Enzyme extraction and activities assay

The fresh leaf of L. japonica under different salt stresses were collected for determining the antioxidant activity. The leaves (0.5 g) were homogenized with 4.5 mL PBS in an ice water bath and centrifuged at 350 g (catalase, CAT), 685 g (superoxide dismutase, SOD and peroxidase, POD), and 5595 g (ascorbate peroxidase, APX) for 10 min, respectively. The supernatant was used to examine the activities of the enzyme.

The activity of SOD was assayed at 550 nm using the hydroxylamine method; the activity of POD was determined at 420 nm according to the colorimetric method^25,26; the activity of CAT was measured at 405 nm based on the ammonium molybdate method, and the activity of APX was determined at 290 nm. All of the experiments were carried out by assay kits. The amount of 200 μL reaction solution was measured by multi-mode microplate reader using UV–Vis spectrophotometer (DENOVIX DS-11, USA).