Microgreens are tender leafy vegetables harvested after two cotyledons have fully developed, with or without the first true leaves, that are becoming popular in the worldwide markets due to their high nutritional value (Treadwell et al., 2016; Xiao et al., 2012). In some regions that have long and cold winters, like Canada, winter production of microgreens in local greenhouses has become an option. The profits of greenhouse production of fruits and vegetables such as tomato and cucumber are decreasing due to the increased costs of greenhouse operation and intense price competition with imported produce. However, it is difficult to import or transport microgreens from other regions to Canada because they are highly perishable products (Mir et al., 2017). Moreover, microgreens have a short growth period (7–20 d), so they can be grown with many cropping cycles in greenhouses throughout the winter.
The low natural light level during winter months is one of the most limiting factors for greenhouse vegetable production in northern regions, such as Canada (Demers and Gosselin, 2002). During the winter months (November–January), the natural daily light integral (DLI) in the northern United States and southern Canada normally ranges between 5 and 15 mol·m−2·d−1, resulting in a daily average photosynthetic photon flux density (PPFD) of 58 to 174 μmol·m−2·s−1 (Faust and Logan, 2018). The available light level in greenhouses could be further reduced by 30% to 60% due to the transmission losses through greenhouse construction and covering materials (Critten, 1993; Llewellyn et al., 2013). Therefore, in these regions, the DLI in greenhouses can be as low as 2 to 6 mol·m−2·d−1 during the winter months (daily average PPFD of 23–69 μmol·m−2·s−1 depending on the greenhouse structure). For microgreens, the recommended minimum DLI has been elusive in the literature. However, for greenhouse vegetable (including microgreens) production in southern Canada, the yield and most quality metrics increased with the increasing DLI within the range of 6.9 to 24 mol·m−2·d−1 (Jones-Baumgardt et al., 2020; Kong et al., 2018a; Kong and Zheng, 2019). Consequently, winter greenhouse production under low natural light conditions is a great challenge for growers due to the decreased yield and quality of horticultural crops, including microgreens.
Supplemental lighting (SL) is a common practice for greenhouse production because it is a way to deal with low natural light issues. Hofstra et al. (1969) found that low-intensity supplemental light is efficient for carbon assimilation and plant growth, and ≈13 μmol·m−2·s−1 supplemental light is five-times more efficiently used during nighttime compared with daytime in terms of CO2 fixation. Overnight SL can be economically beneficial in some regions, such as Ontario in Canada, where the electricity cost during nighttime is almost half that during the daytime. Therefore, overnight SL may benefit crop production more efficiently. Light-emitting diodes (LEDs) have been increasingly used as a SL source in greenhouses because of the many advantages over traditional lamps (Brandon et al., 2012; Gómez et al., 2013). Among the advantages, the adjustable spectral quality enables growers to control plant growth and development using LED lights based on their production purposes. However, the optimal spectral quality of LED is unclear for overnight low-intensity SL during winter greenhouse microgreen production in terms of yield and quality.
Microgreens with longer stems are normally more attractive to most consumers; therefore, plant height is one of the most important microgreen appearance qualities. In addition, plant height or stem length is an important technological quality trait. Most microgreens are harvested with a minimum height of 5 cm (Kyriacou et al., 2016), and inhibition of stem elongation would delay harvest time, thereby extending the crop cycle time. Also, commercial microgreen production has been increasingly switching from hand-harvesting to machine-harvesting to reduce labor costs. Microgreens with plant height less than 5 cm are difficult to harvest with machines (Kong et al., 2019a). Although daytime SL can increase the microgreen yield and some quality traits, it inhibits stem elongation and causes difficulty with machine harvesting (Jones-Baumgardt et al., 2019). Therefore, it would be interesting to investigate whether stem elongation can be promoted by overnight SL without compromising yield and quality during winter greenhouse production.
Recently, our laboratory found that monochromatic blue light (B, 400–500 nm) instead of red light (R, 600–700 nm) promoted stem elongation of indoor-grown microgreens under LED lighting as the sole light source with PPFD of ≈100 or 50 μmol·m−2·s−1 and photoperiods of both 24 and 16 h (Kong et al., 2019a, 2019b). In addition to promoting stem elongation, B compared to R also reduced cotyledon size, changed plant color, and increased biomass partitioning to the stem despite the similar fresh weight (FW) of the stems and leaves (Kong et al., 2019a). We concluded that the promoted stem elongation is a shade-avoidance response mediated by B associated with low phytochrome activity, as indicated by the low phytochrome photostationary state (PPS; <0.6) (Kong et al., 2018b), which may also involve a co-action among the three photoreceptors (phytochrome, cryptochrome, and phototropin) (Kong and Zheng, 2020). However, it is unclear whether a similar promotion effect on stem elongation associated with other responses can be found with overnight supplemental B in winter greenhouse microgreen production.
In a natural light environment, the enriched far-red light (FR) level can also promote stem elongation as a shade-avoidance response by lowering the phytochrome equilibrium (i.e., decreasing the phytochrome activity) (Demotes-Mainard et al., 2016). An increased FR level at the end of day has been shown to enhance stem elongation in many species, including chrysanthemum (Chrysanthemum morifolium) (Lund et al., 2007), poinsettia (Euphorbia pulcherrima) (Islam et al., 2014), and tomato (Solanum lycopersicum) rootstock (Chia and Kubota, 2010). Extending the photoperiod with supplemental FR light is extremely useful to promote shoot elongation of Japanese pear (Pyrus pyrifolia) during the first several months of the seedling stages (Ito et al., 2014). However, plants grown under a light environment with high FR levels might undergo some negative effects, such as decreased chlorophyll content and leaf thickness (Demotes-Mainard et al., 2016). These negative effects may potentially compromise microgreen quality.
It is unknown whether B or FR is more effective as an overnight SL source for promoting elongation while having fewer negative effects on yield and other quality traits for winter greenhouse microgreen production. Therefore, the objective of this study was to evaluate the effects of overnight SL with low-intensity B or FR LED, using no SL as a control, on winter greenhouse production of arugula and mustard microgreens in terms of appearance quality (including stem elongation), crop yield, and phytochemical contents.
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