Responses of yield and appearance quality of four Brassicaceae microgreens to varied blue light proportion in red and blue light-emitting diodes lighting

To optimize blue light proportion in red and blue light-emitting diode (RB-LED) lighting for microgreen production, the yield and appearance quality of cabbage, kale, arugula and mustard were investigated under RB-LED lightings with 5%, 10%, 15%, 20%, 25% and 30% blue light. For each lighting treatment, the total photosynthetic photon flux density was set at ≈ 300 μmol m^-2 s^-1, and the air temperature during light/dark was set at ≈ 21/ 17 °C. As a result, neither fresh nor dry yield was affected by blue light percentage for the tested species except cabbage, which showed quadratic (peaking at 15%) responses in crop yield. Hypocotyl length and cotyledon area of kale and mustard decreased linearly with increasing blue light percentage which, however, did not affect arugula or cabbage in these two traits. For plant color, cotyledons were darker red for mustard and less pure green for the other three species under higher blue light percentage. This was indicated by a negative linear response of hue angle or green chromaticity to increasing blue light percentage. These findings suggested that responses to blue light percentage varied with plant traits and microgreen species. To reach a balance of yield and appearance quality, 15% blue light was recommended for indoor production of cabbage microgreens, while 5% blue light for the other three species, under similar environmental conditions.