This study examines the effect of irradiance level produced by solid-state light-emitting diodes (LEDs) on the growth, nutritional quality and antioxidant properties of Brassicaceae family microgreens. Kohlrabi (Brassica oleracea var. gongylodes, ‘Delicacy Purple’) mustard (Brassica juncea L., ‘Red Lion’), red pak choi (Brassica rapa var. chinensis, ‘Rubi F1’) and tatsoi (Brassica rapa var. rosularis) were grown using peat substrate in controlled-environment chambers until harvest time (10 days, 21/17°C, 16 h). A system of five lighting modules with 455, 638, 665 and 731 nm LEDs at a total photosynthetic photon flux densities (PPFD) of 545, 440, 330, 220 and 110 µmol m−2s−1 respectively were used. Insufficient levels of photosynthetically active photon flux (110 µmol m−2 s−1) suppressed normal growth and diminished the nutritional value of the Brassica microgreens studied. In general, the most suitable conditions for growth and nutritional quality of the microgreens was 330–440 µmol m−2 s−1 irradiation, which resulted in a larger leaf surface area, lower content of nitrates and higher total anthocyanins, total phenols and 2,2-diphenyl-1-picrylhydrazyl (DPPH) free-radical scavenging capacity. High light levels (545 µmol m−2 s−1), which was expected to induce mild photostress, had no significant positive impact for most of investigated parameters.

The purpose of this study was to evaluate the role of 638-nm and 665-nm LEDs on changes of antioxidants of basil (Ocimum basilicum) and parsley (Petroselinum crispum), and to assess the effect of light quality on antioxidative status. Plants were grown in peat substrate for 19 days (21/17 ±2°C, 16 h). Experiments were performed in (I) a controlled-environment: B₄₅₅,R₆₃₈,R₆₆₅,FR₇₃₁(control); B₄₅₅,R*₆₃₈,R₆₆₅,FR₇₃₁; B₄₅₅,R₆₃₈,R*₆₆₅,FR₇₃₁; R₆₃₈; R₆₆₅ (B–blue, R- red, FR–far-red light). PPFD was set from 231 during growth, upto 300 μmol m^-2 s^-1 during 3-day treatment changing R₆₃₈ or R₆₆₅ PPFD level; in (II) greenhouse (November): high-pressure sodium lamps (HPS) (control—300 μmol m⁻²s⁻¹); and HPS + 638 (HPS generated 90 and red LEDs—210 μmol m⁻²s⁻¹). In general, under supplemental or increased red 638 nm light, amounts of tested antioxidants were greater in basil, whereas sole 665 nm or sole 638 nm is more favourable for parsley. Increased or supplemental red light significantly increased contents of phenolics, α-tocopherol, ascorbic acid and DPPH^• but suppressed accumulation of lutein and β-carotene in basil, whereas an increase of β-carotene and DPPH^• was observed in parsley. Hereby, the photoresponse of antioxidant compounds suggests that photoprotective mechanism is stimulated by both light-dose-dependent and wavelength-dependent reactions.

To increase the nutritional value and levels of essential minerals in vegetable food, microgreens are promising targets. The metabolic processes of microgreens can be managed with different cultivation techniques, which include manipulating the properties of light derived by light-emitting diodes (LEDs). In this study Brassicaceae microgreens (kohlrabi Brassica oleracea var. gongylodes, broccoli Brassica oleracea, and mizuna Brassica rapa var. Japonica) were cultivated under different light spectral quality, and the metabolic changes insoluble sugars (hexoses and sucrose), ascorbic acid, β-carotene, and contents of non-heme iron (Fe) and its connection with magnesium (Mg) or calcium (Ca) levels were monitored. Plants grew under the primary LED light spectrum (the combination of blue light at 447 nm, red at 638 and 665 nm, and far-red at 731 nm) or supplemented with LED green light at 520 nm, yellow at 595 nm, or orange at 622 nm. The photoperiod was 16 h, and a total PPFD of 300 µmol m^-2 s^-1 was maintained. Under supplemental yellow light at 595 nm, the content of soluble carbohydrates increased significantly in mizuna and broccoli. Under all supplemental light components, β-carotene accumulated in mizuna, and ascorbic acid accumulated significantly in kohlrabi. Under supplemental orange light at 622 nm, Fe, Mg, and Ca contents increased significantly in all microgreens. The accumulation of Fe was highly dependent on promoters and inhibitors of Fe absorption, as demonstrated by the very strong positive correlations between Fe and Ca and between Fe and Mg in kohlrabi and broccoli, and the strong negative correlations between Fe and β-carotene and between Fe and soluble carbohydrates in kohlrabi. Thus, the metabolic changes that occurred in treated microgreens led to increases in the contents of essential nutrients. Therefore, selected supplemental LED wavelengths can be used in the cultivation of Brassicaceae microgreens to preserve and increase the contents of specific nutritionally valuable metabolites.

We report on a substantial reduction of nitrate concentration in leafy vegetables that were subjected to short-term preharvest treatment by narrow-bandwidth red light of high photosynthetic photon flux density generated by a solid-state illuminator. Lettuce ( Lactuca sativa cv. Grand rapids), marjoram ( Majorana hortensis , Moench.), and green onions ( Allium cepa , L. cv. Lietuvos didieji) were grown to harvest time within a greenhouse under daylight with supplementary lighting provided by standard high-pressure sodium lamps (130 μmol·m ⁻² ·s ⁻¹ ). A subsequent 3-day treatment within a phytotron under 638-nm light-emitting diodes (500 μmol·m ⁻² ·s ⁻¹ ) resulted in the reduction of nitrate concentration by 44% to 65%. The reduction of nitrates was accompanied by an increased concentration of nutritionally valuable carbohydrates, which is also in line with stimulation of expression of nitrate reductase by photosynthetic metabolites. Another indicator of nutritional quality, the content of vitamin C, exhibited some variation that was not directly correlated with the nitrate reduction rate that may be attributed to carbohydrate content variation and leaf tissue aging.