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Four spectrophotometric methods of determination of antioxidant capacity: a method based on the scavenging of the l,1-diphenyl-2-picrylhydrazyl (DPPH) free radical, the "ferric-reducing ability of plasma" (FRAP), a method based on reduction of the 2.2'-azinobis (3-ethylbenzthiazolinesulfonate) free radical (ABTS.+) and a kinetic method based on the oxidation of dihydro-2,7-dichlorofluorescein by 2,2'-azobis(2-amidopropane) (ABAP) were compared with respect to standard antioxidants (ascorbate, glutathione, Trolox and urate) and human blood plasma. Various reactivities of standard antioxidants in different tests were found. glutathione showing a low reactivity in the FRAP assay. Kinetic measurements show that the reduction of indicators, especially by blood plasma, may not be complete at recommended times of the assays and the time of measurement is an important parameter when comparing the results.

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Oxidative stress, defined as a shift of the balance between prooxidative and antioxidative reactions in favor of the former seems to be a common denominator of the action of various agents on living organisms. This review briefly presents the sources of reactive oxygen species and means of antioxidative defense in plants, means of assessment of oxidative stress and exemplary data on the induction of oxidative stress by various environmental and biological factors such as hyperoxia, light, drought, high salinity, cold, metal ions, pollutants, xenobiotics, toxins, reoxygelmtion after anoxia, experimental manipulations, pathogen infection and aging of plant organs.List of abbreviations: OS, oxidative stress; ROS, reactive oxygen species; SOD, superoxide disnmtase

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The current understanding of the complex role of ROS in the organism and pathological sequelae of oxidative stress points to the necessity of comprehensive studies of antioxidant reactivities and interactions with cellular constituents. Studies of antioxidants performed within the COST B-35 action has concerned the search for new natural antioxidants, synthesis of new antioxidant compounds and evaluation and elucidation of mechanisms of action of both natural and synthetic antioxidants. Representative studies presented in the review concern antioxidant properties of various kinds of tea, the search for new antioxidants of herbal origin, modification of tocopherols and their use in combination with selenium and properties of two promising groups of synthetic antioxidants: derivatives of stobadine and derivatives of 1,4-dihydropyridine.

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