The North Carolina Cooperative Extension at North Carolina State University has an entire lab dedicated to researching how plant growth can be manipulated by light. With that lab at his disposal – and with PGRs being under regulatory scrutiny because of how they can harm the environment and growers always looking for ways to impact their growing – assistant professor Ricardo Hernandez set out to find out if LEDs were a viable solution.
“It was easy to justify and exciting to look at,” he says.
Hernandez’s research looking to compare three different types of supplemental lighting – LEDs, LEDs with plant growth regulators (PGRs) and high pressure sodium (HPS) bulbs with PGRs – against a control group of plants receiving no supplemental lighting. The three ornamental crops studied in Hernandez’s research were petunia plugs, dianthus and geraniums. He also studied how cucumber seedlings responded to red and blue flux ratios and the physiological responses of tomato seedlings under similar red and blue lights. Each species in the trial was picked because they are commonly grown and are in high demand.
According to Hernandez’s research, each plant responded differently to different lighting across the spectrum. Under LEDs, some became taller and more compact (in terms of height per unit weight) and colorful. Other species did not. In some cases, species grown under LEDs grew nearly equally to the same species grown under HPS bulbs and with PGRs.
In one case. Hernandez noted that the petunias trialed under LEDs had a "significantly higher" anthocyanin (red, purple, or blue pigments) concentration. Under blue light – six percent blue light in one case and nine percent blue light in another – as opposed to HPS bulbs that have very little blue light and more green light. Hernandez’s theory is that green light reverses the effect of any blue light in that fixture.
“The more we look at this, the more we realize that even the slightest change across the spectrum results in a change in the species,” Hernandez says. “When you compare supplemental LED lighting to no supplemental lighting plus PGRs, we found that plants under LEDs were more compact.”
Additionally, geraniums growing in one of the LED plus PGR trials had an “additive” effect in anthocyanin concentration.
Hernandez believes LEDs will one day completely overtake HPS lighting. But he notes that more trialing and research into which light colors work best with specific cultivars and species needs to be done in order to allow growers to use LEDs with more precision. Additionally, Hernandez believes a more universal spectrum needs to be found in order to make LEDs as viable as possible. But even so, his research indicates that LEDs are a potential alternative to chemical PGRs for dianthus, geranium and petunia plug production.
He adds that his research indicates that all five plants studied performed well under LEDs and with both treatments. Each individual plant, though, responded uniquely under each scenario. So, while it is not a perfect science just yet, LEDs are the future and the results are encouraging.
“It is not a one light spectrum fits all,’ Hernandez says. “But there is a lot of potential.”
Photo: Karen E. Varga