## 3 Results and Discussion
### Ability to Reduce Metal Ions (FRAP and CUPRAC Methods)  
The FRAP and CUPRAC spectroscopic methods, measuring ferric reduction as antioxidant power and cupric reducing antioxidant capacity, respectively, determine the ability of a given antioxidant to reduce an oxidant. The measure of this ability, as a result of a chemical reaction, is the change in the oxidant's color. The degree of color change is correlated with the concentration of total antioxidant capacity [27].  
As can be seen from the data in Figures 4 and 5, the examined extracts demonstrate the ability to reduce metal ions (both iron and copper). It should be added here that the methods used, although measuring the same type of antioxidant capacity (reduction capacity), are not identical. The measurements are performed under different pH conditions (FRAP--acidic, CUPRAC--alkaline) [27]. What is more, the CUPRAC method is dedicated to both hydrophilic and lipophilic antioxidants, while FRAP is more for hydrophilic ones. Our results are in agreement with the literature [7; 25; 28; 29]. The ability to reduce iron ions using the potassium ferricyanide method for ethanolic extracts of _Polygonum cuspidatum_ was determined by [PERSON] et al. [28]. Based on the performed experiments, the authors found a positive correlation between the reducing power and antioxidant activity of the tested extract, which was comparable to resveratrol and butylated hydroxytoluene (BHT) [28].  
Figure 2: Antioxidant activity determined using ABTS method for extracts obtained from Japanese knotweed at 25 and 50 \({}^{\circ}\)C using UASE under the following extraction conditions: (**a**) extraction time of 10 min and a frequency of 37 kHz; (**b**) extraction time of 20 min and a frequency of 37 kHz; (**c**) extraction time of 10 min and a frequency of 80 kHz; (**d**) extraction time of 20 min and a frequency of 80 kHz. Using methanol, methanol/water mixture (50/50%, \(v/v\)), and water as reaction solvents. The same letters indicate data for which the results are not statistically significant.  
Moreover, [PERSON] et al. [29] observed that the extract of _Polygonum cuspidatum_ obtained by the supercritical fluid extraction technique with carbon dioxide also reduced metal ions in a concentration-dependent manner. When the concentration was increased to 250 mg/mL, this reducing ability was remarkable.  
The results from the data show that, in most cases, methanol-water extracts have the best antioxidant properties, while aqueous extracts have the weakest. Extracts obtained at higher frequencies have lower antioxidant properties than extracts obtained at lower frequencies. This trend is visible to a greater or lesser extent in both methods. In most cases, the best antioxidant properties are shown by methanol-water extracts, while the weakest are in water, as in the previous two methods. It seems that the methanol/water mixture is the best extraction medium for isolating compounds responsible for antioxidant properties, mainly polyphenolic compounds. In order to confirm the validity of this hypothesis, in the next step of the research, it was decided to determine the content of polyphenolic compounds in the examined extracts.  
Figure 3: Antioxidant activity determined using DPPH method for extracts obtained from Japanese knotweed at 25 and 50 \({}^{\circ}\)C using UASE under the following extraction conditions: (**a**) extraction time of 10 min and a frequency of 37 kHz; (**b**) extraction time of 20 min and a frequency of 37 kHz; (**c**) extraction time of 10 min and a frequency of 80 kHz; (**d**) extraction time of 20 min and a frequency of 80 kHz. Using methanol, methanol/water mixture (50/50%, \(v/v\)), and water as reaction solvents. The same letters indicate data for which the results are not statistically significant.