In a comparison study, Rensselaer researchers found that Luxeon SunPlus Lime + Purple and Green + Purple LEDs provide a beneficial growth spectra. Furthermore, they found that the growth spectra resulted in high crop yield and high levels of antioxidants critical to human health.
The NSF funded Center for Lighting Enabled Systems and Applications (LESA) at Rensselaer Polytechnic Institute (RPI) in Troy, New York conducted the controlled horticulture lighting study and released the results. The study, which employed a variety of commercial LEDs, compared the effectiveness of the different LEDs on the growth of red lettuce Rouxai. The researchers determined that the best-combined yield and antioxidant concentrations in the lettuce used the Luxeon SunPlus Series LEDs.
“Comparing the growth results using different combinations of LEDs shows that the spectra that performed best for yield (fresh weight), anthocyanin and chlorophyll concentrations were the LUXEON SunPlus Lime + Purple and Green + Purple LEDs from Lumileds,” said LESA’s Dr. Tessa Pocock, a global expert on light and plant physiology. “Green light is necessary for crop growth and the specific wavelengths within the green region matters.”
LESA and Cornell university run the Greenhouse Lighting and Systems Engineering (GLASE) Consortium for which the research work was done. Funding for the research came from the New York State Energy Research and Development Authority (NYSERDA). The consortium was is attempting to find the most effective way to add green light to horticultural fixtures.
The researchers pointed out that if grown properly, salad crops can be an excellent source of nutrients including antioxidants, which help control the level of damaging free radicals in the body. The study monitored and quantified the concentrations of chlorophyll and two significant bioactive antioxidants, the anthocyanins and the carotenoids.
Carotenoids make up a family of yellow and orange pigments including betacarotene (the precursor of vitamin A) and lutein and zeaxanthin, which are yellow pigments that protect the retina against high energy radiation such as blue and UV light. Anthocyanins are red, blue and purple pigments and their dietary uptake is positively correlated with human health including the treatment of vision disorders, the protection against neurological disorders, increase in cognitive ability, the reduction in incidence of cardiovascular disease, and enhancement of antioxidant protection.
The researchers point out that previous studies using commercially available LEDs demonstrated that many leafy greens grown under phosphor converted (PC) LEDs featured large, thin and pale leaves with consistently lower levels of chlorophyll and anthocyanins.
The researchers designed the present study to evaluate the performance of Lumileds’ new commercially available PC LEDs (LUXEON SunPlus Series) designed for horticulture compared to Valoya LED grow lights and custom built direct emission RGB LEDs. The study’s goal was determining whether recently introduced phosphor converted (PC) LEDs could improve the yield of red lettuce while producing high levels of essential nutrients including anthocyanins and carotenoids. The study also examined the relationship or compromise between yield (above ground biomass) and nutritional quality of red lettuce Rouxai.
Lighting Configuration and Methods
The researchers selected spectral photon distributions to provide photosynthetic photon flux densities (PPFDs) at the crop level with identical output in red (600-700 nm), blue (400-500 nm) and green (500-600 nm) wavelength ranges (spectral ratios) between fixtures.
They installed two fixtures in each environmentally controlled growth chamber (Adaptis 1000, Conviron), then they adjusted the distance to the crops and set the light programs to attain PPFDs between 217-242 µmol/m2s. They then measured the spectral photon distributions in the center of the chambers and in a grid spanning the growth area to look at uniformity. The researchers set the spectral ratios to be identical across the photosynthetic active radiation (PAR); 20% blue (400-500 nm), 20% green (500-600 nm) and 60% red (600-700 nm).
The LED lights did differ in PPFD in the far red region (700-800 nm). While far red emission was absent from the RGB direct emission spectra, it reached 19 µmol in the Valoya PC to 25 µmol in the LUXEON SunPlus Lime+ Purple fixture, 28 µmol in the LUXEON SunPlus Green + Purple fixture and 32 µmol in the LUXEON SunPlus Purple fixture.
The photoperiods were 16 hours/day for 14 days, the day/night temperatures were 23°C/18°C and the relative humidity was
between 50% and 70%. The plants were fertilized with a modified Hoagland’s solution. Anthocyanin concentrations were quantified spectrophotometrically, and the carotenoid and chlorophyll concentrations were also quantified.
The researchers observed significantly higher anthocyanin concentrations in Rouxai grown under LUXEON SunPlus Lime + Purple, Green + Purple and RBG spectra compared to Valoya However the light treatments did not affect carotenoid concentrations, and, though not significant, the chlorophyll concentrations were found to be consistently higher under the LUXEON SunPlus Series and the RGB LEDs compared to Valoya grow lights.
“Growers today are experimenting with various LEDs to arrive at the best color combinations for their crops. The LESA study indicates that such experimentation is paying off because a common, nutrient-rich crop can be produced with excellent yield,” said Jennifer Holland, product line director of the LUXEON SunPlus Series.
The study demonstrated that a light spectrum such as the combination of LUXEON SunPlus Lime and Purple, which contains a particular ratio of royal blue, green, deep red and far-red components, is beneficial to growth optimization.