Lighting with Red and Blue Light-emitting Diodes (LEDs) Influences Red Pigmentation of Four Lettuce Varieties

Under low-light greenhouse conditions, such as those found in northern latitudes, foliage of red leaf lettuce (Lactuca sativa L.) varieties is often green and not visually appealing to consumers. Our objective was to quantify the effect of end-ofproduction (EOP; prior to harvest) supplemental lighting (SL) of different sources and intensities on foliage color of four red leaf lettuce varieties, ‘Cherokee’, ‘Magenta’, ‘Ruby Sky’, and ‘Vulcan’. Plants were finished under greenhouse ambient solar light and provided with 16-hours of day-extension lighting from low intensity light-emitting diode (LED) lamps [7:11:33:49 blue:green:red:far red (control)] delivering 4.5 mmol·m·s, or 16-hours of EOP SL from high-pressure sodium (HPS) lamps delivering 70 mmol·m·s, or LED arrays [100:0, 0:100, or 50:50 (%) red:blue] delivering 100 mmol·m·s, or 0:100 blue LEDs delivering 25 or 50 mmol·m·s. Relative chlorophyll content (RCC) and foliage L* (lightness), and chromametric a* (change from green to red) and b* (change from yellow to blue) values were significantly influenced by EOP SL and days of exposure. Generally, RCC of all varieties increased from day 3 to 14 when provided with EOP SL from the HPS lamps and LEDs delivering 100 mmol·m·s. End-of-production SL providing 100 mmol·m·s of 100:0, 0:100, or 50:50 red:blue light for ‡5 days resulted in increasing a* (red) and decreasing L* (darker foliage), b* (blue), and h8 (hue angle; a measure of tone) for all varieties. Our data suggests that a minimum of 5 days of EOP SL providing 100mmol·m·s of 100:0, 0:100, or 50:50 red:blue light enhanced red pigmentation of ‘Cherokee’, ‘Magenta’, ‘Ruby Sky’, and ‘Vulcan’ leaves when plants are grown under a low greenhouse daily light integrals (DLIs) <10 mol·m·d. The production of vegetables and leafy greens in protected and controlled environments can reduce threats associated with field production, and offers the ability to produce crops during the off-season (Zabelitz, 1986). Specifically, greenhouses are controlled environments (Gruda, 2005) where light (quantity, quality, and duration), temperature, relative humidity, CO2 concentration, and water and nutrient availability are adjusted (Gary, 2003) for optimal plant growth and development. However, ambient photosynthetic DLIs are reduced by up to 50% or more from the greenhouse glazing material, superstructure, and shading (Hanan, 1998). Several studies have indicated that in northern latitudes, the most limiting environmental factor in greenhouse vegetable production is low light (Benoit, 1987; Gaudreau et al., 1994; Grimstad, 1987). For example, Benoit (1987) and Gaudreau et al. (1994) reported that low light levels resulted in the formation of loose heads and low fresh weight of lettuce (Lactuca sativa L.), whereas Kleinhenz et al. (2003) reported that shading reduced anthocyanin content of three lettuce cultivars. Therefore, growers can use high-intensity discharge lamps (HID) such as HPS or metal halide lamps for SL and increase the DLI in the greenhouse. High-intensity LEDs are a promising new SL technology that offer many benefits over the commercially available lamps commonly used in horticulture for SL (G omez et al., 2013; Morrow, 2008). They are solid-state, semiconducting diodes that can emit light from 250 to 1000 nm or greater (Bourget, 2008). The development of LEDs with an output of 1 W or greater has created the potential to use aggregates of LEDs (arrays) as supplemental photosynthetic light sources (Currey and Lopez, 2013). Additionally, LEDs provide narrow-spectrum light in wavebands suitable for plant growth and development, including blue (450 nm), red (660 nm), and far red (730 nm). Several studies have investigated growth, development, and physiological responses of lettuce grown under LEDs as sole-source lighting (SSL) in indoor-controlled environments (Johkan et al., 2012; Li and Kubota, 2009; Lin et al., 2013; Yorio et al., 2001) or as an SL in greenhouses (Samuolien e et al., 2012). For example, Son and Oh (2013) found combinations of 65:35, 53:47, and 41:59 red (655 nm):blue (456 nm) SSL LEDs to increase chlorophyll content, total phenolic concentration, flavonoid concentration, and antioxidant capacity of lettuce ‘Sunmang’ (red leaf) and ‘Grand Rapid TBR’ (green leaf), whereas 100:0 red:blue SSLLEDs to influence lettuce morphology and growth. Thus, the use of LEDs for lettuce growth, development, and spectrum-dependent plant photo-physiological responses is well documented (Samuolien e et al., 2012). In addition, anthocyanin