of experiment and precisely controlled spectral modifications. “We’re very grateful to Fluence for designing lighting systems that enable a new frontier of photobiology insights,” says Bugbee. “The LEDs make this research possible.”

An Addition to the PAR Spectrum

“Wavelengths are synergistic. The best analogy I can use is a balanced diet,” says Bugbee. He explains that a robust component of various wavelengths both within and outside of PAR allows greater rates of photosynthesis than narrow-band spectra, achieving an effect that’s greater than the sum of the parts. “You have to have all the nutrients for proper growth.”

Fluence, Bugbee, and his team are providing a more nuanced approach to the PAR spectrum with a deeper exploration of what constitutes a “balanced diet” of light. After decades in the lab, he believes we’ve only begun to understand the interdependence of plants’ metabolic processes.

“We’re in the early days of photobiology. Our previous understanding is incomplete, and we’re doing our best to put the new puzzle pieces together with new technologies like LEDs,” says Bugbee, who intends to continue applying his expertise in areas where he believes it to be the most seminal.

“I tell my graduate students it’s important for our lab to do the fundamental research. Then, we let other people—like Fluence—take it from there apply the results in commercial horticulture.”