Leaf Shape, Growth, and Antioxidant Phenolic Compounds of Two Lettuce Cultivars Grown under Various Combinations of Blue and Red Light-emitting Diodes

Light-emitting diodes (LEDs) of short wavelength ranges are being developed as light sources in closed-type plant production systems. Among the various wavelengths, red and blue lights are known to be effective for enhancing plant photosynthesis. In this study, we determined the effects of blue and red LED ratios on leaf shape, plant growth, and the accumulation of antioxidant phenolic compounds of a red leaf lettuce (Lactuca sativa L. ‘Sunmang’) and a green leaf lettuce (Lactuca sativa L. ‘Grand Rapid TBR’). Lettuce seedlings grown under normal growth conditions (20 8C, fluorescent lamp + highpressure sodium lamp 177 ± 5 mmol·m·s, 12-hour photoperiod) for 18 days were transferred into growth chambers that were set at 20 8C and equipped with various combinations of blue (456 nm) and red (655 nm) LEDs [blue:red = 0:100 (0 B), 13:87 (13 B), 26:74 (26 B), 35:65 (35 B), 47:53 (47 B) or 59:41 (59 B)] under the same light intensity and photoperiod (171 ± 7 mmol·m·s, 12-hour photoperiod). Leaf width, leaf length, leaf area, fresh and dry weights of shoots and roots, chlorophyll content (SPAD value), total phenolic concentration, total flavonoid concentration, and antioxidant capacity were measured at 2 and 4 weeks after the onset of LED treatment. The leaf shape indices (leaf length/leaf width) of the two lettuce cultivars subjected to blue LEDs treatment were similar to the control, regardless of the blue-to-red ratio during the entire growth stage. However, 0 B (100% red LED) induced a significantly higher leaf shape index, which represents elongated leaf shape, compared with the other treatments. Increasing blue LED levels negatively affected lettuce growth. Most growth characteristics (such as the fresh and dry weights of shoots and leaf area) were highest under 0 B for both cultivars compared with all other LED treatments. For red and green leaf lettuce cultivar plants, shoot fresh weight under 0 B was 4.3 and 4.1 times higher compared with that under 59 B after 4 weeks of LED treatment, respectively. In contrast, the accumulation of chlorophyll, phenolics (including flavonoids), and antioxidants in both red and green leaf lettuce showed an opposite trend compared with that observed for growth. The SPAD value (chlorophyll content), total phenolic concentration, total flavonoid concentration, and antioxidant capacity of lettuces grown under high ratios of blue LED (such as 59 B, 47 B, and 35 B) were significantly higher compared with 0 B or control conditions. Thus, this study indicates that the ratio of blue to red LEDs is important for the morphology, growth, and phenolic compounds with antioxidant properties in the two lettuce cultivars tested. The consumption of plant-based foods represents one of the essential components for the nutrition of humans. In this aspect, increasing crop yield is the most fundamental and important issue for farmers and agronomists and will continue to be in the future. The importance of the fruits and vegetables that we ingest daily has been rediscovered, because horticultural crops contain various types of health-promoting phytochemicals, including antioxidant, anticancer, and antiinflammatory substances (Brandt et al., 2004; Pennington and Fisher, 2009). In particular, phenolic compounds, which are one of the most widely occurring groups of phytochemicals, exhibit various types of physiological properties, including antioxidant activity (Balasundram et al., 2006). Many epidemiological studies have shown that the intake of fruits and vegetables maintains and improves human health (Hooper and Cassidy, 2006). Thus, improving the quality of fruits and vegetables is a matter of interest for both consumers and producers. Among several environmental factors affecting crop yield and quality, light is a crucial factor. Basically, light is an energy source for photosynthesis. In addition, various components contribute to light serving as a signal stimulus to plants, including light intensity, light quality, and daylength. Plants perceive light signals through photoreceptors such as phytochromes, cryptochromes, and phototropins. Consequently, most developmental processes that occur throughout the life cycle of plants are dependent on light, including seed germination, phototropism, gravitropism, chloroplast movement, shade avoidance, circadian rhythms, and flower induction (Carvalho et al., 2011; Jiao et al., 2007). In the case of growing a crop under different light sources, each cultural practice must be differentiated, because each light source has a unique light quality that directly affects plant growth and development. Recently, LEDs have been used as sources of artificial lighting in closed-type plant production systems, where environmental conditions are controlled, allowing crops to be produced throughout the year regardless of external weather conditions. In comparison with other conventional artificial