content of lettuce leaves has been investigated and measured (Li and Kubota, 2009; Samuolien e et al., 2012) and is responsible for the red pigmentation in leaves. Additionally, these pigments have been found to act as potent antioxidants and antibacterial agents (Kong et al., 2003; Richards et al., 2004). Anthocyanin concentration in foliage is dependent on environmental conditions, such as light (quality and intensity) and temperature. Synthesis and accumulation of anthocyanins are induced at high light intensities (Steyn et al., 2002); for example, Richards et al. (2004) reported that total anthocyanin content of lettuce ‘Outredgeous’ and ‘Red Sails’ increased as light intensity increased from 150 to 450 mmol·m·s. It has also been found that ultraviolet [ultraviolet-B (290 to 320 nm) and ultraviolet-A (320 to 400 nm)], blue (400 to 480 nm), red (600 to 690 nm), and far red (710 to 760 nm) light are responsible for stimulating anthocyanin production (Mancinelli, 1983; Mol et al., 1996). For example, Stutte (2009) reported the addition of blue (440 nm) light significantly increased anthocyanin concentration of leaf tissue and altered the developmental morphology of lettuce plants. Therefore, light influences the amount and distribution of anthocyanins that contribute significantly to leaf color (Gazula et al., 2007). In horticultural crops, color is a key component that influences and registers with a consumer’s initial perception of product quality (Lightbourn et al., 2008; Ryder, 1999) and appeal (Gazula et al., 2007). For example, leaf color (intensity, distribution, or both) is an important quality parameter in lettuce (Gazula et al., 2005). Leaf color is determined primarily by the spectral properties of leaf pigments. The conventional in vitro Received for publication 23 Dec. 2014. Accepted for publication 18 Feb. 2015. We gratefully acknowledge Rob Eddy, Dan Hahn, and Wesley Randall for greenhouse assistance; Alyssa and Andrea Hilligoss for laboratory assistance; and Judy Santini for experimental design and statistical consultation. We thank Sun Gro Horticulture for substrate and Everris for fertilizer, the USDA-NIFA SCRI grant no: 2010-51181-21369 for funding, and Philips Lighting, Hort Americas, and Orbital Technologies Corp for LEDs. Use of trade names in this publication does not imply endorsement by Purdue University of products named nor criticism of similar ones not mentioned. Associate professor and extension specialist. To whom reprint requests should be addressed; e-mail [email protected]. 676 HORTSCIENCE VOL. 50(5) MAY 2015 methods for measurement are both destructive and time consuming, involving chlorophyll extraction followed by spectrophotometric measurements (Madeira et al., 2003). However, in vivo chlorophyll measurements can be determined with a portable chlorophyll content meter, resulting in nondestructive and rapid measurements of leaf chlorophyll content based on spectral transmittance properties of leaves (Madeira et al., 2003). While, portable tristimulus colorimeters are used to measure the spectral reflectance properties, such as lightness and chromaticity of fruit and leaf color. Madeira et al. (2003) and Le on et al. (2007) demonstrated the feasibility of estimating chlorophyll content and color of sweet pepper (Capsicum annum L. ‘Capistrano’) and butterhead lettuce ‘Lores’, respectively. To our knowledge, no published information exists on EOP (prior to harvest) SL from LEDs to enhance leaf color of greenhousegrown red leaf lettuce. Therefore, the objectives of this study were to quantify and compare the effects of EOP SL from HPS lamps to LEDs of different light intensities, light qualities, and days of exposure on the color of four red leaf lettuce varieties. The four red leaf lettuce varieties selected varied in color, leaf morphology, and are available for commercial production. Materials and Methods Culture and greenhouse environment. On 30 Sept. (Rep. 1) and 14 Oct. 2013 (Rep. 2), seeds of lettuce ‘Cherokee’, ‘Magenta’, ‘Ruby Sky’, and ‘Vulcan’ (Johnny’s Selected Seeds, Winslow, ME) were sown into 72-cell plug trays (30.7 mL individual cell volume; Landmark Plastics, Akron, OH) filled with a commercial soilless medium composed of (by volume) 65% peat, 20% perlite, and 15% vermiculite (Super Fine Germinating Mix; Sun Gro Horticulture, Agawam, MA). Seedlings were irrigated as necessary with acidified water supplemented with water-soluble fertilizer (Jack’s LX 16N–0.94P–12.3K Plug Formula for High Alkalinity Water; J.R. Peters, Inc., Allentown, PA) to provide the following (mg·L): 100 N, 10 P, 78 K, 18 Ca, 9.4 Mg, 0.10 B, 0.05 Cu, 0.50 Fe, 0.25 Mn, 0.05 