CHANGES IN THE ACTIVITIES OF ANTIOXIDANT ENZYMES DURING EXPOSURE OF INTACT WHEAT LEAVES TO STRONG VISIBLE-LIGHT AT DIFFERENT TEMPERATURES IN THE PRESENCE OF PROTEIN-SYNTHESIS INHIBITORS

Changes in activities of the enzymes involved in the metabolism of active oxygen species were followed in homogenates prepared from wheat leaves (Triticum aestivum L.) exposed to strong visible light (600 W m-2). The activities of superoxide dismutase (SOD), ascorbate peroxidase, and monodehydroascorbate reductase increased significantly on prolonged illumination of the leaves, indicating an increase in the rate of generation of active oxygen species. This increase was further exacerbated when high light stress was combined with low temperature (8-degrees-C). Our results indicate that the increase in activities of SOD and ascorbate peroxidase involved de novo protein synthesis that was sensitive to the nuclear-directed protein synthesis inhibitor cycloheximide. The activity of catalase, on the other hand, decreased on exposure to strong light, which could be due to its photolability, particularly at lower temperatures. Ascorbate and total carotenoid contents also increased on light treatment of the leaves. The induction of the enzymes except for catalase and increase in the levels of ascorbate and total carotenoids in response to the stress conditions indicate that they play an important role in the protection of higher plants from the damaging effects of toxic active species.