Growth and morphology responses to narrow-band blue light and its co-action with low-level UVB or green light: A comparison with red light in four microgreen species

Abstract

To explore the action mode of blue light on plant elongation growth, the growth and morphology traits of arugula (Brassica eruca L., ‘Rocket’), cabbage (Brassica oleracea, unknown variety name), mustard (Brassica juncea, ‘Ruby Streaks’), and kale (Brassica napus, ‘Red Russian’) microgreens were compared under four light quality treatments: (1) R, narrow-band red light (660 nm); (2) B, narrow-band blue light (455 nm); (3) BU, broad-band blue light created by mixing B with a low level (≈ 0.7 μmol m−2 s−1) of UVB (310 nm); and (4) BG, broad-band blue light created by mixing B with a low level (≈ 6 %) of green light (530 nm). Continuous (24 -h) light-emitting diode lighting with 100 μmol m−2 s−1 photosynthetic photon flux density was used in the above treatments at ≈ 22 °C. After 7–8 days of light treatment, B, compared to R, had promoted elongation growth (i.e., longer hypocotyls or petioles, faster stem extension), and induced some other typical shade-avoidance responses (e.g., smaller cotyledons, lighter plant color, or greater biomass allocation to stem), which was associated with longer cells in the hypocotyls and lower cell density in the cotyledons except for mustard. Compared with B, the elongation growth was inhibited under BU for arugula and kale, but was promoted under BG for all the species except arugula. If considering all the plant traits together, BU and BG effect was similar to B, but different from R. Therefore, narrow-band blue vs. red light can promote elongation growth as a shade-avoidance response at both plant and cell levels, and adding low-level UVB or green light can only slightly change the narrow-band-blue-light-promoted shade-avoidance response, despite varying sensitivity among species.