Blue Light-emitting Diode Light Irradiation of Seedlings Improves Seedling Quality and Growth after Transplanting in Red Leaf Lettuce

In this study, we determined the effects of raising seedlings with different light spectra such as with blue, red, and blue + red light-emitting diode (LED) lights on seedling quality and yield of red leaf lettuce plants. The light treatments we used were applied for a period of 1 week and consisted of 100 mmol m s of blue light, simultaneous irradiation with 50 mmol m s of blue light and 50 mmol m s of red light, and 100 mmol m s of red light. At the end of the light treatment, that is 17 days after sowing (DAS), the leaf area and shoot fresh weight (FW) of the lettuce seedlings treated with red light increased by 33% and 25%, respectively, and the dry weight of the shoots and roots of the lettuce seedlings treated with blue-containing LED lights increased by greater than 29% and greater than 83% compared with seedlings grown under a white fluorescent lamp (FL). The shoot/root ratio and specific leaf area of plants irradiated with blue-containing LED lights decreased. At 45 DAS, higher leaf areas and FWs were obtained in lettuce plants treated with blue-containing LED lights. The total chlorophyll (Chl) contents in lettuce plants treated with blue-containing and red lights were less than that of lettuce plants treated with FL, but the Chl a/b ratio and carotenoid content increased under blue-containing LED lights. Polyphenol contents and the total antioxidant status (TAS) were greater in lettuce seedlings treated with blue-containing LED lights than in those treated with FL at 17 DAS. The higher polyphenol contents and TAS in lettuce seedlings at 17 DAS decreased in lettuce plants at 45 DAS. In conclusion, our results indicate that raising seedlings treated with blue light promoted the growth of lettuce plants after transplanting. This is likely because of high shoot and root biomasses, a high content of photosynthetic pigments, and high antioxidant activities in the lettuce seedlings before transplanting. The compact morphology of lettuce seedlings treated with blue LED light would be also useful for transplanting. In modern agriculture, the raising of seedlings and the cultivation of crops are often separated. Because seedlings are raised in small areas, farmland is efficiently used. Moreover, control over seedling growth such as achieving high rates of germination and uniformity in seedling morphology is easy to manage in plant nurseries, allowing seedlings of high quality to be raised. The quality of seedlings affects their growth and yield after transplantation. Goodquality seedlings exhibit morphological characteristics such as thick stems, thick leaves, dark green leaves, and large white roots (Oda, 2007). Seedlings with elongated stems, thin leaves, light green leaves, and small undeveloped roots are weak and subject to environmental stress (Seiler and Johnson, 1988). This results in decreased yield. Therefore, raising high-quality seedlings is important for increasing the yield of crops. Plant development and physiology are strongly influenced by the light spectrum, which affects the seedling morphology pattern (McNellis and Deng, 1995). For example, blue light suppresses hypocotyl elongation and induces cotyledon expansion, and red light induces hypocotyl elongation and cotyledon expansion in Arabidopsis seedlings. Blue light is involved in a wide range of plant processes such as phototropism, photomorphogenesis, stomatal opening, and photosynthesis (Whitelam and Halliday, 2007). Most studies with blue light only or blue mixed with red light indicated that blue light-containing irradiation produced higher plant biomass (Matsuda et al., 2008; Yorio et al., 2001). Hanyu and Shoji (2002) suggested that yield and crop quality could be improved by controlling light quality. However, application of the blue spectrum when raising seedlings has rarely been studied. The light spectrum also stimulates the biosynthesis of phenolic compounds. Blue light induced the accumulation of flavonoids (Ebisawa et al., 2008; Kojima et al., 2010) and anthocyanins, which are one class of flavonoid compounds, play a role in antioxidant activity (Duan et al., 2007). Moreover, chlorogenic acid, which had higher antioxidant activities than vitamins C and E (RiceEvans et al., 1997), was also increased by blue light (Awada et al., 2001). Because better growth of plants under environmental stresses was associated with higher antioxidant activity (Rivero et al., 2003), raising seedlings irradiated with blue light can potentially increase crop yield after planting because of the high accumulation of phenolic compounds. LED lights create specific wavelengths and a narrow bandwidth for plant growth compared with filters with broad-spectrum light sources. Therefore, blue LED lights can produce the specific blue spectrum more efficiently than blue filters with other light sources. In this study, we determined the effects of raising seedlings with different light spectra such as with blue, red and blue + red LED lights on seedling quality and yield of red leaf lettuce plants. Photosynthetic pigments, polyphenols, and antioxidant activity of lettuce seedlings treated with different light spectra were also determined. Materials and Methods Plant growth. Seeds of red leaf lettuce (Lactuca sativa L. cv. Banchu Red Fire; Takii Seed Co., Kyoto, Japan) were pregerminated for 1 d at 23 ± 2 C under 100 mmol m s photosynthetic photon flux (PPF) for 14 h with a white FL (FLR110H-W1A; Mitsubishi/ Osram Co., Yokohama, Japan). Germinated seeds were sown in a 200-cell tray (16 mL/cell) filled with a mixture of peatmoss (Tanemaki Baido; Takii Seed Co., Kyoto, Japan) and vermiculite (VS Kako Co., Tokyo, Japan) at a ratio of 2:1 (v/v). The lettuce seedlings were grown in the chamber at 23 ± 2 C under 100 mmol m s PPF for 14 h with FL. All lettuce seedlings were placed 45 cm below the FL. At 10 DAS, the lettuce seedlings were either placed 45 cm below the FL or 20 cm below the blue (peak wavelength: 468 nm, bandwidth at half peak height: 21 nm), red (660 nm, 22 nm), or blue + red [467 + 655 nm, 21 + 20 nm; blue/red light density (PPF) ratio 1/1] LED (ISL-305X302; CCS Inc., Kyoto, Japan) lights (Fig. 1). All seedlings were irradiated for 14 h at 100 mmol m s PPF. The wavelength of the light source was determined by a USB2000 spectrometer (Ocean Optics, Dunedin, FL) (Fig. 2). At 17 DAS, the seedlings were transplanted to plastic pots (diameter, 60 mm; depth, 70 mm) filled with the same substrate and grown in a greenhouse maintained at 25 ± 2 C with a light period of 12 h d; the main light source was sunlight supplemented with a FL (1600 to 1900 HR). The lettuce seedlings Received for publication 24 June 2010. Accepted for publication 4 Oct. 2010. This work was supported by a grant for ‘‘Elucidation of biological mechanisms of photoresponse and development of advanced technologies utilizing light.’’ We thank Ms. Mina Ozaki and Ms. Noriko Nikaido for technical assistance. To whom reprint requests should be addressed; e-mail [email protected]. HORTSCIENCE VOL. 45(12) DECEMBER 201