lighting sources used in plant cultivation, LEDs have the advantages of high light-conversion efficiency with low radiant heat output, semipermanence, and small mass; hence, plants may be irradiated close to the plants. In addition, LEDs are available in a variety of narrow wavebands; hence, it is possible to optimize light quality to improve both crop yield and quality (Morrow, 2008; Yeh and Chung, 2009). Blue and red LEDs are usually used for plant growth because chlorophyll a and b efficiently absorb wavelengths in the blue (maximum absorption at 430 and 453 nm) and red (maximum absorption at 663 and 642 nm) ranges (Hopkins and Huner, 2004). Previous studies assessing the effects of red and blue wavelengths on plants indicated that red LED generally induces plant growth by increasing fresh and dry plant weight, plant height, and leaf area (Heo et al., 2012; Johkan et al., 2010; Wang et al., 2009; Wu et al., 2007). In comparison, blue LED influences photosynthetic function, chlorophyll formation, and chloroplast development rather than having a direct effect on biomass accumulation (Johkan et al., 2010; Savvides et al., 2012; Wang et al., 2009). The synergetic effect was observed when mixtures of blue and red LEDs were used to irradiate plants. Mixed light conditions enhance the growth of various vegetables, including lettuce, more compared with red LEDs alone (Hogewoning et al., 2010; Matsuda et al., 2007; Savvides et al., 2012; Stutte et al., 2009; Yorio et al., 2001). However, other studies have reported the opposite results (Heo et al., 2012; Johkan et al., 2010). It is difficult to understand how plants respond to changes in blue and red light ratios because most existing LED-related studies simply compare plant growth for specific ratios of blue and red LEDs, leading to inconsistent results. Thus, this study aimed at determining the effect of different blue and red LED ratios on Received for publication 18 Mar. 2013. Accepted for publication 11 June 2013. To whom reprint requests should be addressed; e-mail moh@cbnu.ac.kr. 988 HORTSCIENCE VOL. 48(8) AUGUST 2013 the morphological changes, growth characteristics, and the accumulation of antioxidant phenolic compounds of two lettuce cultivars. The results obtained from this study are expected to provide baseline information toward designing artificial lighting sources in closed-type plant production systems. Materials and Methods Plant growth conditions and light spectrum. Seeds of red leaf lettuce (Lactuca sativa L. ‘Sunmang’; Nongwoo Bio Co., Suwon, Korea) and green leaf lettuce (Lactuca sativa L. ‘Grand Rapid TBR’; Asia Seed Co., Seoul, Korea) were sown in a 105-plug tray (32 mL/cell, two seeds per cell) containing a growing medium (Myung-Moon; Dongbu Hannong Co., Seoul, Korea). One of two seedlings per cell was thinned 1 week after sowing. The plants were then grown within a growth chamber (DS-96S; Dasol Scientific Co., Hwaseong, Korea) with normal growing conditions [20 C, fluorescent lamp + high-pressure sodium lamp, photosynthetic photon flux (PPF) 177 ± 5 mmol·m·s, 12-h photoperiod] for 18 d. Fifteen seedling plugs per treatment were transferred to a growth chamber (VS-1203P1; Vision Scientific Co., Daejeon, Korea) equipped with six different blue and red LED ratios and a growth chamber (DS-96S; Dasol Scientific Co., Hwaseong, Korea) to serve as the control. The six lighting sources using LEDs, which were plate type (48 · 48 cm, length · width), were manufactured to generate a combination of blue (456 nm; Itswell Co., Incheon, Korea) and red (655 nm; Bright LED Electronics Co., Seoul, Korea) LEDs. The spectral distribution was initially measured at 25 cm from LED lighting sources to top of the pots and at five points (center and four edges of a tray including pots) using a spectroradiometer (LI-1800; LI-COR, Lincoln, NE), which presented as relative spectral distribution (Fig. 1). All plants were grown at 20 C, PPF of 171 ± 7 mmol·m·s, and a 12-h photoperiod for 4 weeks. The PPF of each LED treatment, which was measured at the top of lettuce plants, was maintained at a similar level by adjusting currents of LED lighting systems. To minimize light distribution being disproportionate within each treatment, the pots were systematically rearranged everyday. For the first 18 d after sowing, distilled water (2 L) was subirrigated to pots in a tray at intervals of 2 to 3 d, and a nutrient solution for lettuce (17.3N–4.0P–8.0K) was subirrigated to the lettuce plants once a week for the rest of the cultivation period. The pH and electrical conductivity of the nutrient solution were 5.5 and 1.16 dS·m, respectively. Growth characteristics. Growth characteristics such as the fresh and dry weights of shoots and roots, shoot/root ratio (S/R ratio), total leaf area, and chlorophyll content (SPAD value) were measured at 4 weeks after the onset of LED treatment. The shoots and roots were dried at 70 C in a drying oven (FS-420; Advantec Co., Tokyo, Japan) for 3 d and were weighed to determine dry weight using a scale (Si-234; Denver In