The study was conducted by Doctor Tessa Pocock, Senior Research Scientist at the Center for Lighting Enabled Systems and Applications (LESA), Rensselaer Polytechnic Institute, Troy, NY.
LEDs are rapidly becoming the light source of choice for indoor horticultural applications due to the ability to customize the light spectra to meet the growth and nutritional needs of the plant of interest—something that legacy horticulture light sources cannot do. In addition, LEDs have superior lifetimes, greater energy efficiency versus fluorescent tubes used in vertical farms and plant factories, require low to no maintenance and emit less heat into the growing environment.
Salad crops are consumed for their nutritive and health benefits rather than for caloric requirements. If grown properly, they can be an excellent source of nutrients including antioxidants, which help control the level of damaging free radicals in the body. In this study, the concentrations of chlorophyll as well as two important bioactive antioxidants—the anthocyanins and the carotenoids—were monitored and quantified. Carotenoids are a family of yellow and orange pigments including beta carotene (the precursor of vitamin A) and zeaxanthin and lutein, which are yellow pigments that protect the retina against high energy radiation (eg. UV and blue light)*. Anthocyanins are red, blue and purple pigments and their dietary uptake is positively correlated with human health such as in the treatment of vision disorders, protection against neurological disorders, reduction in incidence of cardiovascular disease, increase in cognitive ability and enhancement of antioxidant protection*. Previous studies using commercially available LEDs indicated that many leafy greens grown under phosphor converted (PC) LEDs had large, thin and pale leaves with consistently lower levels of chlorophyll and anthocyanins*.
The present plant growth study was conducted to evaluate the performance of new commercially available PC LEDs designed for horticulture from Lumileds (LUXEON SunPlus Series) in comparison to Valoya and custom built direct emission RGB LEDs. The goal was to determine whether recently introduced phosphor converted (PC) LEDs could improve the yield of red lettuce while producing high levels of key nutrients including anthocyanins and carotenoids. The relationship or compromise between yield (above ground biomass) and nutritional quality of red lettuce Rouxai was examined.
Lighting Configuration and Methods
Spectral photon distributions were selected to deliver photosynthetic photon flux densities (PPFDs) at the crop level with identical output in red (600-700nm), blue (400-500nm) and green (500-600nm) wavelength ranges (spectral ratios) between fixtures. Two fixtures were installed in each environmentally controlled growth chamber (Adaptis 1000, Conviron) with PPFDs between 217-242 μmol/m2s by setting light programs and/or adjusting distance to the crop (Figure 1). The spectral photon distributions in the center of the chambers and in a grid spanning the growth area were measured to examine the uniformity (Figure 2). The spectral ratios were designed 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). There was, however, a difference in PPFD in the far red region (700-800nm). It was absent from the RGB direct emission spectra but ranged from 19 μmol in the Valoya PC to 25μmol in the LUXEON SunPlus Lime + Purple fixture, 28μmol in the LUXEON SunPlus Purple + LUXEON 3535L Green fixture and 32 μmol in the LUXEON SunPlus Purple fixture. The light analyses indicated that spectral and PPFD uniformities were poorer on average from side to side of the chambers and significantly better from front to rear. A larger installation of light bars would improve these uniformity discrepancies. 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. Three independent replicates were performed and statistical differences between light trials were determined (KruskalWallis ANOVA, SigmaPlot v11). Samples from growth areas with the greatest light uniformity were harvested for measurements. Anthocyanin concentrations were quantified spectrophotometrically according to the method in Pocock*. The carotenoid and chlorophyll concentrations were quantified according to the method in Lichtenthaler*.
Figure 1: Rouxai red lettuce is grown for 14 days under LUXEON SunPlus Lime + Purple, LUXEON SunPlus Purple + LUXEON 3535L Green and LUXEON SunPlus Purple, Valoya, and RGB fixtures in Adaptis chambers.
