Applying blue light alone, or in combination with far-red light, during nighttime increases elongation without compromising yield and quality of indoor-grown microgreens

To facilitate machine harvest for labor savings, the height of microgreens needs to reach'5 cm. Recent studies indicate that monochromatic blue light (B) can promote stem elongation similar to far-red light (FR). To examine whether nighttime B treatments can promote plant elongation without compromising yield and quality, mustard (Brassica juncea) and arugula (Eruca sativa) microgreens were grown under different light-emitting diode (LED) lighting regimes in a growth chamber. The 16-hour daytime lighting comprised 20% B and 80% red light (R), and had a total photosynthetic photon flux density (PPFD) of 300 mmol_·m^-2_·s^-1 at canopy level. During the 8-hour nighttime, the plants were exposed to the following treatments: 1) dark (D) as one control; 2) 4 hours of B at 40 mmol_·m^-2_·s^-1 followed by 4 hours of darkness (40B-D); 3) 4 hours of darkness followed by 4 hours of B at 40 mmol_·m^-2_·s^-1 (D-40B); 4) 8 hours of B at 20 mmol_·m^-2_·s^-1 (20B); 5) 8 hours of B + FR, and each of them at 20 mmol_·m^-2_·s^-1 (20B20FR); and 6) 8 hours of FR at 20 mmol_·m^-2_·s^-1 (20FR) as another control. The plants were harvested after 11 days of treatment. Nighttime B treatments (40B-D, D-40B, and 20B), compared with D, increased plant height by 34% and 18% for mustard and arugula, respectively, with no difference among the three B treatments. The combination of B and FR (20B20FR), compared with B alone, further increased plant height by 6% and 15% for mustard and arugula, respectively, and showed a similar promotion effect as 20FR. Plant height did not meet the machine harvest requirement for both species with the D treatment, but did so for mustard with the nighttime B treatments and for arugula with the 20B20FR treatment. There was no difference in biomass among all treatments except that 20B, compared with D, increased the fresh weight (FW) of arugula by 12%, showing a similar promotion effect as 20FR. Despite a greater promotion effect on elongation than B alone, 20FR reduced the leaf index compared with D. However, B alone or the 20B20FR treatment increased leaf thickness compared with D, and increased chlorophyll content index (CCI), leaf index, dry matter content, and leaf thickness to varying degree with species, compared with 20FR. Overall, nighttime B alone, or its combination with FR, promoted microgreen elongation without compromising yield and quality.