4. Salicylic acid and ROS detoxification

Salicylic acid is the most studied phytohormone regarding its role in oxidative stress. Due to its multifarious actions, it has been found very effective in detoxifying ROS in plant cells (Figure 2). According to Janda and Ruelland [144], SA-induced tolerance to abiotic stresses such as chilling, heat, heavy metals, osmotic stress, and salinity is involved in activation of the stress-induced antioxidant system. It has been demonstrated that SA could significantly improve both photosynthesis parameters and antioxidant defense system in conferring salt stress tolerance in V. radiata [17]. In O. sativa, exogenous SA significantly reduced the oxidative burst by reducing H2O2 and O2 •� contents under herbicide exposure which was mainly due to the SA-mediated upregulation of antioxidant defense enzymes (SOD, POD, CAT, APX, GR, and GST) and efficient GSH pool. The positive role of SA is mostly dependent on their dose and application methods [145]. Other study also proved that SA-mediated antioxidative defense system was dependent on the concentration used and the method of application [146]. In T. aestivum, 0.25 mM SA resulted in marked increase in the antioxidant enzyme activities (SOD, CAT, POD, GPX, APX, and GR), while the treatment with 2.5 mM SA resulted in a decrease in the activities under water stress [146]. A lower dose of SA also maintains higher AsA pool which in turns significantly scavenged the ROS. This ROS detoxification induced by SA also associated with improved photosystem II (PSII) efficiency. Belkadhi et al. [147] showed that Linum usitatissimum plants showed a lower amount of lipid and protein oxidation and membrane oxidation under Cd stress when pretreated with SA. This protection was availed by the enhanced activities of SOD, GPX, and APX as well as the 2,2<sup>0</sup> -diphenyl-1-picrylhydrazyl (DPPH) scavenging activity and ferricreducing antioxidant power (FRAP). Increases in MDHAR, DHAR, GR, GPX, and CAT activities 53, 64, 49, 82, and 65% were noticed in PEG-treated B. juncea seedlings when sprayed with 50 μM SA [44]. SA-induced upregulation of antioxidant enzymes caused 32 and 26% decrease in MDA and H2O2 content compared to drought (20% PEG, 48 h) alone. One of our research results showed that exogenous SA enhanced the activities of antioxidant enzymes and nonenzymatic antioxidants under salt stress (200 mM NaCl, 48 h). Compared to salt stress alone, NaCl + SA resulted in 41, 107, 25, 37, 44, and 59% increases in MDHAR, DHAR, GR, GST, GPX, and CAT activities [40]. Salicylic acid supplementation also increased AsA and GSH content by 48 and 39%, respectively and enhanced GSH/GSSG ratio by 47% compared to salt stress alone. As a result, MDA and H2O2 contents decreased by 39 and 31% [40]. In Nitraria tangutorum SA mitigated salt-induced oxidative stress (evidenced by a marked reduction in MDA and H2O2 content) by upregulating the activities of SOD, POD, and CAT. The content of MDA in the 1.5 mM SA treated seedlings under 100–400 mM NaCl treatments declined to 2.27–3.59 fold of the control which was a clear sign of the reduction of oxidative stress [148]. However, some of the enzymes like APX activity was inhibited at higher concentrations (1.0 and 1.5 mM) of SAMDA content was measured with 1.5 mM SA applied, and the contents of MDA in the leaves of SAtreated seedlings under 100–400 mM NaCl treatments declined to only 2.27–3.59 fold of the control.
