Light Intensity and Photoperiod Affect Growth and Nutritional Quality of Brassica Microgreens

We explored the effects of different light intensities and photoperiods on the growth, nutritional quality and antioxidant properties of two Brassicaceae microgreens (cabbage Brassica oleracea L. and Chinese kale Brassica alboglabra Bailey). There were two experiments: (1) four photosynthetic photon flux densities (PPFD) of 30, 50, 70 or 90 μmoL·m^-2·s^-1 with red:blue:green = 1:1:1 light-emitting diodes (LEDs); (2) five photoperiods of 12, 14, 16, 18 or 20 h·d^-1. With the increase of light intensity, the hypocotyl length of cabbage and Chinese kale microgreens shortened. PPFD of 90 μmol·m^-2·s^-1 was beneficial to improve the nutritional quality of cabbage microgreens, which had higher contents of chlorophyll, carotenoids, soluble sugar, soluble protein and vitamin C, as well as increased antioxidant capacity. The optimal PPFD for Chinese kale microgreens was 70 μmol·m^-2·s^-1. Increasing light intensity could increase the antioxidant capacity of cabbage and Chinese kale microgreens, while not significantly affecting glucosinolate (GS) content. The dry and fresh weight of cabbage and Chinese kale microgreens were maximized with a 14-h·d^-1 photoperiod. The chlorophyll, carotenoid and soluble protein content in cabbage and Chinese kale microgreens were highest for a 16-h·d^-1 photoperiod. The lowest total GS content was found in cabbage microgreens under a 12-h·d^-1 photoperiod and in Chinese kale microgreens under 16-h·d^-1 photoperiod. In conclusion, the photoperiod of 14~16 h·d^-1, and 90 μmol·m^-2·s^-1 and 70 μmol·m^-2·s^-1 PPFD for cabbage and Chinese kale microgreens, respectively, were optimal for cultivation.