Abstract

A distinct type of postharvest skin browning on apple (Malus domestica Borkh.) fruit called ‘‘stain’’ is a frequent disorder in ‘Fuji’ grown under high light and elevated temperatures. Symptoms typically develop only on sun-exposed sections of the fruit regardless of the presence of sunburn symptoms, and sometimes only in the margins of this area. The role of different antioxidant systems in tissue exposed to different levels of sunlight and having different degrees of sun injury were investigated during cold storage [1 8C, >90% (relative humidity) RH]. Ascorbic acid (AsA) and glutathione (GSH) concentrations, AsA–GSH recycling enzyme activities and gene expression, and flavonoids and carotenoid concentrations were determined every 30 days. ‘‘Stain’’ incidence increased with sun exposure and sunburn level. Both shaded and exposed fruit peel without sunburn symptoms had the highest AsA content. The AsA–GSH recycling enzyme activities and gene expression levels had no clear relationship with sun exposure during cold storage. Chlorophyll a (chl a) and chlorophyll b (chl b) levels diminished overtime and were higher in tissue without any type of sun injury. In contrast, carotenoid levels increased as sun injury incidence increased and remained relatively stable during storage. Total phenolics and quercetin glycoside levels changed coincidently during storage. Results indicate that the AsA–GSH cycle does not have a clear role in ‘‘stain’’ development. Nevertheless, reduced ascorbate levels may reduce the capacity to prevent oxidative stress–provoked damage which may, in turn, result in oxidation of quercetin glycosides, which would then lead to skin browning.