## 3 Results and Discussion
### Bioactive Compounds
Regarding the enzymatic antioxidant compounds in tomato fruits, statistically significant differences were observed in all evaluated enzymes (Table 4). The fruits from the Control showed the highest values of APX and GPX enzymes. Regarding the SOD and CAT enzymes, an increase was observed with at least one Cu NPs treatment compared to the Control. The APX enzyme showed a similar effect, since the 250 mg L\({}^{-1}\) of Cu NPs treatment generated the lowest value, 54.55% lower than the Control. APX is an enzyme that removes H\({}_{2}\)O\({}_{2}\) using ascorbate as an electron donor to reduce H\({}_{2}\)O\({}_{2}\) to water, and it is extremely sensitive to ascorbate concentrations [65]. A decrease in the concentration of APX leads to a higher ascorbate content [66], as observed in Tables 3 and 4. APX plays a central role in the ascorbate-glutathione cycle; this is why the induction of APX is a response to oxidative stress caused in plants by biotic or abiotic stress [67]. The concentration of GPX also decreased with the application of Cu NPs in all concentrations applied. [PERSON] et al. [16] reported that the application of Cu NPs + chitosan in tomato plants showed no differences between the treatments in fruit GPX content, which was attributed to low concentrations used in that study. A decrease in enzymatic antioxidant activity in fruits could be due to the inactivation of more enzymes due to the toxic effects of ROS [68]. This is possible because Cu NPs induce oxidative stress [62]. In addition, due to its redox activity at concentrations higher than the physiological level, Cu can catalyze the overproduction of ROS, causing an oxidative stress [69]. The previous results suggest that the foliar application of Cu NPs in the concentrations shown in this study had a negative effect on APX and GPX enzymes in the tomato fruits, since they decreased their activity in all cases. Conversely, SOD activity increased significantly in tomato fruits (Table 4). The treatments with the application of Cu NPs in 125 and 250 mg L\({}^{-1}\) doses presented an increase of 260% and 86%, respectively, in comparison to the Control. This is because SOD acts as the first line of defense against ROS [45; 70] generated by stress caused by Cu NPs. In addition, the SOD activity is also induced by metal ions such as Cu, or by the increase in singlet oxygen levels [71]. However, the dose of 500 mg L\({}^{-1}\) of Cu NPs presented a decrease of 63.14%. This may be due to the stress caused by the application of Cu NPs that exceeded the response capacity of the plant, causing the observed effect [62]. The CAT enzymatic activity also increased significantly with the application of Cu NPs. The application of 500, 250, and 125 mg L\({}^{-1}\) generated an increase in CAT activity by 211%, 180%, and 155%, respectively, compared to the Control. These results indicate an increasing tendency in the CAT activity as the concentration of Cu NPs in foliar application increased. CAT is one of the enzymes that protects cells against ROS since it converts H\({}_{2}\)O\({}_{2}\) into H\({}_{2}\)O and O\({}_{2}\)[72; 73]. Therefore, its activity is directly related to the stress caused by the application of Cu NPs [62]. An increase in CAT activity is dependent on the concentration of H\({}_{2}\)O\({}_{2}\)[70]. Thus, there is a correlation between the components of the ROS elimination systems (e.g., CAT and SOD) [74]. In tomato fruits, it has been shown that under conditions of high-temperature stress, the CAT enzymatic activity increases [73]. Similar results were obtained in tomato leaves [75]. Therefore, the increase in CAT activity has a positive response in fruits.