Sole-source LED Lighting Impacts Mineral Nutrient Density of Chinese Kale

Previous work has demonstrated narrow-band wavelengths of light from light-emitting diodes (LED) can increase concentrations of primary and secondary metabolites in brassica species.  The objectives of this study were to measure the impacts of: 1) light quality; and 2) different levels of fertility on mineral nutrient density in shoot tissues of Chinese kale (Brassica oleracea var. alboglabra).  ‘Green Lance’ Chinese kale was grown hydroponically in controlled environment chambers under an air temperature of 22 °C with a 16-h photoperiod using a light intensity of 350 µmol×m^-2×sec^-1.  Light quality treatments were established as: 1) fluorescent/incandescent light; 2) 10% blue (447 ± 5 nm) / 90% red (627 ± 5 nm) LED light; 3) 20% blue / 80% red LED light; and 4) 40% blue / 60% red LED light as sole source lighting.  Plants within light treatments were cultured in either a ½ strength or a ¼ strength Hoagland’s #2 nutrient solution.  The experiment was repeated three times.  All plants were harvested 30 d after seeding and shoot tissues were analyzed for K, Mg, Ca, Fe, and Zn concentrations.  Kale under the fluorescent/incandescent light treatment had significantly higher shoot fresh (FM) and dry mass (DM) compared to plants under all other LED light treatments.  The percent shoot tissue DM was 10.7%, 10.5%, 10.5%, and 9.9% for the light treatments of fluorescent/incandescent light, 10% blue / 90% red LED light, 20% blue / 80% red LED light, and 40% blue / 60% red LED light, respectively.  Because of higher total shoot tissue DM, plants under the fluorescent/incandescent light treatment had high mineral nutrient content on a per plant basis.  However, baby leafy greens are typically packaged in bags, individually or in mixed species, for the prepared foods market.  When minerals were expressed on a concentration basis as µg per g DM, all LED light treatments produced higher shoot tissue K, Mg, Ca, Fe, and Zn concentrations than the fluorescent/incandescent light treatment.  Sole source LED lighting resulted in lower kale shoot tissue DM, which lead to lower nutrient content on a per plant basis.  But, when minerals were expressed on a concentrations basis, LED light treatments resulted in higher nutrient density per edible serving.  Sole source LED lighting may provide a nutritional advantage for baby leafy greens produced for the packaged market.