Since microgreens are harvested a few days after germination, and they are consumed for their nutritional importance, the focus of photo-morphogenesis for these high-value crops is on phytochemical concentration.
Microgreens refer to seedlings or immature greens which are harvested two to three weeks after germination. These usually include the cotyledons, the first set of fleshy leaves, and could also include the first set of true leaves. Normal salad greens, leafy vegetables, flowering plants, and herbs, when harvested shortly after germination, can be classified as microgreens (1).
The plants popularly grown as microgreens are members of the Brassica genus since they are easy to germinate such as broccoli, beets, mizuna, kohlrabi, radish, pak choi, tatsoi, arugula, mustard, and kale. Some other species grown as microgreens are cilantro, amaranth, etc. (2, 3).
Microgreens are popular because they contain high quantities of phytochemicals which prevent diseases, and are good for overall health. The concentration of many essential nutrients is also higher in microgreens compared to their mature forms (2).
It is not just the color or quality of light that is important; the intensity of light measured as photosynthetic photon flux densities (PPFD) can also be important. This latter aspect, is of course, also an important economic consideration, and the effort is to try to maximize crop yield and quality with less input of energy or light intensity (4).
Effects of light quality and intensity have been studied for red pak choi (Brassica rapa var. chinensis ‘Rubi F1’), tatsoi (Brassica rapa var. rosularis), mustard (Brassica juncea L ) varieties, kohlrabi (Brassica oleracea var. gongylodes), and mizuna (Brassica rapa var. japonica)
Light intensity has proven to be more important than the color of the LEDs in regulating the plant growth parameters to get the ideal microgreens which have short hypocotyl, large leaves, and more weight.
Combinations of red, green, far-red and blue LEDs, where the percent of red is 74% or more are usually favorable for growing microgreens. Three different combinations red:green:blue 74:18:8 (R74:G18:B8), red:blue 87:13 (R87:B13), or red:far-red:blue 84:7:9 (R84:FR7:B9), had similar effects on the vegetative growth of kohlrabi, mizuna, and mustard microgreens (5). Broccoli microgreens’ growth and biomass were not influenced positively or negatively when grown under blue LED or a combination of red and blue LEDs (6).
Moderate intensities of LED grow lights are more suitable for optimum brassica microgreen vegetative growth in red pak choi, tatsoi, mustard and kohlrabi (4). Low levels of LEDs at 110 μmol m-2 s-1 are not advisable. In kohlrabi, mizuna, and mustard microgreens, hypocotyl length decreased, and dry weight increased when light intensity was increased from 105 to 315 µmol m−2 s−1 (5).
At higher PPFD of 545-440 μmol m-2 s-1, the length of the stem or hypocotyl of mustard, kohlrabi, and tatsoi were shorter compared to those exposed to PPFD of 220 μmol m-2 s-1. Lower PPFD levels of 110 μmol m-2 s-1 produced longer hypocotyls in red pak choi and kohlrabi (4).
Increasing PPFD to 545 and 440 μmol m-2 s-1 increased plant weight in the case of red pak choi and tatsoi; this also increased leaf area in mustard and pak choi (4).
Leaf chlorophyll concentrations were improved by supplying levels of 545-440 μmol m-2 s-1 in red pak choi and mustard (4). This is important for photosynthesis and the appearance of the microgreens, which are green in color. Anthocyanins are important for red colored herbs.
Low light intensities result in lower production of phytochemicals. Moderate amounts are the best, since using very high light intensity of 545 μmol m-2 s-1 does not produce any additional benefit.
Phenols: Moderate levels of PPFD at 440 μmol m-2 s-1 increased the levels of phenols in kohlrabi, pak choi, and tatsoi (4). In mustard, light intensity produced no difference in phenol levels (4).
Anthocyanins: The highest concentrations of anthocyanin were produced at the moderate light intensity of 330 μmol m-2 s-1 in red pak choi and tatsoi. It was only kohlrabi that needed 440 μmol m-2 s-1 for optimum production of anthocyanins (4).
Nitrates: Low levels of light intensity result in higher concentration of nitrates in the microgreens than desired (4).
Sucrose: Sucrose production needed varying levels of light intensity in the different species. High intensity was necessary for mustard and kohlrabi, while moderate levels of light were sufficient for red pak choi and tatsoi (4).
Vitamins and DPPHs: Lower light intensity of 110 μmol m-2 s-1 produced the highest quantities of α tocopherol (Vitamin E) in all the species. (4). Ascorbic acid and DPPH free radical activity concentrations were not affected by the light intensity in any of the species (4).
Essential Nutrients: The concentration of different macro- and micro-nutrients such as glucoraphanin, potassium (K), magnesium (Mg), and iron (Fe) in broccoli microgreens were higher when grown under blue light than a combination of red and blue (6).
Moreover, a short duration exposure to blue LED grow lights before the harvest of broccoli microgreens increased the levels of beta-carotene, glucoraphanin, glucosinolates, epiprogoitrin, and essential nutrients such as macronutrients of calcium (Ca), phosphorus (P), potassium (K), magnesium (Mg), and sulfur (S), increasing their nutritional value (6).
Regardless of the colour of light of the LEDs, blue, red or far-red, or the combinations used, moderate light intensities of 320–440 μmol m-2 s-1 are more suitable to promote both growth and nutritional qualities of microgreens since this “produces larger leaves, higher total anthocyanins, total phenols, DPPH free-radical scavenging capacity and lower content of nitrates” (4).