## 4. Discussion
Proline has been reported to reduce enzymatic denaturation caused by NaCl and other stresses. Accumulation of proline attenuates the reduction in the activity of antioxidant enzymes induced by saline stress. Proline is considered not only as a compatible solute and an osmoprotectant, but also as a scavenger of hydroxyl radicals. Under stressful conditions, proline stabilizes proteins, membranes and sub-cellular structures and protects cellular functions by eliminating ROS [53]. Inoculation reduced the accumulation of proline in the plant grown in the same soil. Studies on the impact of PGPR on proline concentration in stressed plants are contradictory. Many studies have reported that proline levels are higher in inoculated plants compared to uninoculated plants while others have reported lower proline content in plants inoculated with PGPR. These disagreements can be due to different bacterial species, the mechanism of bacterial communication with the plant, the interaction between bacteria, and the intensity of salt stress. Besides its role as an osmoprotectant, proline is also considered to be a stress marker. As a result, inoculated plants may accumulate less proline as stress is reduced [38]. There was no difference in proline content between inoculation treatments and controls under normal conditions, which was consistent with the result published by [PERSON] et al. [54], indicating that PGPR inoculation did not influence proline content under stress-free conditions. Lipid peroxidation measured by MDA levels is an oxidative degradation process of lipids which results in the production of ROS. The most common targets for lipid peroxidation are biological membranes. These modifications lead to functional changes that disrupt cell metabolism [55]. Therefore, increased lipid peroxidation is considered an indication of increased oxidative damage often applied to estimate the tolerance of plants to salinity and drought. In the present study, an increase in lipid peroxidation at the leaf level in the three soils was noted, however, the PGPR treatment reduced the levels of MDA in plants grown in saline soils. Comparable results have been reported in cucumber plants treated with PGPR under drought stress, in white clover [56] inoculated and cultivated under saline conditions. A significant reduction in MDA content had been shown in soybean plants inoculated with _Azospirillum brasilense_ and _A. chrooccum_ compared to uninoculated controls under unstressed and drought-stressed conditions [54]. This reduction in lipid peroxidation because PGPRs reduced cellular damage was caused by abiotic stress and increased tolerance to environmental stress. Antioxidant enzymes play a critical role in detoxifying the destructive effects of ROS created in plants under water or salt stress. Under saline conditions (Soil 2), the activities of APX, GPX, and SOD enzymes were more intense, while those of CAT were little variable. According to, [PERSON] et al. [57], the activity of antioxidant enzymes in wheat was improved with an increasing salt concentration in the soil. The induction of enzymes activities of CAT, COX, and APX under salt stress was higher in salt-tolerant cultivars [58]. The decrease in antioxidant enzymatic activities observed after inoculation with PGPR can be attributed to the enhancement of the adverse effects of salt stress thus reducing the production of ROS. These results were consistent with those reported by [PERSON] et al. [55] who reported that PGPR inoculation reduced SOD activity in susceptible cultivars of _Oryza sativa_ and improved resistance to saline stress. The results of the reduction of the antioxidant enzyme, APX, CAT, and glutathione reductase (GR) following the treatment of wheat with _B. subtilis_ and _Arthobacter_ sp. under stress conditions have been reported by [PERSON] et al. [57]. In contrary, [PERSON] and [PERSON] [59] showed that inoculation of wheat with _Stenotrophomonas maltophilia_ increased the activities of SOD, CAT, and COX under saline conditions.