Biomass, carbohydrates, pigments, and mineral elements in kale (Brassica oleracea var acephala) microgreens respond to LED blue-light wavelength

Light-emitting diodes (LED) are an emerging lighting technology for controlled environments and indoor crop production. Recent research is establishing added benefits to plant quality under narrow band blue and red LEDs. The objective of this study was to compare impacts of different wavelengths of blue light on biomass parameters, nonstructural water-soluble carbohydrates, pigments, and mineral nutrients in kale (Brassica oleracea var acephala) microgreens. A comparison to white LEDs with also made. 'Toscano' kale were seeded into shallow 1020 (25.4 cm x 50.8 cm) flats at a rate of 10 g. Trays were placed into controlled environment chambers at a constant air temperature of 22 degree celsius and germinated in the dark for 24 h. Upon germination, experimental sole source LED light treatments were applied as: (1) white (Orbital Technologies, Madison, WI); (2) 400 nm; (3) 420 nm; (4) 450 nm; and (5) 470 nm (Ray22; Fluence Bioengineering, Austin, TX), with a 14 h photoperiod and a light intensity at plant height of 250 mu mol m(-2) s(-1) for all treatments. All blue light treatments also contained 625 nm and 660 nm wavelengths at a ratio of 40 % Blue/60 % Red light. Three flats were used per light treatment and the experiment was repeated two times. All microgreens were harvested 16 days after seeding and were measured for biomass and concentrations of non-structural water-soluble carbohydrates, tissue pigments, and elemental nutrients. Kale microgreen shoot fresh mass (FM; P <= 0.001) and dry mass (DM; P <= 0.001) varied in response to blue light treatments. The white LED treatment averaged 106.3 g FM per flat, while the 470 nm treatment averaged 54.9 g FM per flat. The highest DM per flat was under the 400 nm treatment (14.8 g DM), while the lowest DM per flat was under the 470 nm treatment (7.2 g DM). The %DM (P <= 0.001) was influenced by blue light treatment, with the highest %DM under the 400 nm treatment (18.8 %DM). Kale water soluble sucrose (P <= 0.001) was influenced by light treatment; however, glucose and fructose were not. Concentration of sucrose (47.5 mg g(-1) DM) for the 420 nm treatment were significantly higher than the white light treatment (18.5 mg g(-1) DM). The lower blue LED wavelengths of 400 nm and 420 nm resulted in lower tissue pigment concentrations and lower elemental nutrient concentrations when compared to the white LED treatments. Even though the lower blue LED wavelengths produced less FM and less pigment and nutrient concentrations, these two blue LED treatments resulted in significantly higher microgreen %DM and soluble sucrose. Results indicate the 400 and 420 nm wavelengths significantly improved textural and flavor quality, which may positively impact consumer preference.