“My goal is to continue to do research with LEDs because we are finding new and exciting results, especially with the indoor production of young plants and microgreens,” said associate horticulture professor Roberto Lopez. “Some of the work that we have been doing has shown the benefits of LEDs.
“If you would have asked me two years ago if I would ever try to produce plugs indoors and not in a greenhouse, I would have said no. If you would have asked me five years ago if I would be working on greens or vegetables, I would have said no. Now I am doing both of those things with LEDs.”
No need for sunlight
Lopez and former graduate student Wesley Randall found that greenhouse-grown seedling plugs of impatiens, marigold, petunia, vinca and zonal geranium did as well or better when supplemented with LEDs compared to plugs supplemented with light from high pressure sodium lamps. What Lopez found surprising was the quality of the plugs produced in a growth room with LEDs as the only light source.“LEDs produce better plugs when they’re grown indoors than when they are grown in a greenhouse with sunlight supplemented with light from LEDs or high pressure sodium lamps,” Lopez said. “It is amazing how good the plugs look grown in an indoor multilayer production system with LEDs. The plugs are compact, sturdier and greener with a similar root and shoot dry mass to greenhouse-grown plants supplemented with light from LEDs or high pressure sodium lamps.”
One crop that Lopez said they are still “tweaking” with LEDs is petunias.
“Petunias, which are long day plants, when moved from an indoor grow room equipped with red and blue LEDs, encountered a slight delay in flowering in the greenhouse,” he said. “We are going to see if exposing the plants to far-red LED light prior to moving them into the greenhouse will induce them to flower.”
Using LEDs to intensify leaf, flower color
Lopez said many of the annual spring bedding plants grown in greenhouses in northern climates are produced under low light levels. The result is that some plants don’t produce the same intense foliage colors that they would if they were grown outdoors.“Plants grown in glass greenhouses are not exposed to the sun’s ultraviolet light because it is blocked by the glass,” he said. “The result is that crops like zonal geraniums and purple fountain grass (Pennisetum setaceum ‘Rubrum’) don’t “color up” like they would outdoors. One of the things we noticed with zonal geraniums was the dark patterns on the leaves stood out much more when the amount of blue light was increased. We hypothesized and found it was a result of an increase in anthocyanin production. We have also looked at geraniums that have very dark foliage and found not only does leaf color darken, but flower color can be made darker by exposing market-ready plants to red:blue LEDs.”
Lopez said the change in leaf color due to anthocyanin production was also dramatic for purple fountain grass.
“Purple fountain grass is a very popular ornamental species produced by many growers,” Lopez said. “Grown in the greenhouse, the leaves appear to be dull green and not very purple. We found that putting the plants under a combination of red and blue LEDs for one to two weeks of what we are calling “end-of-production lighting” resulted in an attractive purple color. UV light is what stimulates anthocyanin synthesis.”
He said in the case of purple fountain grass, only the leaves exposed to the LED lights change color. Those leaves not exposed to the LED light remain green.
Expanding studies to vegetable crops
Seeing the positive results that occurred with LEDs and purple fountain grass, Lopez and PhD student W. Garrett Owen expanded the research to red leaf lettuce to see if they could produce a similar response.“Trying to produce red leaf lettuce can be difficult for greenhouse growers if they are producing crops under low daily light integrals (DLIs),” Lopez said. “Growers producing red leaf lettuce under low DLIs are essentially producing green lettuce.
“We placed red leaf lettuce under the same LED treatments used for purple fountain grass and the plants colored up in three to five days. Based on our research, red leaf lettuce and purple fountain grass can be placed under a 50-50 red and blue LED combination prior to harvesting or shipping triggering anthocyanin formation.”
Based on the results related to LEDs and anthocyanin formation, Lopez said the studies may be expanded to look at the impact of LED light on ornamental cabbage and kale. “Growers, especially those in the South, have a hard time coloring up ornamental cabbage and kale,” he said. “It is primarily a temperature response, as the night temperatures get cooler the plants start to color up.”
Lopez and Owen did a small study placing ornamental cabbage and kale under LEDs that resulted in a minimal color change. When greenhouse-grown plants were grown under cool night temperatures and exposed to LEDs, they exhibited the most intense color.
“What we are proposing is for growers in warmer climates who have access to coolers, is to use a cool temperature/LED treatment,” he said. “We will be conducting this study next fall. Smaller container sizes like 4-inch pots, could be rolled on carts into a cooler and exposed to cool temperatures and LED lights for three to four days prior to shipping enabling the plants to color up.”
Customized microgreens
Another study conducted by graduate students Joshua Gerovac and Joshua Craver looked at the effect of LEDs on the growth of three different microgreen species (kohlrabi, mustard and mizuna) in an indoor multilayer production system. The study included three different light qualities and three different DLIs (light quantity).“Overall what we have seen is as the DLI increases, this is for three microgreen species we trialed, the length of the hypocotyl, basically the height of the microgreen, decreases,” Lopez said. “The more light the plants are provided, the more compact they are. If the plants received 6 moles of light, they were much taller than if they received 18 moles of light. Depending on the growers’ market, some customers might want microgreens that are a little leggier or they might want plants that are more compact. That will depend on market preference.”
The ideal LEDs
Lopez said the ideal vertical LED light module would contain all of the wavelength colors.“The vertical LED light with all the different colors would enable growers to turn them on when they need them and off when they don’t, depending on the stage of plant growth,” he said. “Once flowering begins a grower doesn’t want stem elongation. Far-red light works for flowering so the far-red would be turned on for the minimum amount of time required for flowering. If the grower wants to increase the amount of anthocyanin in the leaves or flowers, he can turn on the red and blue light near the end of the crop. To be able to turn on specific colors when a growers needs them, that is something I envision happening with LEDs.”
For more information
Purdue University
Roberto Lopez, , Department of Horticulture and Landscape Architecture
T: +1 (765) 496-3425
Email: [email protected]
https://ag.purdue.edu/hla/lopezlab/Pages/default.aspx