Tomatoes Grown with Light-emitting Diodes or High-pressure Sodium Supplemental Lights have Similar Fruit-quality Attributes

Light-emitting diodes (LEDs) are an attractive alternative to high-pressure sodium (HPS) lamps for plant growth because of their energy-saving potential. However, the effects of supplementing broad-waveband solar light with narrow-waveband LED light on the sensory attributes of greenhouse-grown tomatoes (Solanum lycopersicum) are largely unknown. Three separate studies investigating the effect of supplemental light quantity and quality on physicochemical and organoleptic properties of greenhousegrown tomato fruit were conducted over 4or 5-month intervals during 2012 and 2013. Tomato cultivars Success, Komeett, and Rebelski were grown hydroponically within a high-wire trellising system in a glass-glazed greenhouse. Chromacity, Brix, titratable acidity, electrical conductivity (EC), and pH measurements of fruit extracts indicated plant response differences between lighting treatments. In sensory panels, tasters ranked tomatoes for color, acidity, and sweetness using an objective scale, whereas color, aroma, texture, sweetness, acidity, aftertaste, and overall approval were ranked using hedonic scales. By collecting both physicochemical as well as sensory data, this study was able to determine whether statistically significant physicochemical parameters of tomato fruit also reflected consumer perception of fruit quality. Sensory panels indicated that statistically significant physicochemical differences were not noticeable to tasters and that tasters engaged in blind testing could not discern between tomatoes from different supplemental lighting treatments or unsupplemented controls. Growers interested in reducing supplemental lighting energy consumption by using intracanopy LED (ICLED) supplemental lighting need not be concerned that the quality of their tomato fruits will be negatively affected by narrow-band supplemental radiation at the intensities and wavelengths used in this study. The negative reputation associated with the quality of off-season tomatoes has long been a subject of investigation (Stevens et al., 1977, 1979). It has been confirmed through sensory studies that consumers tend to be dissatisfied with tomatoes grown during the off-season (Kader et al., 1977; Watada and Aulenbach, 1979) and prefer vine-ripened tomatoes grown in the field during the summer compared with indoor-ripened fruits (Bisogni and Armbruster, 1966). Greenhouse tomatoes grown during the off-season in sunny, warm regions are shipped at least 2300 km to consumers in the northern United States (Pirog and Van Pelt, 2002). Many of these tomatoes are harvested at the mature green stage and forced to ripen in transit or at their destination with ethylene gas. Ethylene ripening treatments, fluctuating storage temperatures, and physical damage all lead to a drastic reduction in postharvest fruit quality (Kader, 1986; Kader et al., 1977). To satisfy growing consumer demand for locally grown, fresh tomatoes during the off-season, greenhouse tomato growers in northern climates increasingly rely on supplemental lighting to compensate for the naturally low solar daily light integral (DLI), or the daily amount of photosynthetically active radiation (PAR) received by a plant, during winter months (Dorais et al., 1991; Korczynski et al., 2002). Greenhouse supplemental lighting is typically provided from overhead HPS (OH-HPS) lamps, emitting an orange-biased spectrum and producing large amounts of radiant waste heat. Some of the most efficient HPS lamps have an energy conversion efficiency of 1.70 mmol·J, which is commensurate with some of the most efficient LED arrays (1.66 mmol·J) presently available (Nelson and Bugbee, 2014). However, the HPS fixtures most widely used by commercial growers have an electrical conversion efficiency of only 1.02 mmol·J. Because energy is the second largest indirect cost for greenhouse crop production (Frantz et al., 2010), there is a clear and present need for more efficient sources of supplemental lighting if highquality produce is to be grown affordably in northern climates during the off-season. Light-emitting diodes are becoming a viable alternative to OH-HPS supplementation because of their relatively high energy efficiency, low radiant heat, long life span, and ability to emit specific narrow wavebands of light (Morrow, 2008; Nelson and Bugbee, 2014). These attributes have enabled scientists to determine metabolic, morphological, and physiological plant responses to specific wavelengths of light. There is great interest in the potential to influence the phytochemical and flavor profile of various high-value crops including but not limited to arugula (Eruca sativa) (Mattson and Harwood, 2012), kale (Brassica oleracea) (Lefsrud et al., 2008), and lettuce (Lactuca sativa) (Li and Kubota, 2009; Lin et al., 2013; Samuoliene et al., 2012, 2013; Stutte et al., 2009; Zukauskas et al., 2011). Some studies with high-light-requiring crops such as