Implication of the antioxidant system in chilling injury tolerance in the red peel of grapefruit

• Increasing lycopene in Star Ruby peel boosted singlet oxygen scavenging capacity.
• Singlet oxygen is involved in the deleterious process originating CI in grapefruit.
• CI tolerance was not related to the antioxidant capacity or GSH or AsA.
• Catalase partially contributes to the CI tolerance in lycopene-enriched peel.

Previous observations have indicated that the red peel areas of grapefruit with high lycopene concentrations were more tolerant to CI than yellow peel areas (Lado et al., 2015a). Because lycopene is a carotene with powerful antioxidant capacity, this study investigated whether the CI tolerance of the lycopene-accumulating rind of grapefruit may be due to an enhancement of the enzymatic and/or non-enzymatic antioxidant systems. Total antioxidant capacity, antioxidant metabolite (GSH and AsA) contents, and antioxidant enzyme (GR [glutathione reductase], ascorbate peroxidase [APX], catalase [CAT] and superoxide dismutase [SOD]) activity and gene expression were measured in the peel of Star Ruby grapefruit with contrasting CI tolerance during storage at 2 °C for up to 58 d. The peel of CI-tolerant fruit exhibited a lower lipid peroxidation level (MDA content). The hydrogen peroxide concentration was similar after 3 weeks of storage, when the differences in chilling damage between sensitive and tolerant fruit were noticeable, suggesting that the increase in H2O2 is a response of flavedo cells to cold stress that is not necessarily related to the development of CI. Moreover, CI tolerance was not associated with enhancement of either total antioxidant capacity or glutathione and AsA contents, indicating that such antioxidant responses may be cold-mediated and not directly linked to chilling tolerance. Analysis of singlet oxygen scavenging capacity by the SOAC assay revealed considerably higher activity in the lycopene-accumulating peel than in the yellow peel at harvest time and throughout the entire cold storage and shelf-life period. Enzymatic activity and gene expression analyses of GR, APX and SOD did not reveal the involvement of these antioxidant enzymes in the protection against CI. However, high CAT activity was detected in the peel of CI-tolerant fruit, although this difference did not correlate with changes in the expression levels of the CAT1 and CAT2 genes. Therefore, the boost in singlet oxygen scavenging capacity is likely the primary factor responsible for CI tolerance in the lycopene-accumulating peel of grapefruit.