## 4 Conclusions  
In conclusion, this study highlights the effectiveness and value of optimizing the extraction parameters for plant materials with limited bioactive compounds, such as water-cross leaves. Utilizing RSM and PLS, this research comprehensively assessed the impact of the extraction temperature, time, solvent composition, and different extraction techniques (PEF, US, and ST). Green extraction methods, when used with an ethanolic solvent, continue to prove their vital importance in improving extraction efficiency. For instance, the TPC of watercress leaves can be increased by 583.38%. Moreover, the combination of PEF, US, and ST techniques, with 50% ethanolic solvent at 80 \({}^{\circ}\)C for 30 min, significantly improved the extraction process, yielding great results for TPC (29.59 mg GAE/g dw), FRAP (58.27 umol AAE/g dw), and DPPH (48.25 umol AAE/g dw). Under these optimized conditions, the extracts also exhibited a rich profile of bioactive compounds, including significant polyphenolic compounds with remarkable medicinal properties, and demonstrated substantial antioxidant activity. This confirms the potential of the extracts as a valuable source of antioxidants. This research emphasizes the efficacy of using combined green extraction methods to enhance the polyphenol content extracted from watercress leaves, providing promising avenues for the development of high-value-added products with enhanced bioactivity, given that the watercress plant already provides a wealth of other essential nutrients.  
**Supplementary Materials:** The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app142210739/s1](https://www.mdpi.com/article/10.3390/app142210739/s1)]([https://www.mdpi.com/article/10.3390/app142210739/s1](https://www.mdpi.com/article/10.3390/app142210739/s1)), Figures S1-S3 comprise the plots for TPC, FRAP, and DPPH that illustrate the comparison between the actual response and the predicted response for each parameter under examination, accompanied by the desirability functions. Figures S4-S6 present the 3D response plots for the responses (TPC, FRAP and DPPH).  
Figure 4: Exemplary HPLC chromatogram at 320 nm of optimal extract of watercress demonstrating identified polyphenolic compounds. (1) Chlorogenic acid; (2) caffeic acid; (3) syringic acid; (4) \(p\)-cumaric acid; (5) ferulic acid; (6) rutin; (7) quercetin 3-\(\beta\)-\(D\)-glucose; (8) luteolin-7-glucoside; (9) narirutin; (10) kaempferol-3-glucose; (11) apigenin-7-O-glucose; (12) myricetin; (13) ros-marinic acid.  
**Author Contributions:** Conceptualization, T.C. and S.I.L.; methodology, V.A. and T.C.; software, V.A.; validation, T.C., K.K. and V.A.; formal analysis, T.C., E.B. and V.A.; investigation, E.N., A.X. and K.K.; resources, S.I.L.; data curation, V.A.; writing--original draft preparation, K.K. and T.C.; writing--review and editing, E.N., A.X., K.K., T.C., V.A., E.B. and S.I.L.; visualization, V.A.; supervision, T.C. and S.I.L.; project administration, S.I.L.; funding acquisition, S.I.L. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research received no external funding.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The original contributions presented in this study are included in the article/Supplementary Material; further inquiries can be directed to the corresponding author.  
**Conflicts of Interest:** The authors declare no conflicts of interest.