Solid-state lighting based on the use of light-emitting diodes (LEDs) is potentially one of the biggest advancements in horticultural lighting in decades. LEDs can play a variety of roles in horticultural lighting, including use in controlled environment research, lighting for tissue culture, and supplemental and photoperiod lighting for greenhouses. LED lighting systems have several unique advantages over existing horticultural lighting, including the ability to control spectral composition, the ability to produce very high light levels with low radiant heat output when cooled properly, and the ability to maintain useful light output for years without replacement. LEDs are the first light source to have the capability of true spectral composition control, allowing wavelengths to be matched to plant photoreceptors to provide more optimal production and to influence plant morphology and composition. Because they are solid-state devices, LEDs are easily integrated into digital control systems, facilitating special lighting programs such as “daily light integral” lighting and sunrise and sunset simulations. LEDs are safer to operate than current lamps because they do not have glass envelopes or high touch temperatures, and they do not contain mercury. The first sustained work with LEDs as a source of plant lighting occurred in the mid-1980s to support the development of new lighting systems to be used in plant growth systems designed for research on the space shuttle and space station. These systems progressed from simple red-only LED arrays using the limited components available at the time to high-density, multicolor LED chip-on-board devices. As light output increases while device costs decrease, LEDs continue to move toward becoming economically feasible for even large-scale horticultural lighting applications.

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Development of a more effective radiation source for use in plant-growing facilities would be of significant benefit for both research and commercial crop production applications. An array of light-emitting diodes (LEDs) that produce red radiation, supplemented with a photosynthetic photon flux (PPF) of 30 µmol·s -1 ·m -2 in the 400-to 500-nm spectral range from blue fluorescent lamps, was used effectively as a radiation source for growing plants. Growth of lettuce (Lactuca sativa L. 'Grand Rapids') plants maintained under the LED irradiation system at a total PPF of 325 µmol·s -1 ·m -2 for 21 days was equivalent to that reported in the literature for plants grown for the same time under cool-white fluorescent and incandescent radiation sources. Characteristics of the plants, such as leaf shape, color, and texture, were not different from those found with plants grown under cool-white fluorescent lamps. Estimations of the electrical energy conversion efficiency of a LED system for plant irradiation suggest that it may be as much as twice that published for fluorescent systems.

Previous research in our group demonstrated that short-duration exposure to narrow-band blue wavelengths of light can improve the nutritional quality of sprouting broccoli (Brassica oleacea var. italica) microgreens. The objective of this study was to measure the impact of different percentages of blue light on the concentrations of nutritional quality parameters of sprouting broccoli microgreens and to compare incandescent/fluorescent light with light-emitting diodes (LEDs). Microgreen seeds were cultured hydroponically on growing pads under light treatments of: 1) fluorescent/incandescent light; 2) 5% blue (442 to 452 nm)/95% red (622 to 632 nm); 3) 5% blue/85% red/10% green (525 to 535 nm); 4) 20% blue/80% red; and 5) 20% blue/70% red/10% green in controlled environments. Microgreens were grown at an air temperature of 24 °C and a 16-hour photoperiod using a light intensity of 250 μmol·m−2·s−1 for all light treatments. On emergence of the first true leaf, a nutrient solution of 42 mg·L−1 nitrogen (N) (20% Hoagland’s #2 solution) was used to submerge the growing pads. Microgreens were harvested after 20 days under the light treatments and shoot tissues were processed and measured for nutritionally important shoot pigments, glucosinolates, and mineral nutrients. Microgreens under the fluorescent/incandescent light treatment had significantly lower shoot fresh mass than plants under the 5% blue/95% red, 5% blue/85% red/10% green, and the 20% blue/80% red LED light treatments. The highest concentrations of shoot tissue chlorophyll, β-carotene, lutein, total carotenoids, calcium (Ca), magnesium (Mg), phosphorus (P), sulfur (S), boron (B), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), zinc (Zn), glucoiberin, glucoraphanin, 4-methoxyglucobrassicin, and neoglucobrassicin were found in microgreens grown under the 20% blue/80% red light treatment. In general, the fluorescent/incandescent light treatment resulted in significantly lower concentrations of most metabolites measured in the sprouting broccoli tissue. Results from the current study clearly support data from many previous reports that describe stimulation of primary and secondary metabolite biosynthesis by exposure to blue light wavelengths from LEDs.

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