Figure 2: Spectral photon distribution measured as photon flux density (PFD) for each light source. To enable uniform comparison, SPD was measured at the center of each chamber at the crop level.
Yield, Plant Health and Antioxidants
Yield, as determined by fresh weight (g), anthocyanin, carotenoid and chlorophyll concentrations and photochemistry in red lettuce cultivar Rouxai grown under the different light treatments were examined. Yields were significantly higher in seedlings grown under LUXEON SunPlus Series Lime + Purple and Purple + Green LEDs and Valoya, followed by LUXEON SunPlus Series Purple only and then RGB (Figure 3). The lack of far red (700-800 nm) output in RGB could account for the low fresh weight of lettuce grown compared to LUXEON SunPlus Lime + Purple and Purple + Green and the Valoya fixtures. However, the low fresh weight in the plants grown under purple that contain high far red does not fit this explanation. It is well accepted that far red induces cell elongation and leaf size. Significantly higher anthocyanin concentrations were observed in Rouxai grown under LUXEON SunPlus Lime + Purple, Purple + Green and RBG spectra compared to Valoya (Figure 4). Carotenoid concentrations were not affected by the light treatments and, though not significant, the chlorophyll concentrations were consistently higher under the LUXEON SunPlus Series and the RGB LEDs compared to Valoya (Figures 5 and 6). Plant health can be measured through the technique of chlorophyll fluorescence using a pulse amplitude modulated chlorophyll fluorometer (PAM 2500, Walz, DE)*. Plant health (FV/FM), the efficiencies of light conversion in the plant (Y(II)) and photoprotection (NPQ) were in healthy ranges indicating that there was no stress on the processes of photosynthesis (Figure 7) in these studies. The plants were well acclimated to all light sources.
Leafy greens are desirable crops for daily eating due to their nutritive value. This study showed that red lettuce Rouxai grown under specific LED spectra can have significantly higher yields as well as greater concentrations of anthocyanins and carotenoids.
The LUXEON SunPlus Lime + Purple LEDs performed best in terms of yield (fresh weight) and antioxidant levels. The spectrum of this light source includes output in the far red range (700-800nm) that can boost lettuce yields. The Valoya PC LEDs produced greens of comparable yield but statistically lower levels of antioxidants, indicating that the spectral photon distribution of phosphor down conversion can have different effects on crop quality. In summary, these data suggest that growth and nutrition are optimized using PC LEDs with strong royal blue, green, deep red and far red components found in the combination of LUXEON SunPlus Lime + Purple LEDs.
The spectra that performed best for both yield (fresh weight) and nutrient concentrations were the LUXEON SunPlus Series Lime + Purple and Purple + Green LEDs. To effectively grow Rouxai red lettuce and similar crops in indoor environments, the use of Lime + Purple and Purple + Green LEDs from Lumileds demonstrated significant advantages.
Figure 3: The yield (fresh weight) of 14 day old red lettuce was produced using LUXEON SunPlus Lime + Purple, Valoya and LUXEON SunPlus Purple + LUXEON 3535L Green fixtures.
Figure 4: Anthocyanin concentration was highest for LUXEON SunPlus Lime + Purple and LUXEON SunPlus Purple + LUXEON 3535L Green LEDs.
Figure 5: All LED light treatments boosted carotenoid concentration.
Figure 6: Chlorophyll concentration was highest for RGB and LUXEON SunPlus Series grown lettuce.
Figure 7: The highest yield (fresh weight) of 14 day old red lettuce was produced using LUXEON SunPlus Lime + Purple, Valoya and LUXEON SunPlus Purple + LUXEON 3535L Green fixtures.
Light grids measured with the JAZ spectrometer within the growth chambers. Plant material used for these trials were grown under and harvested from the center portion of the chambers to ensure the highest light uniformity
WP34 LUXEON SunPlus Series Lime LEDs Produce High Yield and Nutrition in Leafy Greens White Paper 20171213 ©2017 Lumileds Holding B.V. All rights reserved.
* References upon request