ABSTRACT
Plants have developed a range of protective mechanisms to mitigate the deleterious effects of excess light by either scavenging toxic activated oxygen species or preventing their production (Anderson et al., 1997). An increase in the expression of antioxidant genes including ascorbate peroxidase and dehydroascorbate reductase (DHAR) and several unknown genes — either regulatory or of metabolic enzymes — was recently demonstrated by Rossel et al., (2002) in Arabidopsis. This increased gene expression was caused when plants of Arabidopsis were transferred from a light intensity of 100 [i mol m~2s_1 to a high light of 1000 |i mol m"2s_1 for lhour. Therefore, plants respond effectively to oxidative stress and display
increased reased level of antioxidants to mitigate stress. Both enzymatic and non-enzymatic components exist in higher plants related to the detoxification of ROS formed already. Such components include antioxidant non-enzymatic small molecules, ascorbate, glutathione and a-tocopherol (Fryer, 1992). It has been shown that reduced ascorbate and reduced glutathione are involved in preventing photo-oxidation and providing photoprotection (Augusti et al., 2001). However, the results drawn by Xu et al., (2000) have shown in rice leaves that exogenous application of glutathione has suppressed the formation of A and Z and also inhibited NPQ. The results suggested that the action of GSH is on xanthophyll cycle. The lipid soluble antioxidant a-tocopherol is located in the thylakoid membranes and counteracts the effects of ROS (Gonzalez - Rodriguez et al. 2001). The enzymes, including superoxide dismutase, catalase, glutathione reductase and ascorbate peroxidase, are the other group of antioxidants, (Logan et al., 1998; Vranova et al., 2002). Ascorbate peroxidase exists in isoenzymes and plays an important role in the metabolism of H 2 0 2 in higher plants (Shigeoka et al. 2002).
