Antioxidant Enzyme Activities as a Tool to Discriminate Ecotypes of Crithmum maritimum L. Differing in Their Capacity to Withstand Salinity

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Salt stress causes a range of adverse effects in plants, mainly ionic disorders, osmotic stress and nutritional imbalance. A common feature of these effects is the production of reactive oxygen species (ROS) (Ashraf and Foolad, 2007). Thus, salt stress causes stomatal closure, which reduces the CO2/O2 ratio inside leaf tissues and inhibits CO2 fixation (Hernandez et al., 1999). As a consequence, an over reduction of the photosynthetic electron transport chain occurs, which causes the generation of ROS such as singlet oxygen (1O2), superoxide anion (O2.−), hydrogen peroxide (H2O2), and hydroxyl radical (•OH). It is now widely accepted that ROS are responsible for various stress-induced damages to macromolecules and ultimately to cellular structure (Mittler, 2000; Ashraf, 2009). Plants are equipped with a set of non-enzymatic scavengers and of antioxidant enzymes that act in concert to alleviate cellular damage under oxidative stress conditions (Foyer and Noctor, 2000). Superoxide dismutase (SOD) reacts with the superoxide radical at almost diffusion-limited rates to produce H2O2 (Scandalios, 1993). H2O2 is scavenged by peroxidases, especially ascorbate peroxidase (APX), and catalase (CAT). CAT has been found predominantly in leaf peroxisomes where it functions chiefly to remove H2O2 formed in photorespiration or in β-oxidation of fatty acids in the glyoxysomes (Dat et al., 2000). APX, which uses ascorbic acid as a reductant in the first step of the ascorbateglutathione cycle is the most important plant peroxidase involved in H2O2 detoxification (Foyer and Halliwell, 1976; Noctor and Foyer, 1998). ROS are also scavenged nonenzymatically by hydrophilic antioxidants, such as ascorbate and glutathione (GSH), most of them being found in photosynthetic tissues. In the last two decades, a number of reviews have concentrated much on the role of various antioxidant enzymes and metabolites in plant salt tolerance (Jitesh et al., 2006; Ashraf, 2009). Due to considerable variations in the mechanisms of defense against ROS among plant species, it is difficult to generalize the involvement of this phenomenon in salt tolerance