Daily Light Integral and Light Quality from Sole-source Light-emitting Diodes Impact Phytochemical Content of Brassica Microgreens

Multi-layer vertical production systems using sole-source (SS) light-emitting diodes (LEDs) can be an effective alternative to more traditional methods of microgreens production. One significant benefit of using LEDs is the ability to select light qualities that have beneficial impacts on plant morphology and the synthesis of health-promoting phytochemicals. Therefore, the objective of this study was to quantify the effects of SS LEDs of different light qualities and intensities on the phytochemical content of Brassica microgreens. Specifically, the phytochemical measurements included: 1) total anthocyanins; 2) total and individual carotenoids; 3) total and individual chlorophyll; and 4) total phenolics. Purple kohlrabi (Brassica oleracea  var. gongylodes), mustard (Brassica juncea ‘Garnet Giant’), and mizuna (Brassica rapa var. japonica) were grown in hydroponic tray systems placed on multilayer shelves in a walk-in growth chamber. A daily light integral (DLI) of 6, 12, or 18 mol·m^–2·d^–1 was achieved from SS LED arrays with light ratios (%) of red:green:blue 74:18:8 (R₇₄:G₁₈:B₈), red:blue 87:13 (R₈₇:B₁₃), or red:far-red:blue 84:7:9 (R₈₄:FR₇:B₉) with a total photon flux from 400 to 800 nm of 105, 210, or 315 µmol·m^–2·s^–1 for 16-h, respectively. Phytochemical measurements were collected using spectrophotometry and high-performance liquid chromatography (HPLC) methodology. For kohlrabi, with increasing DLIs and light ratios of R₈₇:B₁₃ or R₈₄:FR₇:B₉, total anthocyanins significantly increased compared to those grown under lower DLIs and light ratios of R₇₄:G₁₈:B₈. Conversely, total carotenoids significantly increased under lower DLIs for mizuna and mustard. Light quality increased total integrated chlorophyll under the light ratio of R₈₇:B₁₃ compared to R₈₄:FR₇:B₉ and R₇₄:G₁₈:B₈ for kohlrabi and mustard. Light quality also influenced the total phenolic content of kohlrabi microgreens, with significantly increased levels for light ratios of R₈₄:FR₇:B₉ and R₈₇:B₁₃ compared to R₇₄:G₁₈:B₈ under a DLI of 6 mol·m^–2·d^–1. However, the impacts of DLI on total phenolic content of kohlrabi were not significant. The results from this study may help microgreens producers select light qualities and intensities using SS LEDs to achieve preferred phytochemical contents of Brassica microgreens.