Applying Blue Light Alone, or in Combination with Far-red Light, during Nighttime Increases Elongation without Compromising Yield and Quality of Indoor-grown Microgreens

Abstract

<jats:p>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 (<jats:italic>Brassica juncea</jats:italic>) and arugula (<jats:italic>Eruca sativa</jats:italic>) 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 (<jats:italic>PPFD</jats:italic>) of 300 µmol·m<jats:sup>–2</jats:sup>·s<jats:sup>–1</jats:sup> 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 µmol·m<jats:sup>–2</jats:sup>·s<jats:sup>–1</jats:sup> followed by 4 hours of darkness (40B-D); 3) 4 hours of darkness followed by 4 hours of B at 40 µmol·m<jats:sup>–2</jats:sup>·s<jats:sup>–1</jats:sup> (D-40B); 4) 8 hours of B at 20 µmol·m<jats:sup>–2</jats:sup>·s<jats:sup>–1</jats:sup> (20B); 5) 8 hours of B + FR, and each of them at 20 µmol·m<jats:sup>–2</jats:sup>·s<jats:sup>–1</jats:sup> (20B20FR); and 6) 8 hours of FR at 20 µmol·m<jats:sup>–2</jats:sup>·s<jats:sup>–1</jats:sup> (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.</jats:p>

The Power of Far-Red Light at Night: Photomorphogenic, Physiological, and Yield Response in Pepper During Dynamic 24 Hour Lighting

“…However, the lack of further increase in internode length from 16W + 8BFR (PSS = 0.315) to 16W + 8FR (PSS = 0.135) indicates that there may be a threshold (PSS = 0.315) at which a further reduction in PSS does not increase internode length. A PSS value of 0.315 is comparable to the PSS threshold (PSS = 0.23) determined for arugula and mustard microgreens beyond which no further stem elongation was observed ( Ying et al, 2020 ). Therefore, with respect to plant height and internode length, FR during the night had a larger impact on the morphological outcome as plants subjected to 16W + 8BFR tended to have values more closely to 16W + 8FR ( Figure 9 ).…”

The Efficiency of LED Irradiation for Cultivating High-Quality Tomato Seedlings

“…This increase in plant dry weight was mainly related to differences in light absorption, which in turn were mainly due to differences in leaf area Figure 2C rather than other morphological parameters. The combination of B and R LED lighting increased total dry matter [27], photosynthetic pigment content, stomata number, and reasonable photosynthate distribution in cherry tomato seedlings [28].…”

“…The content of water in plants had no significant difference among all light treatments. Specific leaf area (SLA) under O and G treatments was greater than that under the other respective irradiations of LEDs, while that of RB treatment showed the lowest SLA [27].…”

Growth and morphological responses of gerbera seedlings to narrow-band lights with different light spectral combinations as sole-source lighting in a controlled environment

“…respectively. Total leaf area and longest petiole length were determined from scanned images following the method by Ying et al (2020b) in replicate one. In the second replicate, the total leaf area was measured using a Li-Cor 3100C leaf area meter (Li-Cor Inc., Lincoln, NE, USA) and the longest petiole length was measured with a ruler.…”