Plant pigments have specific wavelength absorption patterns known as absorption spectra. Biosynthetic wavelengths for the production of plant pigments [chlorophylls (Chl) and carotenoids] are referred to as action spectra. Specifically, Chl a and b absorb wavelengths of light strongly in the red (maximum absorption at 663 and 642 nm, respectively) and blue region (maximum absorption at 430 and 453 nm, respectively) with less absorption occurring in the green wavelengths (Hopkins and Huner, 2004). The carotenoid pigments lutein (L) and β-carotene (BC) absorb strongly in the blue region with maximum absorption occurring at 448 and 454 nm (in acetone), respectively (Hopkins and Huner, 2004). Previously, maximum biosynthesis of plant pigments for the action spectrum for wheat (Triticum aestivum L.) occurred at 447 and 646 nm for Chls and BC, respectively (Ogawa et al., 1973). The action spectrum for white corn (Zea mays L.) revealed maximum absorption of Chls and BC to be 445 and 650 nm, respectively (Koski et al., 1951).
Environmental factors such as temperature and irradiance levels can have strong influences on the accumulation of plant pigments and glucosinolates (Antonious et al., 1996; Charron and Sams, 2004; Lefsrud et al., 2005, 2006). Previous research on the physiological impacts of wavelengths on plants has used filters, interference filters, and/or wavelength-specific bulbs. However, a weakness in these techniques has been a reduction in the accompanying irradiance levels with most techniques resulting in irradiance levels less than 13 μmol·m−2·s−1 (Ogawa et al., 1973; Virgin, 1993). Even under decreased irradiance levels, physiological changes have been reported for plants grown under varying wavelengths (Gauthier et al., 1997; Heraut-Bron et al., 2001; Quaderi and Reid, 2005; Walters and Horton, 1995).
With the development of light-emitting diodes (LEDs), specific wavelengths of light can now be applied at higher irradiance levels, thus producing more realistic results on the effects of wavelength on plant physiological responses. However, to date, there is limited information on plant responses to LED lighting. A few studies have demonstrated plant responses to red light using LED. Exposure to only red LED light resulted in both plant elongation and reduced biomass for lettuce (Lactuca sativa L.; Hoenecke et al., 1992) and pepper (Capsicum annuum L.; Brown et al., 1995).
Kale (Brassica oleracea L. var. acephala D.C.) is an excellent source of glucosinolates (GS) (Stoewsand, 1995) and carotenoids. Kale ranks as the highest source of L and BC of any vegetable (U.S. Dept. Agric., 2002). In plants, carotenoids are used as antenna pigments and quench the energetic triplet state of the Chl molecule to prevent damage to the photosynthetic system (Tracewell et al., 2001). In humans, dietary intake of foods rich in carotenoids is associated with reduced risk of lung cancer, cataracts, and age-related macular degeneration (Landrum and Bone, 2001; Le Marchand et al., 1993), whereas diets high in vegetables with GS reduce the risk of cancer (Stoewsand, 1995; van Poppel et al., 1999).
Light is critical for plant growth and development, and wavelengths can easily be controlled by growers in artificial growing environments. Therefore, the goal of this study was to investigate the influences of five different wavelengths of light using LED on plant biomass and accumulation patterns of Chls, carotenoids, and GS compounds in the leaf tissues of kale.
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