Blue versus Red Light Can Promote Elongation Growth Independent of Photoperiod: A Study in Four Brassica Microgreens Species

Abstract

An elongated stem has beneficial effects on microgreen production. Previous studies indicate that under 24-hour light-emitting diode (LED) lighting, monochromatic blue light, compared with red light, can promote plant elongation for some species. The objective of this study was to investigate whether shortened photoperiod can change blue vs. red light effects on elongation growth. The growth and morphology traits of arugula (Brassica eruca, ‘Rocket’), cabbage (Brassica oleracea, unknown variety name), mustard (Brassica juncea, ‘Ruby Streaks’), and kale (Brassica napus, ‘Red Russian’) seedlings were compared during the stage from seeding to cotyledon unfolding under two light quality × two photoperiod treatments: 1) R, monochromatic red light (665 nm) and 2) B, monochromatic blue light (440 nm) using continuous (24-hour light/0-hour dark) or periodic (16-hour light/8-hour dark) LED lighting. A photosynthetic photon flux density of ≈100 μmol·m−2·s−1 and an air temperature of ≈22 °C was used for the preceding treatments. After 7 to 8 days of lighting treatment, regardless of photoperiod, B promoted elongation growth compared with R, as demonstrated by a greater stem extension rate, hypocotyl length, or petiole length in the tested microgreen species, except for mustard. The promotion effects on elongation were greater under 24- vs. 16-hour lighting in many cases. Among the tested species, mustard showed the lowest sensitivity in elongation response to B vs. R, which was independent of photoperiod. This suggests that the blue-light-promoted elongation is not specifically from 24-hour lighting, despite the varying promotion degree under different photoperiods or for different species. The elongation growth promoted by blue LED light under a photoperiod of either 24 hours or 16 hours can potentially benefit indoor production of microgreens.