Light Intensity and Quality from Sole-source Light-emitting Diodes Impact Growth, Morphology, and Nutrient Content of Brassica Microgreens

Abstract

<jats:p>Multilayer vertical production systems using sole-source (SS) lighting can be used for the production of microgreens; however, traditional SS lighting methods can consume large amounts of electrical energy. Light-emitting diodes (LEDs) offer many advantages over conventional light sources, including high photoelectric conversion efficiencies, narrowband spectral light quality (LQ), low thermal output, and adjustable light intensities (LIs). The objective of this study was to quantify the effects of SS LEDs of different light qualities and intensities on growth, morphology, and nutrient content of <jats:italic>Brassica</jats:italic> microgreens. Purple kohlrabi (<jats:italic>Brassica oleracea</jats:italic> L. var. <jats:italic>gongylodes</jats:italic> L.), mizuna (<jats:italic>Brassica rapa</jats:italic> L. var. <jats:italic>japonica</jats:italic>), and mustard [<jats:italic>Brassica juncea</jats:italic> (L.) Czern. ‘Garnet Giant’] were grown in hydroponic tray systems placed on multilayer shelves in a walk-in growth chamber. A daily light integral (DLI) of 6, 12, or 18 mol·m<jats:sup>−2</jats:sup>·d<jats:sup>−1</jats:sup> was achieved from commercially available SS LED arrays with light ratios (%) of red:green:blue 74:18:8 (R<jats:sub>74</jats:sub>:G<jats:sub>18</jats:sub>:B<jats:sub>8</jats:sub>), red:blue 87:13 (R<jats:sub>87</jats:sub>:B<jats:sub>13</jats:sub>), or red:far-red:blue 84:7:9 (R<jats:sub>84</jats:sub>:FR<jats:sub>7</jats:sub>:B<jats:sub>9</jats:sub>) with a total photon flux (TPF) from 400 to 800 nm of 105, 210, or 315 µmol·m<jats:sup>−2</jats:sup>·s<jats:sup>−1</jats:sup> for 16 hours. Regardless of LQ, as the LI increased from 105 to 315 µmol·m<jats:sup>−2</jats:sup>·s<jats:sup>−1</jats:sup>, hypocotyl length (HL) decreased and percent dry weight (DW) increased for kohlrabi, mizuna, and mustard microgreens. With increasing LI, leaf area (LA) of kohlrabi generally decreased and relative chlorophyll content (RCC) increased. In addition, nutrient content increased under low LIs regardless of LQ. The results from this study can help growers to select LIs and LQs from commercially available SS LEDs to achieve preferred growth characteristics of <jats:italic>Brassica</jats:italic> microgreens.</jats:p>

Different Microgreen Genotypes Have Unique Growth and Yield Responses to Intensity of Supplemental PAR from Light-emitting Diodes during Winter Greenhouse Production in Southern Ontario, Canada

“…Although compact growth behaviors in mature plant tissues are commonly associated with increased LI (Poorter et al, 2019;Yeh and Hsu, 2004;Zervoudakis et al, 2012), juvenile plants have shown either reductions or no changes in HL with increasing LI in both greenhouses (Craver et al, 2019;Hern andez and Kubota, 2014;Lopez and Runkle, 2008;Poel and Runkle, 2017;Pramuk and Runkle, 2005) and SS growing environments (Craver et al, 2018;Kitaya et al, 1998;Meng and Runkle, 2019;Potter et al, 1999; Vir sil_ e and Sirtautas, 2013). There were only LI treatment effects in HL in mustard in the present study; while Gerovac et al (2016), Jones-Baumgardt et al (2019), and Samuolien_ e et al (2013) all reported general reductions in HL with increasing SS LI in Brassicaceae microgreens. In mustard, the LI treatment effects on HL were only approximately half the magnitude (over the same TLI range) as was reported in Jones-Baumgardt et al (2019).…”

“…Once the LI responses on biomass accumulation start to level off (i.e., approach saturation) at higher LI, LUE decreases accordingly. The maximum LI in some other studies on microgreens and leafy vegetables were insufficiently high to show saturating responses in FW and DW (Gerovac et al, 2016;Lefsrud et al, 2006;Sago, 2016;Samuolienėet al, 2013). Conversely, Fu et al (2012aFu et al ( , 2012b showed saturation of FW of romaine lettuce grown in SS at LI ranging from 100 to 800 mmol • m -2 • s -1 .…”

“…Increasing LI has generally been associated with increases in fresh weight (FW) and percentage of dry weight (DW), decreases in height, and decreases in specific leaf area (SLA) of juvenile growth stages (including microgreens) of commodities grown in solesource (SS) (Gerovac et al, 2016;Kitaya et al, 1998;Meng and Runkle, 2019;Park and Runkle, 2018;Samuolien_ e et al, 2013; Vir sil_ e and Sirtautas, 2013; Yao et al, 2017) and greenhouse (Boivin et al, 1987;Dorais and Gosselin, 2002;Gaudreau et al, 1994;Oh et al, 2010;Rezai et al, 2018;Torres and Lopez, 2011) production environments. These relationships between LI and plant growth have borne out in seedlings of the majority of herbaceous and woody species that have been investigated worldwide (Poorter et al, 2019).…”

“…Our previous study (Jones-Baumgardt et al, 2019) investigated the SS production of four Brassicaceae microgreens grown under LED LI ranging from 100 to 600 mmol • m -2 • s -1 with a 16-h photoperiod (TLI ranged from 58 to 380 mol • m -2 ) and found linear reductions in HL with increasing LI, but saturating increases in FW and DW with increasing LI. Gerovac et al (2016) and Samuolien_ e et al (2013) also both investigated the influences of SS LED LI on several Brassicaceae microgreens. Gerovac et al (2016) reported that there were LI treatment effects in FW in only one of three genotypes and that the maximum LI was insufficient to saturate bio-mass production in that genotype.…”

“…Gerovac et al (2016) and Samuolien_ e et al (2013) also both investigated the influences of SS LED LI on several Brassicaceae microgreens. Gerovac et al (2016) reported that there were LI treatment effects in FW in only one of three genotypes and that the maximum LI was insufficient to saturate bio-mass production in that genotype. They also reported biomass accumulation on a per plant basis, which is not truly reflective of how microgreens are grown and sold commercially (i.e., in very dense canopies).…”