Microgreens are consumed when cotyledons are fully expanded, retaining their typical color, with or without appearance of the first true leaves (Jones-Baumgardt et al., 2019; Kyriacou et al., 2016). Microgreens can be grown in varying scenarios, including outdoor, greenhouse, and indoor environments (Kyriacou et al., 2016). LED lighting has been increasingly used as a sole light source for indoor production of vegetables such as microgreens (Kozai et al., 2015).
Most microgreens are harvested at 7 to 21 d from seeding with a minimum height of ≈5 cm (Kyriacou et al., 2016). Also, commercial microgreen production has been increasingly switching from hand- to machine-harvesting to reduce labor cost. Microgreens with hypocotyls <5 cm are difficult for machine-harvesting, according to the communication with some Canadian growers. It is well known that both red and blue light can mediate stem elongation (Huché-Thélier et al., 2016). Also, monochromatic red and blue LED lights have been successfully used for microgreen cultivation with the advantage of increasing beneficial phytochemicals such as antioxidants (Kopsell and Sams, 2013; Wu et al., 2007). However, limited information is available on the effect of monochromatic red and blue LED lights on stem elongation of microgreens, especially under different photoperiods, because photoperiod can also affect this plant trait (Bergstrand, 2017).
Previous studies indicated that under LED lighting at a photosynthetic photon flux density (PPFD) of 100 μmol·m−2·s−1 or 50 μmol·m−2·s−1, monochromatic blue vs. red light promoted elongation growth in all the tested bedding plant species, including petunia, calibrachoa, geranium, and marigold (Kong et al., 2018), and some microgreen species such as arugula, cabbage, and kale (Kong et al., 2019). In these studies, a photoperiod of 24 h was used for lighting treatments. Possibly the blue-light-promoted elongation growth is an artifact specifically from 24-h lighting because it is well known that most plants grow naturally under a periodic light/dark environment. However, similar promotion effects by blue vs. red light have also been achieved under a photoperiod of <24 h (i.e., 12–18 h) in other LED studies on seedlings of eggplant (Hirai et al., 2006), cherry tomato (Kim et al., 2014), cucumber (Hernández and Kubota, 2016), marigold (Heo et al., 2002), and sunflower (Schwend et al., 2015) at a PPFD of ≈100 μmol·m−2·s−1. Thus, under a certain range of light levels (e.g., 100 μmol·m−2·s−1), the promoted stem elongation growth by monochromatic blue light, relative to red light, might be a common phenomenon when photoperiod varied between 12 and 24 h. However, this speculation needs confirmation because the preceding studies were performed with different species under different environments. For indoor production, 16-h lighting daily has become popular (Kozai, 2018). When photoperiod is shortened from 24 h to 16 h, it is unknown whether blue vs. red LED lighting at a PPFD of 100 μmol·m−2·s−1 could also promote elongation growth for some microgreen species.
Shortened photoperiod is known to reduce elongation for some species (Bergstrand, 2017; Schüssler and Bergstrand, 2012). Also, a recent study on petunia indicated that the stem elongation was not promoted by blue vs. red light until the exposure duration increased up to 5 d, and the blue light promotion was proportional to the lighting duration time (Fukuda et al., 2016). It is possible that shortening photoperiod within a certain range may reduce, rather than eliminate, promotion effects of blue vs. red light on plant elongation at least in some species. However, in that study, the petunia plants developed expanded true leaves, so both photosynthesis and photomorphogenesis were involved in blue vs. red light effects on plant elongation under different lighting duration. For microgreens without appearance of true leaves, photomorphogenesis is the main contributor to plant elongation. Therefore, it needs confirmation in the microgreens that shortened photoperiod from 24 h to 16 h can reduce blue light promotion effect on elongation to some degree.
Arugula, cabbage, kale, and mustard are popular species used for microgreen production. In a previous study on these microgreens with unfolded true leaves under 24-h lighting, different species varied in their elongation-promotion response to blue vs. red light at a PPFD of 100 μmol·m−2·s−1, showing a lower sensitivity in mustard than other species (Kong et al., 2019). However, it needs confirmation that microgreens without appearance of true leaves (i.e., from seeding to cotyledon unfolding) also differ in the blue light response among species. In addition to promoted elongation, some other typical shade-avoidance responses such as reduced side branch number and cotyledon size and increased biomass allocation to main stem also occurred under blue vs. red light, which varied with different species (Kong et al., 2018, 2019). It was concluded that blue-light-promoted elongation is a shade-avoidance response with varying sensitivity among species (Kong et al., 2018, 2019). Unfortunately, the conclusion was drawn from the studies on some bedding plants and microgreens under 24-h lighting, although the shade-avoidance responses to blue light have been also reported for Arabidopsis under a photoperiod of <24 h (de Wit et al., 2016; Keller et al., 2011; Pedmale et al., 2016). For microgreens, it is still unclear whether the species difference in blue light’s effect on elongation as a shade-avoidance response could also be found under noncontinuous (e.g., 16-h) lighting.
On the basis of the preceding information, the following three hypotheses were proposed for arugula, cabbage, kale, and mustard seedling growth from seeding to cotyledon unfolding. Under LED lighting at a PPFD of 100 μmol·m−2·s−1 with a photoperiod of 24 h or 16 h, 1) shortened photoperiod (16 h) cannot eliminate the blue light promotion effect on plant elongation relative to red light, 2) the elongation promoted by blue light is greater under 24-h than 16-h lighting at least for some species, and 3) species differ in elongation response to blue light and the interspecies difference is unaffected by photoperiod. The objective of this study was to explore the mode of blue light action on plant elongation in four microgreen species by testing the foregoing hypotheses.
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