Postharvest UV-A and UV-B treatments may cause a transient decrease in grape berry skin flavonol-glycoside contents and total antioxidant capacities

Abstract

Controlled postharvest abiotic stresses have already been shown to affect the secondary metabolism of fresh produce. We applied broad band ultraviolet (either 312 nm centred UV-B or 355 nm centred UV-A) radiation on harvested berry clusters of the red table grape Emperor, and measured berry skin antioxidant capacities and phenolic profiles 2, 24 and 48 h after these treatments. There was no significant change in the amount of caftaric acid, the dominant phenolic acid component featuring higher UV-B than UV-A absorbing capacity. On the other hand, but both types of UV treatment decreased the amounts of the two major quercetin-glycosides 2 h after irradiation. This effect was temporal, and concentration recovered during berry storage at 20 °C under low intensity light. Berry skin UV-A and UV-B absorbing capacities showed the same, transitional decrease regardless of the wavelength of the irradiation. In the whole data set, antioxidant capacities and quercetin-glycoside content data showed strong and positive correlations. Berry skin peroxidase activity, as visualized with diaminobenzidine-staining showed a transitional increase after 2 h in response to both UV treatments. These data suggest that the observed UV-induced postharvest changes in phenolic metabolites are related to the oxidation of flavonols as peroxidase substrates rather than to direct UV photo-effects.