## 2 Materials and Methods
### Determination of Defence Enzyme Activities  
Superoxide dismutase activity (SOD, EC 1.15.1.1) was determined using the nitrogen blue tetrazolium (NBT) photoreduction method [26]. Plant leaves (0.5 g) were removed, 1 mL of precooled phosphate buffer (pH 7.8) was added, the mixture was ground into a pulp on an ice bath, and buffer was added to a final volume of 5 mL. Then, the homogenate was centrifuged for 10 min at 1789\(\times\)\(g\) at 4 \({}^{\circ}\)C, and the resulting supernatant was obtained as the crude SOD extract. First, 1.5 mL phosphate buffer (0.05 mol/L), 0.3 mL Met (methionine, 130 mmol/L), 0.3 mL NBT (750 \(\upmu\)mol/L), 0.3 mL EDTA-Na\({}_{2}\) (disodium ethylenediaminetetraacetate, 100 \(\upmu\)mol/L), 0.3 mL riboflavin (20 \(\upmu\)mol/L), 0.05 mL enzyme extract, and 0.25 mL distilled water were added to a transparent test tube. The control tube was used to replace the enzyme extract with buffer. After mixing, one control tube was placed in the dark, and the other tubes were allowed to react for 20 min under 55 \(\upmu\)mol/(m\({}^{2}\cdot\)s) white light. After the reaction was complete, the control tube without light was used as a blank, and the absorbance of the other tubes was measured at 560 nm. SOD activity units were expressed as one enzyme activity unit (U) for 50% inhibition of NBT photochemical reduction.  
Peroxidase activity (POD, EC 1.11.1.X) was determined using the guaiacol method [26]. Plant leaves (0.5 g) were weighed and ground into a homogenate with an appropriate amount of phosphate buffer (pH 5.5). The homogenate was centrifuged at 1006\(\times\)\(g\) for 10 min, after which the supernatant was transferred to a 25 mL volumetric flask. The precipitate was extracted twice with phosphate buffer and the supernatant was fixed at 25 mL and stored at low temperature for further use. The following reaction mixture was used for the enzyme activity assay: 2.9 mL phosphate buffer (0.05 mol/L), 1.0 mL H\({}_{2}\)0\({}_{2}\) (2%), 1.0 mL guaiacol (0.05 mol/L), and 0.1 mL enzyme extract were added sequentially. The enzyme extract, which was heated and boiled for 5 min, was used as the control, and the reaction system was kept warm in a 37 \({}^{\circ}\)C water bath for 15 min immediately after the enzyme extract was added. Then, the mixture was quickly transferred to an ice bath, and the reaction was terminated by adding 2.0 mL trichloroacetic acid (20%). The mixture was centrifuged at 2795\(\times\)\(g\) for 10 min and appropriately diluted, and the absorbance was measured at 470 nm.  
Catalse activity (CAT, EC 1.11.1.6) was determined using the UV absorption method [26]. First, fresh plant leaves (0.5 g) were ground into a homogenate with 3 mL precooled phosphate buffer (pH 7.8) and the homogenate was fixed in a 25 mL volumetric flask. The upper part of the clarified solution was taken and centrifuged at 1789\(\times\)\(g\) for 15 min, and the supernatant was collected as the crude peroxidase extract and stored at 5 \({}^{\circ}\)C for further use. First, 0.2 mL enzyme extract, 1.5 mL phosphate buffer (pH 7.8), and 1.0 mL distilled water were added, and the control blank tubes were cooked in a boiling water bath for 1 min to stop enzymatic activity. Then, all the test tubes were preheated at 25 \({}^{\circ}\)C. Next, 0.3 mL H\({}_{2}\)O\({}_{2}\) (0.1 mol/L) was added to the tubes. The time was recorded immediately after each addition, and the samples were quickly poured into a quartz cuvette. Then, the absorbance was measured at 240 nm, and the readings were taken once every 1 min for a total of 4 min.