Reactive Oxygen Species Metabolism Modulation on the Quality of Apple Fruits Inoculated with Penicillium expansum under Different Ambient pHs

Apple blue mold is a significant postharvest disease caused by Penicillium expansum. pH modification in colonized tissues leads to the production of organic substances, the modulation of enzymes, and then increases fungal pathogenicity. This study evaluated Penicillium expansum-inoculated apple fruits’ quality responding to pH treatments ranging from 2.5 to 8.5 and analyzed the reactive oxygen species (ROS) metabolism modulation in inoculated apple fruits at the same pH. The results showed that the fruit quality of the firmness, total soluble solids, and titratable acid displayed a quick loss at pHs 5.0 and 7.0, compared with 2.5 and 8.5. Similarly, higher disease incidence was observed at pHs 5.0 and 7.0. Apple fruits infected with P. expansum at pHs 2.5 and 8.5 had less content of O2•−, H2O2, and malondialdehyde (MDA); lower enzymatic activity of NADPH oxidase (NOX); and greater cell membrane integrity than those at pHs 5.0 and 7.0. The analysis of the antioxidant enzymatic activities showed upregulation of superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), and glutathione reductase (GR) at pHs 2.5 and 8.5 compared with those at pHs 5.0 and 7.0. Similar trends were shown in ascorbic acid and glutathione. These results support the hypothesis that inoculated apple fruits at pHs 2.5 and 8.5 improve resistance to P. expansum by modulating ROS metabolism, compared with pHs 5.0 and 7.0.