Effects of different mechanical injury treatments on the metabolism of reactive oxygen species in banana peel

Abstract: During harvesting and transportation, bananas are highly susceptible to mechanical damage, which will not only affect the aesthetics, but also act as a pathogen center to further infect normal fruits, leading to secondary infection and causing great losses to the storage and transportation of fruits. At present, the research on mechanical damage of banana mainly focuses on controlling the occurrence of mechanical damage, but there are fewer reports from the mechanism level. Therefore, in this experiment, three different mechanical damage treatments, including scratch, puncture and fall, were used, with untreated as the control group, and stored at (25 ± 1)°C and (80 ± 5)% RH for 8 d, with samples taken at 2 d intervals. Fruit hardness, chlorophyll content, cell membrane permeability, malondialdehyde content, reactive oxygen species content and antioxidant enzyme activities were measured to analyze the relationship between mechanical damage, reactive oxygen species and storage quality and their possible mechanisms. The results showed that the three different mechanical damage treatments accelerated the decrease of banana fruit hardness and chlorophyll content compared with the control; it inhibited the activities of Peroxidase (POD), Superoxide Dismutase (SOD), Catalase (CAT) and Ascorbate Peroxidase (APX) in banana peel, led to the accumulation of reactive oxygen species O2- and H2O2, accelerated the process of cell membrane lipid peroxidation, increased the permeability of banana cell membrane and Malondialdehyde (MDA) content, and then accelerated the deterioration of banana storage quality. The correlation clustering heat map analysis showed that O2- production rate, H2O2 content, MDA content and cell membrane permeability indexes were negatively correlated with each other and hardness, chlorophyll content, SOD activity and APX activity indexes. Among them, H2O2 content was negatively correlated with hardness, chlorophyll content and APX activity (P < 0.01) and with SOD activity (P < 0.05), which was consistent with the gradual decrease of storage quality such as hardness and chlorophyll content with the accumulation of ROS. It was speculated that ROS enrichment might affect fruit softening and chlorophyll degradation, and that CAT, SOD, APX, and POD did not respond to reactive oxygen stress caused by mechanical injury in this experiment. The correlation cluster heat map analysis also revealed that storage time, O2-production rate, H2O2 content, polyphenol oxidase activity, MDA content and cell membrane permeability were positively correlated with each other, which was consistent with the gradual increase of O2- production rate, H2O2 content, MDA content and cell membrane permeability in different treatment groups with the increase of storage time. Notably, polyphenol oxidase activity showed a highly significant positive correlation (P < 0.05) with H2O2 content, MDA content and cell membrane permeability, indicating that polyphenol oxidase may be indirectly involved in mechanical injury stress by responding to reactive oxygen stress. Combined with the principal component analysis, different mechanical injury treatments could accelerate the accumulation of reactive oxygen species in banana peel, which in turn led to the decay and spoilage of banana fruit. Among them, scratch treatment had the greatest effect on banana peel reactive oxygen metabolism, followed by puncture treatment, and falling treatment had the least effect on banana peel reactive oxygen metabolism.