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.
Folta, K.M. 2004 Green light stimulates early stem elongation, antagonizing light-mediated growth inhibition Plant Physiol. 135 1407 1416
Frantz, J.M. 2013 Uptake efficiency of phosphorus in different light environments by zinnia (Zinnia elegans) and vinca (Catharanthus roseus) HortScience 48 594 600
Hoagland, D.R. & Arnon, D.I. 1950 The water-culture method for growing plants without soil. California Agr. Expt. Sta. Circ. 347:32
Kopsell, D.A. & Sams, C.E. 2013 Increase in shoot tissue pigments, glucosinolates, and mineral elements in sprouting broccoli after exposure to short-duration blue light from light-emitting diodes J. Amer. Soc. Hort. Sci. 138 31 37
Kopsell, D.A., Sams, C.E., Barickman, T.C. & Morrow, R.C. 2014 Sprouting broccoli accumulate higher concentrations of nutritionally important metabolites under narrow-band light-emitting diode lighting J. Amer. Soc. Hort. Sci. 139 469 477
Kuehny, J.S., Peet, M.M., Nelson, P.V. & Willits, D.H. 1991 Nutrient dilution by starch in CO2-enriched chrysanthemum J. Expt. Bot. 42 711 716
Li, Q. & Kubota, C. 2009 Effects of supplemental light quality on growth and phytochemicals of baby leaf lettuce Environ. Exp. Bot. 67 59 64
Pettai, H., Oja, V., Freiberg, A. & Lasik, A. 2005 Photosynthetic activity of far-red light in green plants Biochim. Biophys. Acta 1708 311 321
Potter, T.I., Rood, S.B. & Zanewich, K.P. 1999 Light intensity, gibberellin content and the resolution of shoot growth in Brassica Planta 207 505 511
Resh, H.M. 2013 Water culture: Microgreens, p. 135–142. 7th ed. In: H.M. Resh (ed.). Hydroponic food production. CRC Press, Boca Raton, FL
Samuoliené, G., Brazaityte, A., Jankauskiene, J., Virsile, A., Sirtautas, R., Novickovas, A., Sakalauskiene, S., Sakalauskaite, J. & Duchovskis, P. 2013 LED irradiance level affects growth and nutritional quality of Brassica microgreen Cent. Eur. J. Biol. 8 1241 1249
Thapper, A., Mamedov, F., Mokvist, F., Hammarstrom, L. & Styring, S. 2009 Defining the far-red limit of photosystem II in spinach Plant Cell 21 2391 2401
Treadwell, D.D., Hochmuth, R., Landrum, L. & Laughlin, W. 2010 Microgreens: A new specialty crop. Univ. Florida IFAS Ext. Bul. HS1164
United States Department of Agriculture 2016 Fruit and vegetable market news. 19 Jan. 2016. <https://www.marketnews.usda.gov/>
Wollaeger, H.M. & Runkle, E.S. 2014 Growth of impatiens, petunia, salvia, and tomato seedlings under blue, green, and red light-emitting diodes HortScience 49 734 740
Xiao, Z., Lester, G.E., Luo, Y. & Wang, Q. 2012 Assessment of vitamin and carotenoid concentrations of emerging food products: Edible microgreens J. Agr. Food Chem. 60 7644 7651
Yeh, N. & Chung, J. 2009 High-brightness LEDs—Energy efficient lighting sources and their potential in indoor plant cultivation Renew. Sustain. Energy Rev. 13 2175 2180