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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.

BACKGROUND The effect of light quality on phytochemicals in romaine baby leaf lettuce ‘Thumper’ was investigated in (I) a closed environment and (II, III) a greenhouse (16 h, 21/17 °C): (I) basal (638, 455, 660, 735 nm) LEDs supplemented with UV (380 nm), green (510 nm), yellow (595 nm) or orange (622 nm) LEDs (PPFD of ∼175 µmol m−2 s−1); (II) high‐pressure sodium (HPS) lamps (90 µmol m−2 s−1) supplemented with blue (455, 470nm) or green (505, 530nm) LEDs (30 µmol m−2 s−1); (III) at 3 days before harvesting, HPS lamps (90 µmol m−2 s−1) supplemented with red (638 nm) LEDs (210 µmol m−2 s−1). RESULTS (I) Supplemental UV or orange light enhanced phenolic compounds, supplemental UV or green light enhanced α‐carotene, and supplemental green light enhanced anthocyanins. All supplemental LED colours had a negative effect on tocopherol and ascorbic acid levels. (II) HPS lighting supplemented with different LEDs was not efficient, since the increase in some compounds did not compensate the decrease in major tested phytochemicals. (III) Short‐term irradiation with supplemental 638 nm LEDs before harvesting in the greenhouse did not have a significant effect on phytochemical contents, apart from enhancing tocopherols. CONCLUSION Wavelength control using LED technology affects the production of secondary metabolites, as the metabolism of many nutrients is light‐dependent. The narrow‐bandwidth supplemental light effects were diminished by broader‐spectrum HPS light or natural daylight in the greenhouse. © 2013 Society of Chemical Industry