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

Author
item GEROVAC, JOSHUA - Purdue University
item CRAVER, JOSHUA - Purdue University
item LOPEZ, ROBERTO - Purdue University
item Boldt, Jennifer

Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/3/2016
Publication Date: 5/1/2016
Publication URL: https://handle.nal.usda.gov/10113/5801827
Citation: Gerovac, J.R., Craver, J.K., Lopez, R.G., Boldt, J.K. 2016. Light intensity and quality from sole-source light-emitting diodes impact growth, morphology, and nutrient content of Brassica microgreens. HortScience. 51(5):497-503.

Interpretive Summary: Micro-greens are a relatively new specialty crop appearing in many upscale markets and restaurants. In addition to enhancing the flavor, color, and texture of foods, several species of microgreens contain high concentrations of health-promoting phytochemicals. Multi-layer vertical production systems using sole-source lighting can be used for microgreen production; however, traditional fluorescent lighting can consume large amounts of electrical energy. Light-emitting diodes (LEDs) offer many advantages over conventional light sources, including a high efficiency at converting electricity to light, the ability to define specific wavebands of light to include in an LED array, low heat output, and adjustable light intensities. The objectives of this study were to quantify the effects of sole-source LEDs providing different light intensities and qualities on the growth, morphology, and nutrient uptake of Brassica microgreens, as well as quantify electrical energy consumption. Purple kohlrabi, mizuna, and mustard microgreens were grown in hydroponic tray systems placed on multi-layer shelves in a walk-in growth chamber. They were grown at three different light intensities (6, 12, or 18 mol·m–2·d–1) under three different light ratios provided by sole-source LED arrays [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)]. Both light quantity and light quality impacted microgreen growth, morphology, and nutrient uptake. As light quantity increased, microgreen height and leaf area generally decreased and microgreen fresh weight and leaf greenness (relative chlorophyll content) increased. The fresh weight of microgreens grown under a light ratio of R84:FR7:B9 was greater than those grown under R87:B13, which could be economically important since microgreens are typically sold on a fresh weight basis. Microgreen nutrient concentration, however, generally decreased as light quantity increased. Very few differences between LED light treatments occurred at low light intensities, but were more prevalent at high light intensities. For example, at the highest light intensity (18 mol·m–2·d–1), mizuna accumulated more nitrogen, potassium, and calcium; mustard accumulated more potassium, calcium, and boron; and kohlrabi accumulated more boron and iron when grown under a light ratio of R74:G18:B8 compared to those grown under R84:FR7:B9. This may have practical implications for both the production of microgreens and their potential nutrient composition and quality at harvest. Results from this study can help producers select light qualities and intensities from sole-source LEDs to achieve preferred growth characteristics of Brassica microgreens.

Technical Abstract: 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.