Continuous Lighting Increases Yield and Nutritional Value and Decreases Nitrate Content in Brassicaceae Microgreens

Microgreens of four species of the family Brassicaceae (broccoli, mizuna, radish, and arugula) were grown under 16- and/or 24-h photoperiod conditions. In the first series of experiments, the daily light integral (DLI) was different (15.6 and 23.3 mol m –2 day –1 at PAR 270 µmol m –2 s –1 ), while it was the same (15.6 mol m –2 day –1 at PAR 270 µmol m –2 s –1 and 180 µmol m –2 s –1 ) in the second. In the third series of experiments, continuous lighting was used only in the last three days before harvesting. The results obtained showed that broccoli, mizuna, radish, and arugula plants in the early phases of growth are resistant to continuous lighting and do not show typical signs of leaf photodamage. In all three series of experiments, microgreens of all four species grown under 24-h photoperiod had a higher yield and nutritional value (higher content of substances with antioxidant properties—anthocyanins, flavonoids, carotenoids, and proline—as well as increased activity of antioxidant enzymes) and a lower content of nitrates compared to plants grown under 16-h photoperiod. It was concluded that it is possible through the use of continuous lighting without increasing energy costs (while maintaining the DLI) to increase the yield and nutritional value of the studied species of microgreens and reduce their nitrate content compared to the standard 16-h photoperiod. In addition, an increase in nutritional value and a decrease in nitrate content is also possible with the use of continuous lighting (as an agricultural practice) for several days immediately before harvesting.