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 (Gómez 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 (Samuoliené 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 SSL LEDs 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 (Samuoliené et al., 2012).
In addition, anthocyanin content of lettuce leaves has been investigated and measured (Li and Kubota, 2009; Samuoliené 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 μmol·m−2·s−1. 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 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 León 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 greenhouse-grown 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.
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