Shoot Tissue Pigment Concentrations in Broccoli Microgreens Are Higher under Narrow-Band Wavelengths from LEDs than under Fluorescent/Incandescent Light

Light-emitting diodes (LEDs) provide the ability to measure physiological responses to narrow-band wavelengths in plants.  Previous research in our group demonstrated short-duration exposure to narrow-band blue (455–470 nm) wavelengths can improve the nutritional quality of broccoli (Brassica oleracea) microgreens.  The objectives of this study were to measure the impact of different percentages of blue light on the concentrations of nutritionally important pigments in broccoli microgreens, and to compare LEDs with incandescent/fluorescent light.  Sprouting broccoli seeds were cultured on growing pads submerged in deionized water and grown under the light treatments of: 1) fluorescent/incandescent light; 2) 5% blue (455–470 nm)/95% red (627–630 nm); 3) 5% blue/85% red/10% green (525 nm); 4) 20% blue/80% red; and 5) 20% blue/70% red/10% green in controlled environments.  The light intensity was set at 250 µmol·m^-2·s^-1 for all light treatments with a 16-h photoperiod and air temperature of 24 °C.  Upon emergence of the first true leaf, a complete nutrient solution (20% Hoagland’s #2 solution) was used to submerge the growing pads.  Microgreens were harvested after 20 days under the light treatments.  Shoot tissues were freeze dried and measured for chlorophyll and carotenoid pigments.  Pairwise contrasts revealed that LED light treatments increased shoot tissue beta-carotene, lutein, total carotenoids, chlorophyll a, chlorophyll b, and total chlorophyll when compared to the incandescent/fluorescent light treatment.  Broccoli microgreens grown under the incandescent/fluorescent light treatment had higher concentrations of violaxanthin and neoxanthin.  Higher blue light percentages also increased shoot tissue chlorophyll, but did not impact carotenoid pigments.  Narrow-band wavelengths from LEDs increased many of the nutritionally important pigments in broccoli microgreen tissues, which would be beneficial when consumed in the diet.