Effects of Different Light-Emitting Diode Illuminations on Bioactive Compounds in Ruby Streaks Mustard Microgreens by Ultra-High Performance Liquid Chromatography–High-Resolution Mass Spectrometry

Mustard (Brassica juncea var. "Ruby Streaks") is a popular microgreen species with rich phenolic and glucosinolate contents. Light-emitting diodes (LEDs) have been increasingly used to promote the content of bioactive compounds in commercially grown plants, particularly in controlled environment agriculture (CEA) settings. The present research aimed to investigate the effect of LED light wavelengths on the phytochemical profiles of the major secondary metabolites [i.e., anthocyanins, non-anthocyanin phenolics, and glucosinolates (GLs)] of Ruby Streaks mustard microgreens. Twenty-six anthocyanins, thirty-five non-anthocyanin phenolics, and six GLs were characterized in Ruby Streaks mustard extracts. Among them, three hydroxycinnamic acid-acylated Cyanidin 3-O-sophoroside-5-O-diglucosides and eight hydroxycinnamic acid-acylated flavonol-O-pentosylglucosides were newly discovered compounds in mustard microgreens. Three light treatments (blue, red, and magenta) were investigated and compared to the white (2700k) control. The red (660 nm) light and magenta (450 + 650 nm) light promoted the accumulation of both total and individual anthocyanin/aliphatic GLs, while the blue (450 nm) light was found to be the dominant factor in the accumulation of non-anthocyanin phenolics. The results suggested the significant and complex effect of lighting conditions on phytochemical accumulation in Ruby Streaks mustard microgreens, thus the potential of modulating nutritional profiles by varying wavelength of the light source.