## 4 Discussion
### Physiological Parameters  
According to our study, proline content increased significantly under drought stress compared to the control group. On the other hand, we observed that SA, MA, and CA treatment induced more accumulation of the free proline in the water-stressed and well-wated plants (Figure 4B). Drought stress increased the proline level of wheat [100; 101; 102], sunflower [103], tomatoes, amaranth [104; 105], bean [106], and linesed [107], the level of which has increased further by the exogenous application of SA. These results show that salicylic acid treatment may stimulate proline biosynthesis and is involved in protective mechanisms mitigating stress. It is known that drought stress induces rapid and excessive production of ROS (i.e., superoxide anion and hydrogen peroxide), leading to lipid peroxidation and, consequently, membrane damage [108; 109; 110]. Superoxide anion radical formation is the primary cause of oxidative damage. Our research shows that its level significantly increased under various stress intensities compared to the level of H\({}_{2}\)O\({}_{2}\), which insignificantly decreased (Figure 3). These results suggest that H\({}_{2}\)O\({}_{2}\) may play a secondary role in the drought stress signalling network by inducing defence pathways in the late drought phase. The end products of lipid peroxidation, like malondialdehydedehyde (MDA) or trans-4-hydroxynonenal (4-HNE), can change the physical properties of cell membranes, disrupting their integrity and, thus, destroying the cell's protective function. In this vein, mapping the movement of cell apoplastic ion and metabolite patterns could offer novel insights into the function of membrane integrity, particularly during stressful conditions. This understanding could shed light on how cells maintain their protective barriers and respond to environmental challenges [111]. The increase in MDA content in cells indicates oxidative stress in plants and disorders in the functioning of antioxidant defence, which shows cell damage [112; 113; 114; 115]. When the ROS level reaches above the threshold, enhanced lipid peroxidation takes place in both cellular and organellar membranes, which, in turn, affects normal cellular functioning. The most damaging oxidative effect is lipid peroxidation of the biological membranes, which results in the concomitant production of malondialdehydedehyde (MDA). Under a strong stress factor, the activity of enzymes and antioxidant compounds may be insufficient to counteract the generated ROS, which may consequently lead to membrane damage manifested by an increased level of MDA [116]. Our results revealed that lipid peroxidation, measured in terms of malondialdehydehyde content, was prevented using acids. Exogenous lettuce application of salicylic acid (SA),maleic acid (MA), and citric acid (CA) significantly decreased the MDA, superoxide anion, and H\({}_{2}\)O\({}_{2}\) levels in stressed plants. Our results supported that the decrease in membrane damage may be related to the induction of antioxidant responses by acids that protect the cell from oxidative damage. A similar mechanism was responsible for SA-induced stress tolerance in bean and tomato plants [106; 117]. Salicylic acid treatment also prevented lipid peroxidation in _Ctenanthe setosa_[118], raise [119] and wheat [102]. Pretreatment of sensitive and tolerant drought maize cultivars with salicylic acid almost totally prevented a lipid peroxidation increase. Additionally, the salicylic acid foliar application significantly declined MDA content in the _Cucurbita pepo_ plant leaves under normal and stress conditions [120]. Based on the literature data, it was shown that exogenous CA improved the drought tolerance of _Lilium_ Cv, _G. barbadense_, cotton plants, and _Brassica oleracea_[121; 122]. In addition, CA affected the H\({}_{2}\)O\({}_{2}\) and phosphorus metabolism levels in cabbage [123]. Our results showed that CA reduced H\({}_{2}\)O\({}_{2}\) and MDA levels in lettuce leaves compared to untreated plants, which confirms the above research results.