Mo, and 0.25 Zn. On 21 Oct. (Rep. 1) and 04 Nov. (Rep. 2), 21 d old seedlings were transplanted into 12.7-cm (885 mL) diameter containers (ITML Horticultural Products, Middlefield, OH) filled with a commercial soilless medium comprised of (by volume) 65% peat, 20% perlite, and 15% vermiculite (Fafard 2; Sun Gro Horticulture, Agawam, MA). Plants were irrigated as necessary with acidified water supplemented with a combination of two water-soluble fertilizers [3:1 mixture of 15N–2.2P–12.5K and 21N–2.2P–16.6K, respectively (Everris, Marysville, OH)] to provide the following (mg·L): 200 N, 26 P, 163 K, 50 Ca, 20 Mg, 1.0 Fe, 0.5 Mn and Zn, 0.24 Cu and B, and 0.1 Mo. Plants were grown in a glass-glazed greenhouse at Purdue University, West Lafayette, IN (lat. 40 N) with exhaust fan and evaporative-pad cooling, radiant hot water, and retractable shade curtains controlled by an environmental control system (Maximizer Precision 10; Priva Computers Inc., Vineland Station, ON, Canada). Amplified quantum sensors (SQ-212; Apogee Instruments, Inc., Logan, UT) measured photosynthetic photon flux (PPF) every 15 s and the average of each sensor was logged every 15 min by a data logger (WD 2800; Spectrum Technologies, Inc., Aurora, IL). Air temperature was monitored and recorded by a separate aspirated Priva sensor. The greenhouse day and night air temperature set points were 20/18 C (12 h/12 h). The photoperiod was a constant 16 h (0600 to 2200 HR) consisting of natural daylengths with day-extension lighting from HPS lamps (e-system HID; PARSource, Petaluma, CA) that delivered a supplemental PPF of 70 mmol·m·s at plant height. The DLI and average daily temperatures (ADTs) for Reps. 1 and 2 were 8.2 ± 1.2 and 8.3 ± 1.6 mol·m·d and 19.1 ± 2.1 and 19.0 ± 1.6 C, respectively. End-of-production greenhouse environment and supplemental lighting treatments. On 4 Nov. (Rep. 1) and 18 Nov. 2013 (Rep. 2), 35-d-old plants were moved to a glass-glazed greenhouse where the day and night air temperature set points were a constant 18 C. Five plants of each variety were placed under a 16-h photoperiod consisting of either ambient solar light plus day-extension light (control; no EOP SL) or EOP SL from 0600 to 2200 HR. Dayextension lighting consisted of two (7:11:33:49 blue:green:red:far red) low intensity LED lamps (Philips GreenPower Flowering deep red/white/ far red LED lamp; Koninklijke Philips Electronics N.V., The Netherlands). Supplemental light was delivered from a 150 W HPS lamp (PL 2000; P.L. Light Systems Inc., Beamsville, ON, Canada) or one of five LED arrays (Orbital Technologies Corporation, Madison, WI) providing monochromatic red [100:0 red (660 nm):blue], monochromatic blue [0:100 red:blue (460 nm)], or a combination of red and blue (50:50 red:blue) light (Table 1). Spectral scans of the control and EOP SL treatments were taken at night at the beginning of each replication with a spectroradiometer (PS100; StellarNet, Inc., Tampa, FL). Spectral quality of light sources is shown in Figure 1. Amplified quantum sensors (SQ-110; Apogee Instruments, Inc., Logan, UT) measured solar PPF every 15 s and the average of each sensor was logged every 15 min by a data logger (WD 2800; Spectrum Technologies, Inc.). The average solar DLI and air temperature after 14 d for Reps. 1 and 2 were 7.1 ± 3.8 and 6.6 ± 2.1 mol·m·d and 18.5 ± 1.3 and 18.4 ± 0.9 C, respectively. The supplemental DLI for each treatment was calculated and is reported in Table 1. Data collection. At 0, 3, 5, 7, and 14 d after initiating EOP SL, total chlorophyll (a+b) content (i.e., RCC) was estimated using a SPAD chlorophyll meter (SPAD-502; Konica Minolta Sensing, Inc., Osaka, Japan) by measuring two recently matured leaves of each plant under each lighting treatment. Leaf color of the same two recently matured leaves was measured using a portable tristimulus colorimeter (CR-200; Konica Minolta Sensing, Inc., NJ) equipped with a measuring head with a self-contained light source that provided diffuse, uniform light over an 8-mm diameter measuring area. The analyzer was calibrated to a standard white reflective plate (L* = 97.5, a* = 0.40, b* = 1.90) using the CIE (Commission Internationale de l’Eclairage) 1976 (L*a*b*) color coordinates. The L* value indicates darkness and lightness (black: L* = 0; white: L* = 100). Chromametric a* value is the ratio between greenness and redness (green: a* = –60; red: a* = +60) and chromametric b* value is the ratio between blueness and yellowness (blue: b* = –60; yellow: a* = +60). On a circular scale, hue angle (h ) or tone indicates redness (0 ), yellowness (90 ), greenness (180 ), or blueness (270 ) and were calculated as