## 3 Results and Discussion  
### Extraction Optimization  
The main parameters affecting the extraction process of ST and the use of green extraction methods (US and PEF) were assessed to determine the maximum TPC yield. In order to minimize the number of conducted experiments and obtain a better overview, an RSM approach was employed. Using the RSM approach, a multi-factor system was constructed, as shown in Table 2, resulting in 20 different design points. From the results, the combination of the most suitable extraction parameters were identified to ensure the preparation of an extract from dried watercress leaves with the highest possible concentration of bioactive compounds and antioxidant activity. The experimental design used approximately 1 g of dried watercress plant and 20 mL of solvent for each extraction. This proportion was deliberately chosen to ensure the efficient extraction of all components. Preliminary research indicated that using a smaller volume of solvent led to significant absorption by the plant, whereas volumes exceeding 20 mL did not result in increased extraction yields. The results obtained from the analyses of the 20 extracts (presented in Table 2) indicate that different combinations have a major impact on the measured responses. In particular, design point 13 is considered the most appropriate for acquiring the highest amount of total polyphenols, while design points 11 and 2 were considered the most suitable for enhancing the antioxidant activity by the FRAP and DPPH methods, respectively. Among the most interesting results is that neither PEF, US, nor the use of 100% ethanol was found to enhance the FRAP activity. In general, ethanol is considered to favor the efficiency of an extraction, especially for bioactive compounds characterized by medium polarity, such as polyphenols [30]. This was substantiated in the present study, since, for maximum isolation of the total polyphenols, the use of 50% ethanolic solvent was found to be the most appropriate. Nevertheless, in a previous study, it was proven that the use of water as the solvent was the best extraction medium concerning the antioxidant activity in all studied fruits [31]. This finding was corroborated in the current study, where water emerged as the most suitable solvent for enhancing the antioxidant capacity of watercress leaves, as measured by the FRAP method. Despite the advantages associated with green extraction techniques, it is important to note that extraction processes, such as US at elevated temperatures, may lead to the degradation of heat-sensitive compounds, resulting in the loss of valuable elements [32; 33], which occurred in the case of the antioxidant capacity via the FRAP method. Table 3 displays the statistical parameters, second-order polynomial equations, and coefficients (R\({}^{2}\) > 0.97) for each model, indicating a strong fit for the developed models. The plots comparing the actual response to the predicted response for each parameter, along with the desirability functions, are shown in Figures S1-S3. Three-dimensional response surface plots for TPC are depicted in Figure S4, and the remaining responses (FRAP and DPPH) are in Figures S5 and S6. Figures S4-S6 feature six 3D surface plots, labeled A to F. Each plot illustrates the relationships among three variables, with the x and y axes representing different variables (\(X_{1}\), \(X_{2}\), \(X_{3}\), and \(X_{4}\)), and the z-axis corresponding to the response. The plots use a color gradient from blue (indicating low values) to red (indicating high values) to denote the response values. Figure S4 demonstrates how the TPC changes with various combinations of \(X_{1}\), \(X_{2}\), \(X_{3}\), and \(X_{4}\), which is beneficial for comprehending the interactions among these variables and for optimizing conditions to attain specific TPC levels. \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline \multirow{3}{*}{ \begin{tabular}{c} **Design** \\ **Point** \\ \end{tabular}  } & \multicolumn{3}{c}{