Microgreens and baby greens are a relatively new specialty crop appearing in many upscale markets and restaurants. Collectively, these crops consist of vegetables and herbs consumed at a young growth stage. The main difference between the two is that microgreens are harvested at the base of the hypocotyl when the first set of true leaves start to emerge, while baby greens are harvested after the first set of true leaves has developed, generally ≥21 d after germination (Treadwell et al., 2010). Microgreens are mainly used by chefs and consumers to enhance the flavor, color, and texture of various foods (Treadwell et al., 2010). In addition, several species of microgreens contain high concentrations of health-promoting phytochemicals (Xiao et al., 2012). Commercial greenhouse growers have recently become interested in producing microgreens because of their high market value. Wholesale prices currently range from US$60 to $100 per kg for microgreens packaged in clamshell containers (Resh, 2013; Treadwell et al., 2010). In comparison, the wholesale price of greenhouse-grown boston lettuce (Lactuca sativa L.) packaged in plastic containers was about US$12 to $16 per kg (United States Department of Agriculture, 2016). Specifically, microgreens of the genus Brassica have become a popular choice due to the ease of germination, relatively short production time (7 to 21 d), and wide offering of intense flavors and colors (Xiao et al., 2012).
Several commercial growers are currently producing microgreens in greenhouses using soilless media in trays. Microgreens are also produced hydroponically using capillary mats placed in troughs, similar to those used in a nutrient film technique system. Another technique being used is a combination of hydroponics and SS lighting in multilayer vertical growing systems (Resh, 2013). Multilayer vertical growing systems using SS lighting were first developed and implemented commercially in Japan in the early 2000s (Goto, 2012). Although fluorescent lamps were initially used as the standard light source, growers have begun replacing them with LED arrays. Several operations worldwide have implemented this technology as LEDs have become more economically viable due to increased efficiency and decreased cost (Goto, 2012).
Multilayer vertical growing operations have substantial energy costs due to the amount of electricity required for SS lighting and temperature management (Goto, 2012). Light-emitting diodes offer many advantages over conventional light sources, including high photoelectric conversion efficiencies, narrowband spectral distribution, low thermal output, and adjustable LIs (Yeh and Chung, 2009). These advantages may become even more prevalent and defined as technology and research continue to improve.
Another potential advantage of using LEDs is the ability to select light qualities and intensities that have beneficial effects on plant growth and photomorphogenesis (Goto, 2012). The ability to impact growth of microgreens has been recently investigated using SS LEDs at different LIs. Samuoliené et al. (2013) found that increasing the photosynthetic photon flux (PPF) of SS LED lighting with a red:far-red:blue light ratio (%) of 91:1:8 (R91:FR1:B8) led to significantly reduced hypocotyl elongation of kohlrabi (B. oleracea var. gongylodes ‘Delicacy Purple’), tatsoi (B. rapa var. rosularis), and mustard (B. juncea L. ‘Red Lion’), and increased percent DW of red pak choi (B. rapa var. chinensis ‘Rubi F1’) and tatsoi microgreens. In regard to LQ, Li and Kubota (2009) reported that white light supplemented with far-red light significantly increased fresh weight (FW), DW, stem length, leaf length, and leaf width, compared with white light alone, of baby leaf lettuce ‘Red Cross’. An additional study of LQ was conducted by Kopsell and Sams (2013) on broccoli (B. oleracea var. italica Plenck) microgreens 13 d after sowing. Seeds were placed under either a red:blue light ratio (%) of 88:12 (R88:B12) at a continuous PPF of 350 µmol·m−2·s−1 or 0:100 (R0:B100) at 41 µmol·m−2·s−1. Specifically, they found that microgreens grown under the R0:B100 light ratio produced significantly higher levels of all essential nutrients compared with those grown under R88:B12 19 d after sowing.
Although previous reports have indicated that LI or LQ from SS LEDs had an effect on the growth of microgreens and baby greens, to our knowledge, little work has been published on the interaction between LI and LQ on the growth and nutrient content of Brassica microgreens. Therefore, the objective of this study was to quantify the effects of SS LEDs providing different LIs and LQs on the growth, morphology, and nutrient content of Brassica microgreens.
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