Plants, as well as other aerobic organism constantly produce reactive oxygen species (ROS). At regulated low concentrations ROS may serve as signal molecules, while in excessive amounts these may cause oxidative damage to biomolecules. Actual cellular concentrations are controlled by a network of various antioxidants, and acclimation to stress conditions is achieved by a dynamic balance of ROS production and neutralization. Accordingly, plant stress physiology studies generally include an array of methods testing the occurrence of ROS as well as evaluating antioxidant capacities. The aim of the present work is to provide an overview of these methods, with special emphasis on avoiding errors that can possibly lead to either inaccurate data or misinterpretations of otherwise correct measurements.

Harvested mature berry clusters of two white table grape cultivars were used to study the effects of postharvest UV irradiation. One cultivar, 'Queen of Vineyard' (QV), had higher light-acclimated PSII quantum yield, higher phenolic contents, and stronger total antioxidant capacities than the other, 'White Sultana' (WS). These differences were maintained throughout the experiment. Responses of the two cultivars to a 30-min UV irradiation were also different. Antioxidant capacities and flavonol, especially quercetin-3-O-glucuronide, contents were lower 2 h after the UV treatment in both cultivars and recovered in QV but not in WS berry skins later on. Our data demonstrate that mature grapevine berries have photosynthetically active tissues capable of dynamic changes even several hours after harvest and suggest that changes in photochemistry may contribute to postharvest metabolic responses of berry skins. Results also support the potential of postharvest manipulation of fruit qualities with UV irradiation. Highlights• Postharvest UV irradiation affects grapevine berry skin photochemistry • The cultivar with higher photochemical yield was more UV tolerant • Berry skin photosynthesis may support changes in antioxidative metabolites