## 5 Drought Stress and Antioxidant Defense System  
Most of the plant defensive system is devoted to contrast the adverse consequences of drought-triggered ROS. In this context, a prompt, powerful and efficient antioxidant system is of pivotal importance to provide drought tolerance [75]. This machinery involves enzymatic and non-enzymatic detoxification moieties, which lessen and repair injury triggered by ROS. Enhancement  
Figure 2: Schematic representation of effect of drought stress on photosynthesis (modified from [PERSON] et al. [17]).  
of the antioxidant apparatus helps in ROS scavenging that decreases electrolyte leakage and lipid peroxidation, therefore maintaining the vitality and integrity of organelles and cell membrane [76].  
It is well recognized that drought induces oxidative stress by generating ROS, for instance O\({}_{2}\)\({}^{\bullet-}\), hydroxyl radicals (OH\({}^{\bullet}\)), singlet oxygen (\({}^{1}\)O\({}_{2}\)) and H\({}_{2}\)O\({}_{2}\)[77]. The proportion of ROS generation and antioxidant enzyme activities regulates the cell redox state, thereby resulting in ROS control or cell injury and cell death when ROS exceed the physiological levels [78]. Numerous studies conducted under water deficit conditions found enhanced activities of pivotal antioxidant enzymes, namely CAT, SOD, POD and APX [79]. Usually, tolerant species/varieties/genotypes have an enhanced antioxidant enzymes activity in comparison to non-tolerant plants, which is supportive for their essential role in drought tolerance, especially to control H\({}_{2}\)O\({}_{2}\) and O\({}_{2}\)\({}^{\bullet-}\) production and diffusion in leaf tissues [80].  
Production of O\({}_{2}\)\({}^{\bullet-}\) and H\({}_{2}\)O\({}_{2}\) were controlled by superoxide dismutase (SOD), peroxidase (POX) and catalase (CAT) action, whose activity was enhanced for example in drought-tolerant potato genotypes [81]. Ascorbate peroxidase (APX) also participates as excess ROS scavenger (APX uses ascorbate as a substrate to stimulate the conversion of H\({}_{2}\)O\({}_{2}\) to H\({}_{2}\)O), and its activity is usually elevated under stress conditions [82]. Alteration in APX activity in leaves is more common than in fibrous roots because APX mainly occurs in the chloroplast and cytoplasm and is a crucial enzyme for scavenging H\({}_{2}\)O\({}_{2}\) in chloroplasts [83]. Activities of SOD, POD, CAT and APX were altered and played a key role in protecting peny plants against acute water deficit [84]. The amount of non-enzymatic antioxidants (ascorbic acid, reduced glutathione and \(\alpha\)-tocopherol) and antioxidant enzymes (SOD, CAT and APX) activities were simultaneously enhanced in _Coleus pretentthus_ in drought stress conditions [85]. SOD, CAT and COX enzymes activities were stimulated by limited water availability in _Vicia faba_[70]. Increase of SOD, POX and CAT activities was observed in drought-tolerant genotype, in comparison to the drought sensitive plants of faba bean [86]. The amount of enzymatic and non-enzymatic antioxidants improved in drought tolerant plants under mild and moderate water deficit conditions [87]. CAT, SOD, POD and APX activities increased in _Adonis amurensis_ and _Adonis pseudoamurensis_ subjected to drought, indicating that improved functioning of these enzymes helps to lower the level of ROS and mitigate the drought generated oxidative stress [88]. Water deficit boosted the levels of SOD and POD in _Vigna mungo_ and the authors concluded that increased levels of these enzymes stimulate tolerance against drought stress and are vital to reduce its adverse effects [71]. Water deficit increased the CAT, POX and SOD levels in leaves of _Glycyrrhiza glabra_ L., which aimed at counteracting the spread of H\({}_{2}\)O\({}_{2}\)[89].