## 4 Discussion
### Chlorophyll Fluorescence
The F\({}_{\text{V}}\)/F\({}_{0}\) indicates the photochemical and non-photochemical quantum efficiency [88]. The application of acids positively affected this parameter in III (PEG 5%) or IV (7.5%), indicating that the efficiency of oxygen evolution in PS II was improved. All aerobic organisms use molecular oxygen to generate ATP, which is the chemical energy useful for life. Oxygen has a central role in the evolution of complex life on Earth, mainly because of the biochemical symmetry of oxygenic photosynthesis and aerobic respiration (H\({}_{2}\)O \(\rightarrow\) O\({}_{2}\)\(\rightarrow\) H\({}_{2}\)O) [89]. The appearance of oxygenic photosynthetic organisms on Earth suggests that low and/or localised levels of photosynthetically produced oxygen necessitated the emergence of ROS scavenging mechanisms to protect life [90]. Nevertheless, oxygen may also be toxic and mutagenic through the production of reactive oxygen species (ROS). Free radical generation can be considered a double-edged sword because, on the one hand, O\({}_{2}\)-dependent reactions and aerobic respiration have significant advantages. Still, on the other, the generation of ROS has the potential to cause damage [89]. The effect may indicate that organic acids play a role in alleviating reactive oxygen species caused by stress [91], consistent with the findings of [PERSON] et al. [92] regarding castor beans. The maximum photochemical quantum PSII (F\({}_{\text{V}}\)/F\({}_{\text{M}}\)) is a vital metric to ascertain the health of plants under varied stress. It indicates the maximum quantum yield of the PSII [93]. According to [PERSON] and [PERSON] [94], the F\({}_{\text{V}}\)/F\({}_{\text{M}}\) for unstressed tissues in plants ranges from 0.75 to 0.85. In this study, the average value F\({}_{\text{V}}\)/F\({}_{\text{M}}\) of stressed (up to the application of PEG 7.5%) and unstressed ranges from 0.83 to 0.86. This indicates that water stress did not depict damage to the photosynthetic apparatus, though there was evidence of a decline in its functionality. The decreasing tendency in F\({}_{0}\) amidst a stable F\({}_{\text{V}}\)/F\({}_{\text{M}}\) signals no adverse photodamages and inhibitions in the lettuce tissues [95]. This may indicate that the F\({}_{\text{V}}\)/F\({}_{\text{M}}\) is not a reliable metric for analysing plant drought stress, as suggested by many authors. The performance index (PI\({}_{\text{ABS}}\)) estimates the all-inclusive functionality of the flow of electrons through Photosystem II, indicating plants' homeostasis [96]. The decreasing PI\({}_{\text{ABS}}\) can be primarily associated with the decreased functionality of the photochemistry and the photochemical efficiency of electron transport correlated with fluctuations in the DI0/RC [97]. Our research showed an increasing PI\({}_{\text{ABS}}\) trend under the influence of acid application, particularly CA and MA (PEG 5%, end of experiment) or CA, SA, and MA (PEG 7.5%; third term of fluorescence measurement). Obtained results correlate with the trends of F0 changes, indicating a positive aspect of acid application. ABS/RC quantifies the flow of energy via the reaction centre. It is determined by the ratio of inactive/active reaction centres [98]. Increasing drought stress generally causes decreased energy absorption in the reaction centres. An increase in ABS/RC for SA, MA, and CA (PEG 7.5%, IV T) treatment indicates enhanced energy flow due to the conversion efficiency of excited energy (TR\({}_{0}\)/RC). These findings were in line with [PERSON] et al. [99], who observed enhanced energy flow for SA (2 mM) on salinity-stressed radish. A generally decreasing tendency of TR0/RC under drought stress may gradually impede the effective conversion of excited energy in the PSII. What is important is that the application of acids at PEG 7.5 had a stimulating effect--that is, it alleviated plant stress.