Antioxidant defence in halophytes under high salinity.

Abstract

Abstract Understanding the various mechanisms of salt-stress tolerance is important for crop improvement, especially when approximately 20% of agricultural land and 50% of crop land in the world is under salt stress. Stomatal closure due to salt stress reduces the CO2:O2 ratio inside the leaf tissues and inhibits CO2 fixation. This inhibition leads to over-reduction of the photosynthetic electron transport chain and causes the generation of reactive oxygen species (ROS) such as singlet oxygen (1O2), superoxide anion (O2.-), hydrogen peroxide (H2O2) and hydroxyl radical (OH.). These ROS are responsible for various stress-induced damages to macromolecules and ultimately to cellular structure. Hence, to avoid excessive ROS accumulation during stress and maintain the correct levels of ROS for signalling, plants possess a complex antioxidant defence system including non-enzymatic antioxidants such as ascorbic acid, glutathione, tocopherols and carotenoids; and enzymatic antioxidants such as superoxide dismutase (SOD), catalase (CAT), peroxidase (POX) and ascorbate peroxidase (APX). Although ROS production and defence mechanisms are common in glycophytes and halophytes, detoxification strategies may vary in response to salinity with regard to total antioxidant activity and type of iso-enzymes expressed. In this chapter we describe salinity, the classification of saline soils, genetic variability within halophytes, sources and overproduction of ROS under high-salinity conditions, the antioxidant defence system, enzymatic antioxidant system and nonenzymatic antioxidant system.