Drought and Excess Uv-b Irradiation Differentially Alter the Antioxidant System in Cucumber Leaves

Almost all environmental stresses induce oxidative stress in plants as an early and rapid response (Bolkhina et al., 2003). Under drought stress, overproduction of highly reactive oxygen species (ROS) in chloroplasts, represented by the superoxide anion (O2 ·-), hydrogen peroxide (H2O2), hydroxyl radical (·OH) and singlet oxygen (O2) has been reported (Smirnoff, 2002; Foyer and Noctor, 2003). This effect was also observed in chloroplasts of plants under enhanced UV-B irradiation (Santos et al., 2004). Plants have an endogenous mechanism to protect cellular and subcellular systems from the cytotoxic effects of ROS. Antioxidant enzymes such as superoxide dismutase (SOD), catalase (CAT), Halliwell-Asada pathway enzymes and peroxidases (POX) are widely distributed in all higher plants and are involved in decomposition of O2 ·and H2O2 (Foyer and Noctor, 2000). Peroxidases, especially their anionic forms located in cell walls, are engaged in peroxidation of phenolic compounds in the presence of H2O2 (Grabber et al., 1997). These processes cause lignification of the cell wall, creating a barrier against pathogen attack (Lewis and Yamamoto, 1990), and also increasing the rigidity of the cell wall under drought conditions (DaMatta et al., 2002). Enhanced UV-B radiation or drought can decrease net photosynthetic capacity and lead to the reduction of root, stem and leaf biomass and yield. Such effects were shown to be greater under a combination of the two stresses than under single ones (Feng et al., 2007). The results suggested that the costresses of supplementary UV-B irradiation and drought functioned synergistically, and that one of them could differentially affect the inhibitory effects of the other. Recent studies indicate that the response of a plant to a combination of two different abiotic stresses is unique and cannot be extrapolated from the response to each stress applied alone. Tolerance to a combination of stress conditions should be the focus of research aimed at developing plants with enhanced tolerance to naturally occurring environmental conditions. Surprisingly, the co-occurrence of different stresses is rarely addressed by molecular biologists, and its physiological aspects are also insufficiently understood (Mittler, 2006). This study was intended to determine how two environmental stresses, water deficit and UV-B radiation, applied alone or together, can alter the activity of the antioxidant enzymes SOD, glutathione