Fortunately, plants have developed various protective mechanisms to cope orreduce the potential danger of ROS, which are effective at different levels ofstress-induced deterioration (Baek and Skinner, 2003). The toxic effects of ROSare counteracted by enzymatic as well as nonenzymatic antioxidative system suchas: superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX),guaiacol peroxidase (G-POD), glutathione reductase (GR), Ascorbic acid (AsA),Tocopherol, Glutathione and phenolic compounds etc. (Ahmad et al., 2008). SOD,which can be found in various cell compartments and it catalyses thedisproportion of two O2- radicals to H2O2 and O2 (Scandalios, 1993). H2O2 iseliminated by various antioxidant enzymes such as CAT and G-POD (Scandalios,1993) which convert H2O2 to water. APXs have high substrate specificity forascorbate and are the primary H2O2 scavenging enzymes in the chloroplasts andcytosol of plant cells (Asada, 1992).
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