*2.2.2 Oxidative stress and antioxidant metabolism*

Elevated salinity-induced ionic and osmotic stresses are bound to cause oxidative stress, which is a physiological condition of elevated cellular generation of reactive oxygen species (ROS) and or diminished scavenging (antioxidant-mediated) of ROS leading to impaired cellular redox homeostasis. In turn, elevation in the generation of ROS and impaired ROS metabolism/scavenging has been widely reported in plants under soil salinization. The list of major ROS includes O2 •−, •OH, H2O2, and 1O2 are produced in different including chloroplast, mitochondria, apoplast, and peroxisome, under normal conditions of stress such as salinity. If not metabolized and/or scavenged, most elevated cellular ROS may lead to cytotoxicity and severe damage to biological molecules such as proteins, lipids, and nucleic acids. In order to avoid the harmful effects of oxidative stress, plants have triggered an antioxidant defense

system based on two types of machinery [12, 25]. Fortunately, plants are endowed with an antioxidant defense system comprising (enzymes and non-enzymes/ antioxidant metabolites), which in isolation and/or combination tend to scavenge/ metabolize most ROS and thereby avert elevated ROS-caused consequences [26, 27]. Major enzymatic antioxidants comprise superoxide dismutase (SOD), catalase (CAT), guaiacol peroxidase (GPX), glutathione sulfotransferase (GST), ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR) and glutathione reductase (GR). On the other hand, ascorbate (AsA), glutathione (GSH), carotenoids, tocopherols, and phenolics are among the major non-enzymatic antioxidants in the plant stress defense system [27–34].

Given the rising demands on crop yield for keeping up with the burgeoning human population, strategies for sustainably maintaining an optimum crop plant health under increasing soil salinization and climate change and the development of salt-tolerant crop varieties are being exhaustively explored.
