Fig. 2. Changes in the levels of accumulated ROS and activities of ROS-scavenging enzymes in apple leaves under heat stress.

a In situ accumulation of H₂O₂ and superoxide radical (O₂⁻) before (left panels) and after (right panels) heat treatment revealed by DAB and NBT staining, respectively. Quantitative measurement of b H₂O₂ and c O₂⁻ concentrations in apple leaves treated with and without high temperature. Activities of d superoxide dismutase (SOD), e peroxidase (POD) and f catalase (CAT) after 4 h of heat treatment. Data are shown as the means of three replicates with SEs. Different letters indicate significant differences between treatments, according to one-way ANOVA and Tukey’s multiple range test (P < 0.05).