Effect of Temperature Variation and Blue and Red LEDs on the Elongation of Arugula and Mustard Microgreens

Abstract

Recent studies using LED lighting at low to modest intensity have indicated that compared with red light, blue light can promote plant elongation in many crops as a shade avoidance response despite varying sensitivity with light environments, plant species, and growth stages. Currently, there is limited understanding of how temperature affects the blue light-mediated plant response. To clarify this point, two microgreen species (arugula and mustard) were grown indoors under two light quality × two temperature treatments: red LED light (peak at 670 nm) and blue LED light (peak at 450 nm) at 18 °C or 28 °C. A photosynthetic photon flux density of 110 µmol m−2 s−1 and a photoperiod of 12 h d−1 were used for all treatments. After 6 to 8 days of treatment, at both temperatures, blue vs. red light promoted plant elongation, as demonstrated by a greater plant elongation rate, final plant height, and hypocotyl length, in arugula but not in mustard. Blue vs. red light also promoted some shade-avoidance responses such as decreased cotyledon size in both species and increased petiole length and dry mass partitioning to hypocotyls in arugula only. The elongation promotion in arugula by blue light was greater at 18 °C than at 28 °C, showing interactions between light and temperature on most plant traits. For mustard, plant elongation was promoted at 28 °C compared to 18 °C independent of light treatment, showing no interactions between light and temperature on most plant traits. These results suggest that the blue light-mediated elongation as a shade-avoidance response is not reversed by high temperature, despite the varying sensitivity with temperatures and species.

Magic Blue Light: A Versatile Mediator of Plant Elongation

“…We found that there are reported discrepancies in the morphological responses to blue vs. red LED light for plants under conditions other than an in vitro environment. Specifically, blue LED light was found to promote stem or leaf elongation in eggplants [ 23 , 24 ], cherry tomatoes [ 25 ], cucumber [ 26 , 27 ], watermelon [ 28 ], sesame [ 29 ], arugula [ 30 , 31 , 32 , 33 , 34 , 35 , 36 ], kale [ 30 , 32 , 33 , 34 ], cabbage [ 30 , 32 , 33 , 34 , 37 ], sunflowers [ 16 , 38 ], peas [ 39 ], calibrachoa [ 19 , 40 , 41 , 42 , 43 ], petunias [ 17 , 21 , 22 , 40 , 41 , 42 , 43 , 44 ], marigolds [ 18 , 40 , 42 , 43 ], geraniums [ 40 , 42 , 43 ], tulips [ 45 ], and wild Arabidopsis [ 46 , 47 , 48 ], compared to ...…”

“…Specifically, blue LED light was found to promote stem or leaf elongation in eggplants [ 23 , 24 ], cherry tomatoes [ 25 ], cucumber [ 26 , 27 ], watermelon [ 28 ], sesame [ 29 ], arugula [ 30 , 31 , 32 , 33 , 34 , 35 , 36 ], kale [ 30 , 32 , 33 , 34 ], cabbage [ 30 , 32 , 33 , 34 , 37 ], sunflowers [ 16 , 38 ], peas [ 39 ], calibrachoa [ 19 , 40 , 41 , 42 , 43 ], petunias [ 17 , 21 , 22 , 40 , 41 , 42 , 43 , 44 ], marigolds [ 18 , 40 , 42 , 43 ], geraniums [ 40 , 42 , 43 ], tulips [ 45 ], and wild Arabidopsis [ 46 , 47 , 48 ], compared to red LED light ( Table 1 ). On the other hand, blue LED light was observed to inhibit stem or leaf elongation in l...…”

“…The promotional effects of BL on elongation were often more pronounced under 24 h vs. 16 h lighting. In a further study, with a photoperiod of 12 h d −1 , B vs. R LED light also promoted plant elongation for arugula [ 31 ]. These findings suggest that the promotion of elongation growth by BL is not solely dependent on the 24 h lighting cycle, despite varying promotional magnitudes under different photoperiods.…”

“…In our initial experiments, only four ornamental species (petunias, calibrachoa, geraniums, and marigolds) were evaluated [ 40 , 41 , 42 , 43 ]. However, subsequent experiments expanded the scope to microgreens such as arugula, mustard, cabbage, and kale [ 30 , 31 , 32 , 33 , 34 , 35 ], and other microgreens/baby greens such as sunflower, cilantro, celtuce, basil, and pak choi (unpublished data; Figure 2 ), as well as the model plant Arabidopsis [ 46 , 47 , 48 ]. Most of the tested species exhibited increased plant elongation under BL relative to R light when exposed to continuous (24 h d −1 ) lighting at a PPFD of 100 µmol m −2 s −1 , except for mustard, cilantro, and celtuce.…”

“…Therefore, it is possible that the cryptochrome activity under B at a light intensity of 100 µmol m −2 s −1 was high enough to inhibit hypocotyl elongation but not main stem elongation. However, unlike the Arabidopsis seedlings, species such as arugula, cabbage, and kale exhibited increased hypocotyl elongation under BL relative to R light [ 30 , 31 , 32 , 33 , 34 , 35 ]. It appears that whether or not the growth stage affects BL-mediated plant elongation varies with species.…”