EFFECTS OF ANTIOXIDANTS

Effects of Antioxidants in Wheat on Lipid Peroxidation

Effects of Antioxidants in Wheat on Lipid Peroxidation

Introduction

Wheat (Triticum aestivum L.) is one of the most important crops in Iran, which plays a special role in people's nutrition. But unfortunately abiotic stresses, such as salinity, decrease wheat growth and productivity by reducing water uptake and cause nutrient disorders and ion toxicity in this region. Reactive oxygen species (ROS) are regarded as the main source of damage to cells under biotic and abiotic stresses (Candan and Tarhan, 2003; Bor et al., 2003; Gara at al., 2003; Mittler, 2002; Vaidyanathan et al., 2003). ROS's are partially reduced forms of atmospheric oxygen, which are produced in vital processes such as photorespiration, photosynthesis and respiration (Mittler, 2002; Uchida et al., 2002). To produce water in these processes, four electrons are required for perfect reduction of oxygen. But ROS typically results from the transference of one, two and three electrons, respectively, to O2 to form superoxide (O2 ·-), peroxide hydrogen (H2O2) and hydroxyl radical (HO·) (Mittler, 2002). These species of oxygen are highly cytotoxic and can seriously react with vital biomolecules such as lipids, proteins, nucleic acid, etc, causing lipid peroxidation, protein denaturing and DNA mutation, respectively (Breusegem et al., 2001; Scandalios, 1993; Quiles and Lopez, 2004). Evidence suggests that membranes are the primary sites of salinity injury to cells and organelles (Candan and Tarhan, 2003) because ROS can react with unsaturated fatty acids to cause peroxidation of essential membrane lipids in plasmalemma or intracellular organelles (Karabal et al., 2003; Stewart and Bewley, 1980). Peroxidation of plasmalemma leads to the leakage of cellular contents, rapid desiccation and cell death. Intracellular membrane damage can affect respiratory activity in mitochondria, causing pigment to break down and leading to the loss of the carbon fixing ability in chloroplasts (Scandalios, 1993). Fortunately, plants have developed various protective mechanisms to eliminate or reduce ROS, which are effective at different levels of stress-induced deterioration (Beak and Skinner, 2003).The enzymatic antioxidant system is one of the protective mechanisms including superoxide dismutase (SOD: EC 1.15.1.1), which can be found in various cell compartments and it catalyses the disproportion of two O2 ·- radicals to H2O2 and O2 (Scandalios, 1993). H2O2 is eliminated by various antioxidant enzymes such as catalases (CAT: EC 1.11.1.6) (Kono and Fridovich, 1983; Scandalios, 1993) and peroxidases (POX: EC 1.11.1.7) (Gara et al., 2003; Jablonski and Anderson, 1982) which convert H2O2 to water. Other enzymes that are very important in the ROS scavenging system and function in the ascorbate-glutathione cycle are glutathione reductase (GR: EC1.6.4.2), monodehydro ascorbat reductase (MDHAR: EC 1.6.5.4) and dehydroascorbate reductase (DHAR: EC 1.8.5.1) (Candan and Tarhan, 2003; Yoshimura et al, 2000). Moreover, ROS are inevitable byproducts of normal cell metabolism (Martinz et al., 2001). But under normal conditions production and destruction of ROS is well regulated in cell metabolism (Mittler, 2002). When a plant faces harsh conditions, ROS production will overcome scavenging systems and oxidative stress will burst. In these conditions, ROS attack vital biomolecules and disturb the ...