Roles of the ascorbate–glutathione cycle, pigments and phenolics in postharvest ‘sunscald’ development on ‘Granny Smith’ apples (Malus domestica Borkh.)

Abstract

Postharvest sunscald on apples is a physiological disorder that develops on the skin during cold storage. The symptoms develop only on sun-exposed sections of the fruit with or without sun-injury and they appear as superficial dark-brown patches. In order to determine the role of different antioxidant systems in sunscald development, fruit with different sun exposures and sun-injury levels on the tree were harvested and stored at 0°C for up to 4 months. Ascorbic acid (AsA), glutathione (GSH), AsA–GSH recycling enzymes activities and transcripts levels, and flavonoids and carotenoid concentrations were monitored monthly. Unexposed fruit (shaded) did not develop sunscald. On sun-exposed sections of the fruit, sunscald incidence increased and appeared closer to harvest, as sunburn severity increased. Total ascorbic acid (AsA) was the highest on fruit with no sunburn symptoms (shaded and sun-exposed), but over 90% of it was in its oxidized form. There were no clear differences for any of the AsA–GSH recycling enzyme activities or transcription levels between sun exposures levels during cold storage.All quercetin glycosides were higher in fruit with sun-injury compared to those sun-exposed or shaded throughout the storage period. In severe sunburned or shaded fruit, quercetin glycosides represented 73% and 10% of total phenolic content, respectively. Quercetin glycosides contents at harvest were highly correlated with sunscald appearance in storage.Sunscald symptoms appearance was associated with lower quercetin glycosides concentration on fruit skin. The results indicate that the AsA–GSH cycle does not play a direct role in ‘sunscald’ development on Granny Smith apples. Instead, quercetin glycosides appear to be directly related with sunscald expression postharvest, i.e. the higher their concentration in the tissue, the faster the accumulation of their brown oxidative products.