Water Deficit — Induced Oxidative Stress and Antioxidative Defenses in Rice Plants

Abstract

Summary The disturbance of cellular balance between prooxidants and antioxidants in photosynthetic tissues encountering water-limited environments and the resultant oxidative stress symptoms were studied, using rice seedlings hydroponically cultured and water stress-treated in 30 % polyethylene glycol. Water deficit induced lipid peroxidation, chlorophyll bleaching and loss of small molecular antioxidants including ascorbate, glutathione, α-tocopherol and carotenoids in rice shoots. The activities of active oxygen-processing enzymes, such as superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APOX) and nonspecific peroxidase (POX) decreased either slightly (in SOD, APOX, and POX), or substantially (in CAT) during water deficit. In contrast, monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR) and glutathione reductase (GR) showed significantly increased activities in waterstressed shoots. Such response of the ascorbate-glutathione cycle enzymes was not seen in rice seedlings when pretreated with cycloheximide. The steady state level of H2O2 in rice shoots was markedly lowered by the stress treatment despite the decreased activities of H2O2 - scavenging enzymes. No significant increase was seen in the content of ‹catalytic Fe› that presumably promotes nonenzymatic breakdown of H2O2 during water deficit in plants. Various water stress symptoms were effectively suppressed when light was removed from the growth environment and exacerbated with increasing intensities of illumination even under low irradiance conditions. The results imply that water deficit-induced oxidative stress in plants arises from an increase in formation of a certain strong prooxidant in excess of the antioxidative capacity of photosynthetic cells and that this prooxidant, requiring light for its formation, is neither superoxide nor H2O2. It is further suggestive that the increased production of the responsible prooxidant and its effects in rice plant cells during water deficit are mainly countered by the actions of small molecular antioxidants, most importantly ascorbate in conjunction with a-tocopherol, and thus the increase in the capacity of ascorbate regeneration system in cells by de novo synthesis of MDHAR, DHAR and GR is one of the primary responses of plants to water deficit so as to mitigate oxidative stress.