Increases in Shoot Tissue Pigments, Glucosinolates, and Mineral Elements in Sprouting Broccoli after Exposure to Short-duration Blue Light from Light Emitting Diodes

By D. Kopsell, C. Sams. 2013

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Microgreens are specialty leafy crops harvested just above the roots after the first true leaves have emerged and are consumed fresh. Broccoli (Brassica oleacea var. italica) microgreens can accumulate significant concentrations of cancer-fighting glucosinolates as well as being a rich source of other antioxidant phytochemicals. Lightemitting diodes (LEDs) now provide the ability to measure impacts of narrow-band wavelengths of light on seedling physiology. The carotenoid zeaxanthin has been hypothesized to be a blue light receptor in plant physiology. The objective of this study was to measure the impact of short-duration blue light on phytochemical compounds, which impart the nutritional quality of sprouting broccoli microgreens. Broccoli microgreens were grown in a controlled environment under LEDs using growing pads. Seeds were cultured on the pads submerged in deionized water and grown under a 24-hour photoperiod using red (627 nm)/blue (470 nm) LEDs (350 mmol m s) at an air temperature of 23 8C. On emergence of the first true leaf, a complete nutrient solution with 42 mg L of nitrogen (N) was used to submerge the growing pads. At 13 days after sowing, broccoli plantlets were grown under either: 1) red and blue LED light (350 mmol m s); or 2) blue LED light (41 mmol m s) treatments for 5 days before harvest. The experiment was repeated three times. Frozen shoot tissues were freeze-dried and measured for carotenoids, chlorophylls, glucosinolates, and mineral elements. Comparing the two LED light treatments revealed the shortduration blue LED treatment before harvest significantly increased shoot tissue b-carotene (P £ 0.05), violaxanthin (P £ 0.01), total xanthophyll cycle pigments (P £ 0.05), glucoraphanin (P £ 0.05), epiprogoitrin (P £ 0.05), aliphatic glucosinolates (P £ 0.05), essential micronutrients of copper (Cu) (P = 0.02), iron (Fe) (P £ 0.01), boron (B), manganese (Mn), molybdenum (Mo), sodium (Na), zinc (Zn) (P £ 0.001), and the essential macronutrients of calcium (Ca), phosphorus (P), potassium (K), magnesium (Mg), and sulfur (S) (P £ 0.001). Results demonstrate management of LED lighting technology through preharvest, short-duration blue light acted to increase important phytochemical compounds influencing the nutritional value of broccoli microgreens. 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; Samuolien _ e 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 energyefficient 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 narrowband 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 (Chl) to dissipate excess energy and bind singlet oxygen (O2) 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 b-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 Received for publication 13 Sept. 2012. Accepted for publication 19 Oct. 2012. This research was made possible through support from the University of Tennessee Institute of Agriculture. Associate Professor. Professor. Corresponding author. E-mail: dkopsell@utk.edu. J. AMER. SOC. HORT. SCI. 138(1):31–37. 2013. 31 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 (Tla1ka 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 b-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. Materials and Methods SPROUTING BROCCOLI CULTURE AND HARVEST. Hydroponic culture using growing pads (Sure to Grow, Beachwood, OH) of polyethylene teraphthalate fibers (25.4 · 24.7 · 0.89 cm) were used to germinate and grow sprouting broccoli (Mountain Rose Herbs, Eugene, OR) microgreen plants (Kopsell et al., 2012). A 7-g sample of sprouting broccoli seeds was sown evenly onto the growing pad, and two pads were set in a perforated tray (26 · 52 · 6 cm). Perforated trays were set into solid-bottom trays (26 · 52 · 6 cm) and filled with deionized water to create a hydroponic tray system for microgreen culture. Four tray systems were placed randomly into each of two controlled environment chambers (Model E15; Conviron, Winnipeg, Manitoba, Canada) to germinate seeds at 23 ± 1 C in darkness. Deionized water was added at a rate of 200 mL d to the bottom tray. Tray systems were equally divided to act as two experimental replications, and chambers acted as experimental treatments. Three complete experimental runs were conducted on 8 Apr. 2011, 28 Apr. 2011, and 23 May 2011. After 3 d, the photoperiod was set at 24 h using LED panels with 470and 627-nm diodes (Orbital Technologies, Madison, WI). Photosynthetically active radiation for LED lighting was measured with a spectroradiometer (Model SPEC-ultraviolet/PAR; Apogee Instruments, Logan, UT) at the center of each panel, and panel heights from the growing pads were adjusted to maintain an average light intensity of 350 ± 10 mmol m s for the chambers composed of 12% blue and 88% red light. On emergence of the first true le

Subfields: Molecular Biology, Plant Science

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