## 5 Conclusions  
Drought stress reduced the fresh and dry matter yields of plants. The acid treatment caused increasing tendencies in the fresh weight yield but the response varied depending on the level of drought stress and acid. Mainly MA treatment also enhanced the nutrient uptake of stressed plants. Experimental conditions did not vary the chlorophyll content of leaves. Meanwhile, drought stress increased leaf superoxide anion (O2\({}^{\bullet-}\)) and malondialdehydehehyde (MDA) levels but decreased H\({}_{2}\)O\({}_{2}\). Proline (Pro) and phenolic compounds (TFC) accumulated more in stressed plants. Drought stress also affected the chlorophyll fluorescence. In conclusion, acid foliar spray improved the resistance of plants against drought stress by upregulating the antioxidant defence system and reducing the oxidative damage caused by reactive oxygen species.  
The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app14125119/s1](https://www.mdpi.com/article/10.3390/app14125119/s1)]([https://www.mdpi.com/article/10.3390/app14125119/s1](https://www.mdpi.com/article/10.3390/app14125119/s1)), Table S1: Summary of ANOVA results plant yielding by PEG level and various kinds of foliar spray; Table S2: Summary of ANOVA results for uptake of elements by PEG level and various kinds of foliar spray; Table S3: Results of the Pearson coefficient (r) and \(p\)-value between plant yielding and elements. Correlation coefficients are significant with \(p<0.05\); Table S4: The impact of the application of organic acids on selected parameters of chlorophyll fluorescence (F\({}_{0}\), F\({}_{\mathrm{M}}\)); Table S5: The impact of the application of organic acids on selected parameters of chlorophyll fluorescence (F\({}_{\mathrm{V}}\), F\({}_{\mathrm{V}}\)/F\({}_{\mathrm{M}}\)); Table S6: The impact of the application of organic acids on selected parameters of chlorophyll fluorescence (F\({}_{\mathrm{V}}\)/F\({}_{0}\), PI\({}_{\mathrm{ABS}}\)); Table S7: The impact of the application of organic acids on selected parameters of chlorophyll fluorescence (ABS/RC, TR\({}_{0}\)/RC); Table S8: Summary of ANOVA results for selected parameters of chlorophyll fluorescence by PEG level and various kind of foliar spray; Table S9: Summary of ANOVA results for physiological parameters by PEG level and various kinds of foliar spray; Table S10: Results of the Pearson coefficient (r) and \(p\)-value between physiological parameters. Correlation coefficients are significant with \(p<0.05\); Scheme S1. Acquisition of lettuce plant material for determination.  
Conceptualization, T.K. ([PERSON]); methodology, [PERSON] ([PERSON]), T.K. ([PERSON]), T.C. and M.F.-L., formal analysis, [PERSON] ([PERSON]), T.C., M.F.-L. and J.L.L.; investigation, [PERSON] ([PERSON]), T.C., M.F.-L., J.L.L. and T.K. ([PERSON]); data curation, [PERSON] ([PERSON]), T.C. and M.F.-L.; writing--original draft preparation, [PERSON] ([PERSON]), T.C. and M.F.-L.; statistical analyses, T.K. ([PERSON]) and T.C.; visualisation, T.C.; supervision, project administration, funding acquisition, [PERSON] ([PERSON]). All authors have read and agreed to the published version of the manuscript.  
This research was financed from the subsidy received from the Ministry of Science and Higher Education of Poland.  
Not applicable.  
The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.  
The authors declare no conflicts of interest.