All organisms are exposed to reactive oxygen species such as hydrogen peroxide (H2O2), superoxide anion (O2· –) and hydroxyl radical (·OH) as unavoidable products of oxidative metabolism. Many biotic and abiotic factors such as: pathogens, air pollutants, salts, xenobiotic, heavy metals, UV radiation initiate ROS formation. The main sites of ROS formation in plant cells are chloroplasts, peroxisomes and mitochondria. ROS can cause wide-ranging damage to many macromolecules including proteins, lipids and nucleic acids, eventually leading to cell death. Recently considered positive biologi- cal roles of ROS act as signal-transducting molecules. To minimize the damaging effects of ROS, aerobic organisms evolved several antioxidant defense mechanisms, including catalase, superoxide dismutase, peroxidases and alternative oxidase, and non-enzymatic molecules such as: ascorbic, glutathione, homo- glutathione, cysteine.