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Light intensity and quality from sole -source light emitting diodes impact growth, morphology, and nutrient content of Brassica microgreens

GEROVAC, Joshua R., CRAVER, Joshua K., LOPEZ, Roberto G., BOLDT, Jennifer K.
Hortscience 2016 v.51 no.5 pp. 497-503
Brassica juncea, Brassica oleracea, Brassica rapa subsp. nipposinica, chlorophyll, electric power, growth chambers, hydroponics, hypocotyls, kohlrabi, leaf area, light intensity, light quality, lighting, microgreens, nutrient content, nutrient uptake, photons, plant growth, production technology
Multi-layer vertical production systems using sole-source (SS) lighting can be used for microgreen production; however, traditional SS lighting can consume large amounts of electrical energy. Light-emitting diodes (LEDs) offer many advantages over conventional light sources including: high photoelectric conversion efficiencies, narrow-band spectral light quality, low thermal output, and adjustable light intensities. The objectives of this study were to: 1) quantify the effects of SS LEDs of different light qualities and intensities on growth, morphology, and nutrient uptake of Brassica microgreens; and 2) quantify the electrical energy required to operate SS LEDs of different light qualities and intensities. Purple kohlrabi (Brassica oleracea L. var. gongylodes L.), mizuna (Brassica rapa L. var. japonica), and mustard [Brassica juncea (L.) Czern. ‘Garnet Giant’] were grown in hydroponic tray systems placed on multi-layer 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 total photon flux (TPF) from 400 to 800 nm of 105, 210, or 315 µmol·m–2·s–1 for 16-h. Regardless of light quality, as DLI increased from 6 to 18 mol·m–2·d–1, hypocotyl length decreased and percent dry weight increased for kohlrabi, mizuna, and mustard microgreens. With increasing DLI, leaf area of kohlrabi generally decreased and relative chlorophyll content increased. Additionally, nutrient uptake increased under low DLIs regardless of light quality. Similarly, regardless of light quality, as DLI increased, electrical energy consumption progressively increased. The results from this study can help producers select light qualities and intensities from SS LEDs to achieve preferred growth characteristics of Brassica microgreens.