cucumber (Cucumis sativa) and pepper (Capsicumannum) have been conducted to quantify crop yield when grown with ICLEDs (Hao et al., 2012; Jokinen et al., 2012). However, little fruit quality-attribute work with LEDs has been done on a long duration, full grow-out of tomatoes. Kowalcyzk et al. (2012) compared tomatoes grownwith OH-HPS supplemental lighting to those grown under overhead LED arrays and reported that tomatoes harvested from plants grown under either type of supplemental lighting were ranked sweeter and had a higher overall quality score compared with control fruits (grown only with ambient solar radiation). That study used overhead LED arrays that likely illuminated the upper portion of the high-wire crop canopy. Gomez et al. (2013) used vertical IC-LED towers that irradiated both foliage as well as developing fruit clusters throughout the canopy, showing that comparable yield can be achieved with IC-LED supplemental lighting using only a fraction the energy of OH-HPS supplemental lamps. With tomato fruit photosynthesis accounting for up to 15% of total photosynthate within the fruit (Hetherington et al., 1998), we hypothesized that incident light on tomato fruits from ICLED supplemental lighting would alter fruit metabolism and the sensory quality of fruits. The objective of this study was to determine whether IC-LED supplemental lighting could affect perceived or measured quality attributes of greenhouse tomato fruits compared with OH-HPS or unsupplemented plants and determine how. Materials and Methods Plant material. Tomato varieties ‘Komeett’ (De Ruiter Seeds, Bergshenhoek, Received for publication 24 July 2015. Accepted for publication 19 Aug. 2015. Funding was sourced from the USDA NIFA-SCRI program. We gratefully acknowledge the assistance of Judy Santini for statistical consulting as well as Bruce Bordelon and Gioia Massa for consultation regarding physicochemical testing and organoleptic sensory panels, respectively. The greenhouse component would not have been possible without the help of Rob Eddy, Dan Hahn, and Eric Whitehead. Graduate Research Assistant. Post-doctoral Research Associate. Professor. Corresponding author. E-mail: cmitchel@purdue.edu. 1498 HORTSCIENCE VOL. 50(10) OCTOBER 2015 TheNetherlands) (Expts. 1, 2, and 3), ‘Rebelski’ (De Ruiter Seeds) (Expt. 2), and ‘Success’ (De Ruiter Seeds) (Expt. 1) were grown hydroponically in a randomized block design using high-wire trellising in a glass-glazed greenhouse in West Lafayette, IN (40 N, 86 W). Expts. 1 and 3 were conducted under naturally decreasing DLI (summer to winter; 4 months in length), and Expt. 2 was conducted under naturally increasing DLI (winter to summer; 5 months in length), with supplemental lighting treatment locations rerandomized for each experiment. Daily fertigation used a commercial fertilizer (4.5N–14P–34K; CropKing, Lodi, OH) mix with irrigation intervals adjusted to maintain a 30% leaching fraction. Tomato plants were grown either with natural solar radiation only (control), natural solar radiation plus supplemental lighting from 600-W OH-HPS lamps (PL Lighting Systems, Beamsville, ON), or natural solar radiation plus supplemental light from IC-LED towers (ORBITEC,Madison,WI). The IC-LED towers emitted red (peak wavelength = 627 nm) and blue (peak wavelength = 450 nm) light with a 95:5 red:blue ratio. Global photosynthetic photon flux (PPF) of the IC-LED towers was adjusted equivalent to that of the OH-HPS treatment in the vertical profile below HPS lamps using a line quantum meter (QMSW-SS; Apogee Instruments, Inc., Logan, UT), correcting for yield photon flux with a spectroradiometer (EPP-2000; StellarNet Inc., Tampa, FL). Supplemental DLI was adjusted monthly based on latitudinal outdoor data (Korczynski et al., 2002) reduced by 50% to account for attenuation by greenhouse glass and infrastructure. Total DLI (solar + supplemental light) was therefore relatively constant throughout each experiment with a target total DLI of 25 mol·m·d, which is adequate for tomato production (Dorais, 2003; Jones, 2008; Moe et al., 2006). Table 3 details the monthly supplemental DLIs used in these studies. Treatments were separated by white polyethylene curtains between rows that were deployed only during supplemental lighting periods, thereby allowing maximal solar radiation to reach all sections of the greenhouse while preventing light pollution across treatments. Plants were leaned and lowered to maintain constant plant height as well as harvested and pruned using practices standard in the industry. Data collected during the experimental period for greenhouse ambient DLI and greenhouse ambient temperature are presented in Figs. 1 and 2, respectively. Physicochemical testing. Tomato fruits from ripening clusters were sorted based on ripeness stage. Samples were collected over 4 weeks and harvest time was determined to be nonsignificant. Stage 5 tomatoes (>60% of fruit is red) (USDA Tomato Ripeness Classification) were labeled with a two-digit identifier code corresponding to the