## 3 Results and Discussion
### Ability to Neutralize Free Radicals (ABTS and DPPH Methods)  
Antioxidant activity can be studied using various analytical techniques, referring, on the one hand, to different properties of the tested compounds and, on the other hand, to different stages of the oxidation process and a different understanding of the role of antioxidants (different types of antioxidant activity) [23]. Particularly popular are studies using stable free radicals ABTS and DPPH, which can be easily determined spectrophotometrically, and the antioxidant properties of the substance can be established based on the color change. Whenever possible, it is advisable to use both of the above methods to determine the antiradical activity of the tested samples because the radicals used in them are not chemically identical. In the literature [24], one can find data on antioxidant properties determined using DPPH and ABTS for extracts obtained in various ways from _Polygonum cuspidatum_. [PERSON] et al. [24] studied the antioxidant properties of extracts from knotweed, which were obtained by maceration with 50% ethanol. The results obtained by them showed that the IC\({}_{50}\) values of the _Polygonum cuspidatum_ extract were about 110 ug/mL, while the IC\({}_{50}\) values of (+)-catechin and ascorbic acid were about 35 ug/mL and about 50 ug/mL, respectively. This indicates that the tested extract has a lower ability to neutralize the DPPH radical than these standard compounds. The antioxidant properties of ethanolic extracts of _Polygonum cuspidatum_ were also confirmed in experiments performed by [PERSON] et al. [25]. As in the previous studies, the ability to neutralize the ABTS and DPPH color radicals of the extracts they tested was, however, much lower than the standard antioxidants they used (such as BHA and vitamin C).  
Figures 2 and 3 show the antioxidant activity expressed as % inhibition of the ABTS cation radical (see Figure 2) and the DPPH radical (see Figure 3) for Japanese knotweed extracts obtained under UASE conditions with different ultrasound time and frequency (10 or 20 min and 37 or 80 kHz, respectively) using methanol, water and a mixture of these two liquids as an extraction medium at temperatures of 25 \({}^{\circ}\)C and 50 \({}^{\circ}\)C. As before, for statistical analysis, Figure 2a (Figure 3a) was compared with Figure 2b (Figure 3b), and Figure 2c (Figure 3c) with Figure 2d (Figure 3d). Lowercase letters a-g (a-h) and uppercase letters A-F (A-J) in the figures indicate a lack of statistical significance. Comparing the data in both figures, it is clear at first glance that the tested extracts exhibit different antioxidant activity. Higher % inhibition values are observed in the ABTS method than in the DPPH method. It should be noted here that this is most likely related to the availability of antioxidants in the sample to the sterically protected DPPH radical [26] and, consequently, poorer neutralization of this radical during the measurement (60 min). Moreover, in almost every case studied, the greatest antioxidant properties are shown by water-methanol extracts and the weakest by water extracts. However, there is no clear trend regarding the influence of individual variables on the antioxidant properties of extracts. An exception in this respect is the effect of temperature in the DPPH method. In this case, we can say that the higher the temperature, the better the antioxidant properties. Undoubtedly, this part of the research should be summed up as follows: the greatest antioxidant properties are not observed in the case of the extract characterized by the highest resveratrol content (i.e., methanol-water extract, extraction temperature of 50 \({}^{\circ}\)C, time of 10 min, and frequency of 80 kHz), so not only resveratrol is responsible for the antioxidant properties.