## 3 Results and Discussion
### Bioactive Compounds
This result is consistent with that observed in the protein content. Regarding the total phenols content of fruits, treatment with the application of 250 mg L\({}^{-1}\) of Cu NPs generated an increase of 5.43% in relation to the Control, while treatment with 125 mg L\({}^{-1}\) of Cu NPs showed a decrease of 14.44%. The rest of the treatments were similar to the Control (Table 3). Jalapeno pepper fruits showed an increase in the content of the total phenols (5.9%) when 2.0 mg of Cu NPs + Chitosan-PVA were applied [17]. Additionally, foliar application of nano-fertilizers of Zn and B increased the phenols content in pomegranate fruits [55]. Phenols are antioxidant compounds that trigger the synthesis of a series of secondary metabolites from the shikimic acid pathway or through phenylpropanoids under conditions of abiotic stress [56; 57]. In tomato fruits, the main flavonoid compounds are ruin, naringenin, chalcone naringenin, quercetin, hydroxycinnamic, chlorogenic, and caffeic acids [3]. Their function as antioxidants in fruits is due to the reduction of free radical levels in cells [41; 58]. Therefore, the observed response may be related to the ROS formation due to Cu NPs. In addition, an accumulation of this type of compound is beneficial for fruit quality. The flavonoid content in fruits also showed statistically significant differences between treatments (Table 3). The highest content of flavonoids in the tomato fruits was obtained with the application of 50 mg L\({}^{-1}\) of Cu NPs, exceeding the Control by 36.14%. Additionally, fruits of treatments with 250 and 125 mg L\({}^{-1}\) of Cu NPs presented a flavonoid content that surpassed the Control by 15.71% and 10.33%, respectively. However, there was a clear decreasing tendency in the flavonoid content as the Cu NPs dose increased. When 500 mg L\({}^{-1}\) of Cu NPs was applied, it was observed that the flavonoid content in fruits decreased by 12.48% compared to the Control. These results agree with those observed in jalapeno pepper fruits, where the flavonoid content increased 13% with Cu NPs + Chitosan-PVA [17]. In addition, in _A. Thaliana_, the concentration of anthocyanins increased when CuO nanoparticles were applied [59]. These metabolites act as antioxidants, protecting plants from oxidative stress by the elimination of H\({}_{2}\)O\({}_{2}\) and singlet oxygen generated under biotic or abiotic stress [60; 61]. Therefore, the observed increase in flavonoids content is probably related to oxidative stress caused by Cu NPs [62]. The results obtained clearly indicate that the application of Cu NPs induced a higher content of non-enzymatic antioxidant compounds in tomato fruits. This is consistent with the increase in the antioxidant capacity observed in fruits with treatments containing 50 and 125 mg L\({}^{-1}\) of Cu NPs (Table 3)--increases of 8.18% and 6.49%, respectively, in the ABTS antioxidant capacity, were observed compared to the Control. [PERSON] et al. [63] reported that the application of ZnO NPs to the soil (3, 20, and 225 mg kg\({}^{-1}\)) in bean and tomato crops stimulated the antioxidant capacity considerably. Moreover, the results indicated that Cu NPs application can generate certain stress in tomato plants. This produced an increase in ROS, which activates a defense mechanism [54; 62], increasing the production of antioxidant compounds such as ascorbate, glutathione, carotenoids, and flavonoids, among others [64]. The same response has been observed under conditions of mild stress caused by various NPs. However, the response changes when high doses are applied due to an overproduction of ROS, resulting in a decrease of antioxidants [62]. In the present study, the results indicated that the foliar application of 500 mg L\({}^{-1}\) of Cu NPs had a slightly adverse effect. On the other hand, lower doses generally showed positive effects in the content of the different non-enzymatic antioxidant compounds (Table 3).