## 3 Results and Discussion
### Bioactive Compounds  
Statistically significant differences were found for all non-enzymatic antioxidant compounds evaluated in tomato fruits, observing that at least one treatment with Cu NPs surpassed the Control (Table 3). The content of total proteins was positively affected by the application of Cu NPs. An increase of 99.74% was observed with the application of 50 mg L\({}^{-1}\) of Cu NPs compared to the Control. However, as the dose increased, a decrease in protein content was consistently observed, and no effects were observed at a Cu NPs concentration of 500 mg L\({}^{-1}\). This may be related to the plant's \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Treatment**} & **Firmness** & \multirow{2}{*}{**pH**} & **EC** & **TSS** & **TA** & **ORP** \\  & **(kg cm\({}^{-2}\))** & & **(mS cm\({}^{-1}\))** & **(\({}^{\circ}\)Brix)** & **(\% citric acid)** & **(mV)** \\ \hline
500 mg L\({}^{-1}\) Cu NPs & 4.78 a & 4.37 ab & 3.60 ab & 5.10 b & 0.53 a & 165.20 a \\
250 mg L\({}^{-1}\) Cu NPs & 5.13 a & 4.35 ab & 3.78 a & 5.25 ab & 0.51 a & 157.33 b \\
125 mg L\({}^{-1}\) Cu NPs & 4.92 a & 4.41 a & 3.42 bc & 5.30 ab & 0.41 b & 165.00 a \\
50 mg L\({}^{-1}\) Cu NPs & 4.73 a & 4.43 a & 3.59 ab & 5.22 ab & 0.50 a & 164.17 a \\ Control & 3.98 b & 4.30 b & 3.22 c & 5.40 a & 0.49 a & 166.83 a \\ CV (\%) & 12.83 & 1.81 & 6.69 & 3.45 & 8.43 & 2.27 \\ \hline \hline \end{tabular}  
* Each unit of data is the average of six replicates. CV: coefficient of variation. Different letters per column indicate significant differences between treatments according to the LSD Fisher test (\(p\leq 0.05\)). EC: electrical conductivity. TSG: total soluble solids. TA: titratable acidity. ORP: oxidation–reduction potential.  
\end{table} 
Table 2: The effect of the application of Cu NPs on tomato fruit quality. response to different types of stress, including the alteration of the protein gene expression. The stress caused by nanoparticles can produce ROS, in turn modifying the activity of antioxidant enzymes, some transcription factors, and other proteins involved in the response to stress [43; 44]. ROS have a double role that depends on duration, site, and concentration. A low concentration acts as a signal and generates a stress response in plants, while a higher concentration interrupts cellular homeostasis which produces oxidative damage to proteins, DNA, and lipids [45]. This explains the observed results, where the total proteins increased when Cu NPs were applied in concentrations of 50, 125, and 250 mg L\({}^{-1}\), in opposition to the concentration of 500 mg L\({}^{-1}\). The vitamin C content in tomato fruits significantly increased in all treatments. The highest value of vitamin C was found in fruits from plants treated with 250 mg L\({}^{-1}\) of Cu NPs, resulting in a 121.97% increase (Table 3). The doses of 125, 50, and 500 mg L\({}^{-1}\) of Cu NPs showed increases of 87.64, 37.53, and 35.18%, respectively, compared to the Control. The results obtained showed that the application of Cu NPs significantly increased the vitamin C content, which in turn increased the nutraceutical quality of tomato fruits. This can be attributed to the induction of antioxidant compounds, reported as one of the functions of Cu NPs [17]. Vitamin C is essential in the human diet, and must be ingested from fruits rich in its content, since the human body cannot produce it [24]. In plants, vitamin C has many cellular functions, mostly linked to its ability to donate electrons [46]. Specifically, in tomato fruits, vitamin C is one of the most important compounds [3] which may act directly as an antioxidant by trapping reactive oxygen species (ROS), preventing or minimizing oxidative damage [47]. In addition, it can also regenerate the glycoprotein andtocopherol radicals, and act as a cofactor for many enzymes (e.g., APX) [2]. It plays an important role in photosynthesis as an enzymatic cofactor (including the synthesis of ethylene, gibberellins, flavonoids, and anthocyanins) [48].