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Abstract ABSTRACT Mohamed Mahmoud Aboul Fotouh Mesalhi: Biochemical and Molecular Responses Associated with UV-Induced Resistance in Plant. Unpublished M.Sc. Thesis, Department of Agricultural Biochemistry, Faculty of Agriculture, University of Ain Shams, 2014. Ultraviolet radiation is known to be one of the environmental stresses that disturb biochemical and physiological processes in plant. However, many studies declared that irradiation of plant tissues with low doses of UV radiation should stimulate adaptive mechanisms, providing better protection to other stresses. In the present investigation, dry and germinated seeds of cucumber and green bean seeds were exposed to UVC (254 nm) for periods of 7, 15, 30 and 60 min, then grown plants were subjected to salinity stress (50 mM NaCl) as a challenge. The ability of different doses to induce tolerance to salinity stress was evaluated by determination of fresh weight, dry weight and root to shoot ratios. Also, biochemical changes associated with UV-C induced tolerance were investigated by determination of lipid peroxidation, total phenols concentration, proline concentration, phenylalanine ammonia lyase (PAL) activity and antioxidant enzymes activities (SOD, CAT, GPOD, APX and PPO) in leaves and roots. As well, the behavior of POD isozymes in leaves was also examined. Results showed that plants grown from UV treated seeds were less affected by salinity stress which was obvious in increased fresh and dry weights of shoots and roots and increased root to shoot ratios under saline conditions as compared with control. Also, one of the consequences of salinity stress is increased MDA levels. UV treatment reduced MDA levels in leaves and roots of cucumber seedlings under different conditions which was not observed in the case of green bean plants in spite of their better growth than control. Reduced MDA levels were explained by the induced antioxidant system (total phenols, PAL, SOD, CAT, G-POD, APX and PPO), while in green bean, the enhanced tolerance despite the increased MDA was explained by the effect of MDA in induction of survival mechanisms in plant. Also, elevated proline concentration in leaves and roots contributed to protection from osmotic shock resulted from salinity stress. The isozymes pattern of POD of cucumber leaves declared that UVC seed treatment did not result in synthesis of new POD isoform. However, it was found that the increments in POD activity were attributed to upregulation of the existing isoforms. The isozymes pattern of POD of green bean leaves showed that the increments caused by treatment of dry seeds was attributed to upregulation of the existing isoform, whereas treatment of germinated seeds led to synthesis of a new isoform under saline conditions. Both treatments enhanced G-POD under saline and non saline conditions. Generally, it could be concluded that UV-C seed treatment activated the antioxidant system in both plants, leading to alleviation of adverse effects of salinity stress. Keywords: UV radiation, Salinity stress, Seed treatment, Cucumber, Green bean, Antioxidant system. |