ZnSO4 spraying affects the growth and phytochemicals of Chinese kale microgreens

Chinese kale microgreens are gaining interest for its phytochemical compounds and anti-cancer properties mainly from glucosinolates. Zinc is an essential for plant growth and is involved with phytochemicals accumulation. This study investigated the different concentration of ZnSO4 on the enhancement of the phytochemicals and quality of Chinese kale microgreens. Three-day-old seedlings were sprayed with 2 and 4 mM of ZnSO4 until harvesting on days 4, 5, 6 and 7. ZnSO4 significantly decreased the fresh weight of 6 and 7-day-old microgreens and dry weight on the 7(th) day. The total glucosinolates, total phenolic compounds and total ascorbic acid, gradually rose as the age of microgreen increased. When the 3-day-old microgreens were sprayed at 6, 12 and 18 mM, the fresh and dry weight and hypocotyl length of 7-day-old microgreens significantly decreased as the concentration increases. ZnSO4 at 18 mM was the most effective in enhancing phytochemicals, resulting in increased DPPH radical scavenging activity and ferric reducing antioxidant power. This concentration also induced phenylalanine ammonia lyase activity resulting to an increase in total phenolic compounds. Moreover, ZnSO4 at 18 mM induced H2O2 content but are sufficiently countered with the increase in antioxidant enzymes namely superoxide dismutase, catalase, and ascorbate peroxidase. In conclusion, the 3-day-old microgreen was the most effective age for the start of the daily spraying of ZnSO4 before harvesting at 7-day-old. Higher concentrations of ZnSO4 (6-18 mM) enhanced the phytochemical compounds and enzymatic antioxidant activities but reduced growth. In contrast, lower concentration (< 4 mM) only improved microgreens growth.