## 3 Results and Discussion
### Fruit Quality  
The results of tomato fruit quality (firmness, pH, electrical conductivity (CE), Total soluble solids (TSS), Titatable Acidity (TA), and ORP) showed statistical differences in all the evaluated variables (Table 2). The fruit firmness increased with all treatments of Cu NPs (500, 250, 125, and 50 mg L\({}^{-1}\)) with respect to the Control. The treatment with 250 mg L\({}^{-1}\) of Cu NPs increased the firmness by 28.90%, and the treatment with 125 mg L\({}^{-1}\) Cu NPs produced a 23.62% increase in the firmness with respect to the Control. The other two treatments (500 and 50 mg L\({}^{-1}\)) showed an increase of 20.10% and 18.85%, respectively. [PERSON] et al. [16] reported that the application of Cu NPs + chitosan increased the firmness of tomato fruits by 9%. This is consistent with the results shown in this study. It has been shown that cell wall lignification occurs in plants with copper stress [37]. Additionally, there is a relationship between the accumulation of copper in the cell wall and the responses of lignifying enzymes such as phenylalanine ammonia lyase (PAL). This response catalyzes the transformation of L-phenylalanine to cinnamic acid, which is the first step of the phenylpropanoid pathway leading to the synthesis of lignin [37,38]. [PERSON] and [PERSON] [14] observed root lignification when CuO NPs were applied, and mentioned that NPs could have translocated through the vascular tissues, and their subsequent dissolution to Cu ions could have given rise to the observed lignification. Thus, the increase in the firmness of tomato fruits may be the result of an increase in the pericarp cell walls' lignification due to Cu NPs application. The fruit pH in the treatments of 125 and 50 mg L\({}^{-1}\) Cu NPs, were 2.50 and 2.94% higher than that of the Control (Table 2). This agreed with [PERSON] et al. [16], who registered a pH increase of 2.79% in tomato fruits with Cu NPs + chitosan application. However, this differs from [PERSON] et al. [17], who reported a pH decrease with the application of Cu NPs + Cs-PVA in jalapeno pepper. The pH range in tomatoes is normally found between 4.0 and 4.5. Fruit quality is highly influenced by pH, and less-acidic fruits are more appreciated by consumers for their better flavor feature [39,40]. The increase in fruit pH is due to the transformation of the organic acids in the vacuoles of the vegetables into simple sugars that are later used in cellular respiration to obtain energy. This causes a decrease in the acidity, and therefore an increase in pH [41]. The results seem to \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Treatment** & **K** & **Na** & **Cu** \\  & **(g kg\({}^{-1}\) DW)** & **(g kg\({}^{-1}\) DW)** & **(mg kg\({}^{-1}\) DW)** \\ \hline
500 mg L\({}^{-1}\) Cu NPs & 21.21 a & 1.22 b & 26.59 a \\
250 mg L\({}^{-1}\) Cu NPs & 20.88 a & 0.88 b & 35.17 a \\
125 mg L\({}^{-1}\) Cu NPs & 21.05 a & 0.98 b & 12.97 a \\
50 mg L\({}^{-1}\) Cu NPs & 20.27 ab & 1.08 b & 11.70 a \\ Control & 18.31 b & 2.16 a & 48.78 a \\ CV (\%) & 6.13 & 17.17 & 144.69 \\ \hline \hline \end{tabular} Each unit of data is the average of six replicates. DW: dry weight. CV: coefficient of variation. Different letters per column indicate the significant differences between treatments according to the LSD Fisher test (\(p\leq\) 0.05).  
\end{table} 
Table 1: The mineral content in tomato fruits with copper nanoparticles (Cu NPs) application. indicate that the application of Cu NPs induced the transformation of organic acids to simple sugars in tomato fruits. The application of Cu NPs increased the EC of tomato fruits, indicating a higher salt content (Table 2). The lowest values for EC were observed in the Control fruits, while fruits in a treatment with 250 mg L\({}^{-1}\) Cu NPs showed increases in the EC by 17.39% (i.e., 3.78 mS cm\({}^{-1}\)). This can be attributed to the increase in K, since this element is one of the most important inorganic solutes in plants [36].