## 4 Discussion
### Physiological Parameters
The cells' self-defence against ROS involves activating or synthesising antioxidant enzymes that deactivate ROS. It was reported that SA application modulates the activities of superoxide dismutase, peroxidase, and catalase enzymes via activating a defensive system, triggering redox changes in signal transduction pathways [124; 125; 126; 127]. Our research has proven that SA treatment during stress conditions causes a significant increase in the level of non-enzymatic antioxidants, like total phenolic compounds (TPCs), flavonoids (Fla), and carotenoids (Car), as well as the accumulation of ascorbic acid (AsA). Of course, our results revealed that drought stress alone significantly increased these antioxidant levels in lettuce leaves and spraying them with SA, CA, and MA further boosted this increase (Figures 2A, 3, 4A). Other studies confirm drought stress significantly increases the content of these plant metabolites [128; 129; 130; 131; 132; 133]. The exogenous application of SA also significantly induced these accumulations in various plant species [134; 135; 136; 137; 138; 139; 140; 135; 134; 136; 137]. All these antioxidant metabolites help regulate metabolism and reduce oxidative damage. The upregulation of antioxidants in stressed conditions indicates stress tolerance [138]. Moreover, it turns out that ascorbate is essential for synthesising proline, which acts as one of the potential osmoprotectants in response to water stress [139]. It is known that salicylic acid is a promising compound for the reduction of stress sensitivity in plants. Moreover, SA is vital in producing plant bioactive compounds [140; 141; 142]. The protective function of SA on plants is associated with its role in regulating the biosynthesis of secondary metabolites, which plays an essential role in plant resistance [143]. Usually, in plants subjected to drought stress, chlorophyll content considerably decreases. It is associated with pigment biosynthesis inhibition, which reduces plant growth [144; 145; 146; 147]. The adverse effects of water deficit on the chlorophyll pigments are caused by the increase in chlorophyll enzyme activity, which destroys chloroplasts and causes chlorophyll degradation [148]. Our research recorded that chlorophyll a and b amounts did not change under applied experimental conditions (Figure 4A). The exogenous application of acids probably reduced chlorophyllase activity and increased carotenoid content, thus participating in energy dissipation and stabilising photosynthetic complexes [149]. It turns out that ascorbic acid (AsA) can also reverse the adverse effects of water deficit stress by significantly improving the chlorophyll contents [129]. In many studies, the exogenous application of SA under water stress reduces the chlorophyll degradation process, corroborating the hypothesis that SA, CA, and MA may mitigate the effect of water deficiency [150; 151; 152]. Our results indicate that the foliar spray of acids did not change chlorophyll pigment levels under stress and control conditions (Figure 4A). The beneficial effects of SA could be attributed to its stimulatory effects on Rubisco activity and, ultimately, to increased photosynthetic rate.