The production of secondary metabolites in plant tissues is determined by an interaction of environmental growth factors with biochemical, physiological, and genetic characteristics (Goldman et al., 1999; Kopsell et al., 2004; Kurilich et al., 1999). The light environment is a significant influential factor on plant secondary metabolite production (Kopsell et al., 2012; Lefsrud et al., 2006), and physiological changes will occur in plants when exposed to varying wavelengths of light (Johkan et al., 2010; Li and Kubota, 2009; Loreto et al., 2009; Samuolienė et al., 2012; Stutte et al., 2009). Irradiance levels can also influence secondary metabolite production in plants such as carotenoid pigment and glucosinolate concentrations (Charron and Sams, 2004; Lefsrud et al., 2006).
Near monochromatic LEDs are one of the most energy-efficient and rapidly developing lighting technologies. One developing application of LED technology is for horticultural plant production in controlled environments (Martineau et al., 2012). Capacities such as spectral composition control, high light output, and little radiant heat emissions make LEDs the most significant advancement in horticultural lighting since the development of high-intensity discharge lamps (Morrow, 2008). LEDs now provide the ability to measure impacts of narrow-band wavelengths of light on seedling morphology and physiology (Lefsrud et al., 2008).
Carotenoids are produced in the plastids and protect photosynthetic structures by quenching excited triplet Chl (3Chl) to dissipate excess energy and bind singlet oxygen (1O2) to inhibit potential oxidative damage (Demmig-Adams et al., 1996; Frank and Cogdell, 1996; Tracewell et al., 2001). Although there is a good understanding of the biosynthetic pathways for carotenoid production in plants (Cunningham, 2002; Kopsell and Kopsell, 2006), the regulatory means by which different plants can alter levels of individual carotenoids largely remains uncertain (Farnham and Kopsell, 2009). Chlorophyll and carotenoid pigments function in light harvesting and photoprotection in plants. Maximum absorption of chlorophyll a (Chl a) and b (Chl b) pigments are in the red (663 and 642 nm, respectively) and blue regions (430 and 453 nm, respectively) of the visible light spectrum. In contrast, absorption of the carotenoid pigments of lutein (LUT) and β-carotene (BC) are highest in the blue region at 448 and 454 nm, respectively (Lefsrud et al., 2008; Lichtenthaler, 1987). Light signals are perceived by plants through photoreceptor proteins called phototropins (Briggs and Huala, 1999). Two such phototropins, called phot-1 and phot-2, have been identified in Arabidopsis thaliana and work together to respond to blue light stimuli (Baum et al., 1999; Briggs and Christie, 2002; Fuchs et al., 2003). Blue light exposure during plant growth is qualitatively required for normal photosynthesis and facilitates quantitative leaf responses similar to those normally associated with higher light intensities (Hogewoning et al., 2010). Xanthophyll carotenoid pigments, specifically zeaxanthin (ZEA), can modulate blue light-dependent responses in plants (Tlałka et al., 1999). Moreover, ZEA is believed to be an important photoreceptor for blue light-activated stomatal operations (Briggs and Huala, 1999). As a result of their light absorption capabilities, specialized pigments have evolved to be vital to light perception and responses in plant species.
Cruciferous vegetables are relatively abundant sources of antioxidants with potential anticarcinogenic activity (Kurilich et al., 1999). The bioactive compounds in Brassicas include lutein and β-carotene carotenoids, glucoraphanin and glucobrassicin glucosinolates, quercetin and kaempferol flavonoids, and C and E vitamins (Jeffery and Araya, 2009). Plant carotenoids are the most important source of provitamin A in the human diet. There is increasing evidence that carotenoids can also protect humans against certain specific chronic ailments including cancer, cardiovascular disease, and age-related macular degeneration (Giovannucci, 1999; Mayne, 1996). The most studied bioactive components in the Brassicas are the glucosinolates and their hydrolyzed isothiocyanates (Stoewsand, 1995). Isothiocyanates possess anticarcinogenic activity and may be useful as chemopreventative agents in the human diet. The major aliphatic glucosinolate found in broccoli is glucoraphanin. The isothyocyanate derived from glucoraphanin is sulforaphane, which induces upregulation of phase II detoxification enzymes and is the central cancer-preventative agent in broccoli (Fahey et al., 1997). The vegetable Brassicas are also consumed for their nutritional values of Ca, Mg, P, K, and Fe (Farnham et al., 2000), and many members of the genus are identified as good dietary sources of these nutrients. Calcium and K are two important elements that play critical roles in bone development and cellular metabolism in human nutrition. Brassicas are high in Ca content and low in oxalate compounds that can bind to Ca and reduce absorption (Weaver and Heaney, 1999). Microgreens are specialty leafy crops harvested just above the roots after the first true leaves have emerged and usually consumed fresh as a salad green (Kopsell et al., 2012). Broccoli microgreens are valued for significant concentrations of cancer-fighting glucosinolates (GSs) as well as being a rich source of carotenoid phytochemicals and essential mineral elements. Because of the impacts blue light wavelengths can have on plant physiology and secondary metabolite production, the objective of this study was to measure the impact of short-duration blue light from LEDs on the nutritionally important phytochemical compounds in sprouting broccoli microgreens.
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