Glucosinolate Concentrations of Broccoli Microgreens Are Greater under Specific Narrow Wavelength LED Light Regimes than under Conventional Fluorescent/Incandescent Light in Controlled Environments

We have previously shown that specific narrow-band wavelengths of light from light-emitting diodes (LEDs) stimulate production of glucoraphanin and several other secondary metabolites that have significant dietary implications. The objective of this research was to determine the effects of different percentages of specific narrow wavelengths of light on glucosinolate production in broccoli microgreens (Brassica oleracea). Broccoli microgreens were sprouted on fiber pads submerged in deionized water.  Following the appearance of the first true leaf, a 20% Hoagland’s #2 solution was used to maintain saturation of the pads providing a nutrient source for the plants.  After the first 24 h, light treatments were initiated on a 16 h photoperiod with a constant air temperature of 24 ^oC.  Light intensity was maintained at 250 µmol·m^-2·s^-1 for all light treatments.  The following light treatments were applied: 1) fluorescent/incandescent light (control);  2) 5% blue (455–470 nm) and  95% red (627–630 nm);  3) 5% blue, 85% red, and 10% green (525 nm);  4) 20% blue, 80% red;  and 5) 20% blue, 70% red, and 10% green.  Each of the light treatments were replicated four times.  Microgreens were harvested after 20 days under the light treatments, frozen, lypholized, ground in liquid nitrogen and stored at –80 °C.  The plants were then extracted with methanol and analyzed for aliphatic, aromatic, indole and total glucosinolate content by HPLC using a water-acetonitrile mobile phase gradient.  They were also analyzed specifically for glucoraphanin, an important dietary glucosinolate that has been reported to have anti-carcinogenic properties. Plants grown under the 20% blue, 70% red, and  10% green and the fluorescent/incandescent treatments did not differ in any of the glucosinolates measured.  However, plants grown under all of the other narrow wavelength LED light treatments were significantly higher in glucosinolates than the fluorescent/incandescent control.  The concentrations of glucoraphanin, aliphatic, indole, aromatic, and total gluconinolates  were all 200% to 300%  greater than that of plants grown under the control  light treatment.  Data from this experiment indicate that narrow wavelength specific LED light may be useful in improving the nutritional quality of broccoli microgreens and other leafy greenhouse vegetables grown in controlled environments.