*2.2.3 Detoxifying enzymes*

Drought as other kinds of abiotic and biotic stresses may produce oxidative stress. The oxidative stress is caused by at least two processes: (i) an imbalance of production and detoxification of reactive oxygen species (ROS) and (ii) de novo ROS biosynthesis as a response for defense and adaptation to environment. These processes produce significant variations in the general cellular redox state [53, 73]. The term ROS includes substances with one or more activated atoms of oxygen that can be radicals. Some free radicals do not contain oxygen atoms. The key ROS are triplet oxygen, singlet oxygen, superoxide anion radical, hydrogen peroxide, and hydroxyl radical [53]. Oxidative stress causes lipid peroxidation that impairs membranes and induces loss of their barrier function, and consequently a breakdown of organelles occurs [74]. Plants have evolved defense systems against excess of ROS. Those systems include nonenzymatic and enzymatic responses. The enzymatic system consists of several enzymes such as superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), glutathione reductase (GR), and others that eliminate or scavenge ROS [75].

Plants native to arid lands may have antioxidant defense systems to limit the deleterious effects of ROS. For example, in *Oudneya africana*, a Saharan plant, water deficit caused variations in enzymatic and nonenzymatic antioxidants, differentially affecting the concentration of SOD and POX [76]. Transgenic poplar expressing the eukaryote translation initiation factor 5A (TaeIF5A1) from *Tamarix androssowii* showed greater superoxide dismutase (SOD) and peroxidase (POD) activities, lower electrolyte leakage, and improved tolerance to abiotic stresses [77]. Six plant species from semiarid Loess Hilly Region of China showed significant differences in SOD and POD activities when subject to drought along 3 months. Also there were significant interactions of SOD, CAT, and POD activities and MDA content between months and species [78]. A transcriptomic study under drought stress showed that *Prunus mongolica* Maxim, a species widely distributed in the Gobi Desert, increased transcription of iron superoxide dismutase and manganese superoxide dismutase which promoted drought stress tolerance [31].
