Effects of multi-corona discharge seed treatment on antioxidant activity and bioactive compound production in kale microgreens

Microgreens are nutrient-dense seedlings known for their high concentrations of antioxidants and bioactive compounds. This study investigated the effects of multi-corona discharge plasma treatment on kale (Brassica oleracea L. var. sabellica) seed germination, growth, antioxidant activity, and metabolite composition. Seeds were exposed to positive and negative DC corona discharges at voltages of ±15, ±22.5, and ±30 kV for 30, 60, and 90 s. While non-thermal plasma treatment reduced growth parameters compared to the untreated control, it significantly enhanced antioxidant activity and bioactive compound content. Under the +30 kV for 90 s treatment with positive polarity, DPPH radical scavenging capacity increased from 43.29 ± 0.46 % to 61.24 ± 0.42 %, FRAP value from 9.62 ± 0.04 to 11.48 ± 0.02 mg Fe(II)/g DW, total phenolic content from 15.59 ± 0.02 to 17.94 ± 0.01 mg GAE/g DW, and total flavonoid content from 58.01 ± 0.12 to 64.35 ± 0.14 mg RE/g DW. Additionally, H₂O₂ content increased significantly in treated seeds (24.94 ± 0.46 vs 18.51 ± 0.23 nmol/g FW), indicating mild oxidative stress. LC-MS/MS analysis revealed a substantial upregulation of metabolites related to antioxidant pathways, including glucosinolates, amino acids, phenolic acids, flavonoids, and nucleosides. Notably, metabolites such as sinapine, 4-methoxyglucobrassicin, and isorhamnetin 3-galactoside were enriched in treated samples. These findings suggest that plasma-induced reactive species act as abiotic stressors, activating the phenylpropanoid and secondary metabolic pathways, ultimately enhancing phytochemical accumulation. This study supports the use of corona plasma treatment as a novel seed priming strategy to improve the nutritional value of microgreens without causing oxidative damage, albeit with some reduction in early growth, underscoring the need to optimize treatment intensity.