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  • Narrow Wavelength LED Light is New Rx for Picky Plants

BioPhotonics
Mar 2016
JAMES SCHLETT, EDITOR, james.schlett@photonics.com

It does not take a brown thumb to know plants are fickle. But this winter many green-thumb gardeners growing fruits or perennials from seed — in a spare bedroom or whatever doubles as their greenhouse — will not realize just how picky their plants are. Even if the seeds sprout, leaves unfurl and buds flower, their fragrance may not reach its full potential. That’s because fluorescent and high-pressure sodium (HPS) grow lights commonly used to support photosynthesis often fail to adequately deliver the wavelengths of light that result in optimum levels of aromatic volatile compounds.

Exposure to narrow wavelengths of LED light can make strawberries more flavorful and aromatic, but different cultivars of the fruit favor different wavelengths.

Exposure to narrow wavelengths of LED light can make strawberries more flavorful and aromatic, but different cultivars of the fruit favor different wavelengths. Courtesy of Thomas A. Colquhoun.

A growing body of research is showing plants will reach optimum levels for specific volatile compounds that influence aroma and flavor when they are exposed to narrow wavelengths of LED light or combinations of such wavelengths. Full spectrum lighting just will not do the trick for most plants, partly because wavelength intensity is not consistent throughout the spectrum.

The research is showing that to achieve optimum flavor and aroma, most plants want their wavelengths à la carte. And one cultivar, or variety, will not favor the same wavelength or wavelengths favored by another cultivar. In other words, the wavelengths that make one type of strawberry exceedingly flavorful and aromatic will not have the same effect on another type of strawberry, according to Thomas A. Colquhoun, an assistant professor of plant biotechnology at the University of Florida.

“We have to know the code for the plants to get what you want out of it, and that’s just a process of trial and error,” said Colquhoun.

In 2013, Colquhoun published one of the first — if not the first — papers showing light modulation of volatile metabolites in flowers. The paper, published in Postharvest Biology and Technology, showed how a far-red light treatment prompted a spike in the levels of 2-phenylethanol, an important floral volatile. More recently, Colquhoun was involved in the study of a basil variety grown in a greenhouse with exposure to natural light, and in an indoor setting under a combination of narrow-bandwidth LED lights. Compared to the basil plants grown in the greenhouse, those subjected to a specific LED light treatment had a 10 to 15 percent greater yield, had 30 to 40 percent more volatiles and 15 to 20 percent more antioxidants.

“We were able to outcompete the sun,” said Colquhoun.

Last fall, Gothenburg, Sweden-based LED grow-light manufacturer Heliospectra AB also conducted a feasibility study on the impacts lighting had on the aroma of basil. For this study, the plants were subjected to four different lighting regimens, with one consisting of HPS lights and the others involving LEDs. The research last December earned Heliospectra a 458,000 SEK (about $54,000) research grant from Vinnova, the Swedish innovation agency, to continue its research on using LED lighting to enhance plant quality.

“A significant characteristic of an herb is its aroma. This is a factor often overlooked when trying to maximize the crop production,” said Rebecca Nordin, a marketing and communications associate for Heliospectra.

Colquhoun, too, believes the enhancement of plant quality in terms of aroma and flavor is vital for the future of agriculture. Even though conventional grow lights’ blue and red wavelengths, which result in a telltale pink hue, do a good job at increasing biomass, they will not result in optimum plant aroma and flavor.

“People want more flavor in their tomatoes. They want more flavor in their strawberries. If we can make them more flavorful, people will eat more,” he said.

And, who knows, if one day the optimum narrow wavelengths are discovered for cultivars in the Rosaceae family, people will actually want to stop and smell the roses.


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