## 3 Results and Discussion
However, under refrigeration, the fruits of the plants with NaCl presented significantly higher antioxidant capacity, while the fruits of the T0 plants present the lowest value. The same was observed after 23 days of storage under refrigeration. These results suggest that when the fruits are maintained at room temperature (24 \({}^{\circ}\)C), the stress caused by the application of NaCl negatively affects the antioxidant capacity evaluated by ABTS. Under this same condition, the application of Cu NPs can increase the antioxidant capacity at least at some point in storage. However, when fruits are stored in refrigeration (4 \({}^{\circ}\)C), the opposite is observed. Application of NaCl at 15 and 23 days of storage significantly increases ABTS antioxidant capacity (50-150%) compared to T0. Since the ABTS technique measures the hydrophilic compounds [40], it is probable that the refrigeration condition modified the behavior of these antioxidants present in the fruits. The behavior observed in antioxidant activity was different for both DPPH and ABTS methods, and no correlation between these results was observed. This is due the differences between both methods: the ABTS technique measures the hydrophilic compounds [40], while the DPPH technique can be used to examine both hydrophilic and lipophilic antioxidants [41]. There are also differences in reaction times, and it is specific for each type of antioxidant [28; 42], which results in differences in the quantification of antioxidant activity as observed here (Tables 6 and 7). For this reason, a single method is not suitable for measuring all antioxidants in plants, and there is no shortcut approach to determine antioxidant activity as mentioned by [41]. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Storage** & **Treatment** & **0 ds** & **8 ds** & **15 ds** & **23 ds** \\ \hline \multirow{4}{*}{Room temperature} & NPs Cu & 2.33 \(\pm\) 0.03 b & 2.93 \(\pm\) 0.04 a & 2.34 \(\pm\) 0.08 c & not done \\  & NaCl & 2.17 \(\pm\) 0.03 c & 2.34 \(\pm\) 0.01 e & 2.02 \(\pm\) 0.06 d & not done \\  & NPs Cu + NaCl & 2.21 \(\pm\) 0.03 c & 2.04 \(\pm\) 0.04 f & 2.08 \(\pm\) 0.04 d & not done \\  & T0 & 2.53 \(\pm\) 0.03 a & 2.79 \(\pm\) 0.02 b & 2.55 \(\pm\) 0.11 b & not done \\ \hline \multirow{4}{*}{Refrigeration} & NPs Cu & not done & 2.40 \(\pm\) 0.07 e & 2.34 \(\pm\) 0.04 c & 2.86 \(\pm\) 0.05 b \\  & NaCl & not done & 2.65 \(\pm\) 0.04 c & 3.84 \(\pm\) 0.09 a & 3.29 \(\pm\) 0.06 a \\ \cline{1-1}  & NPs Cu + NaCl & not done & 2.53 \(\pm\) 0.05 d & 1.57 \(\pm\) 0.05 e & 2.67 \(\pm\) 0.05 c \\ \cline{1-1}  & T0 & not done & 2.76 \(\pm\) 0.04 b & 1.52 \(\pm\) 0.02 e & 2.26 \(\pm\) 0.06 d \\ \hline \hline \end{tabular}  
* ds: days of storage. NPs Cu: plants without stress and with application Cu nanoparticles to 250 mg L\({}^{-1}\). NaCl: plants with stress by NaCl and with application Cu nanoparticles at 250 mg L\({}^{-1}\). T0: absolute control without application of Cu nanoparticles and without stress by NaCl. Each value is expressed as mean \(\pm\) standard deviation (\(n=3\)). Different letters by column indicate statistical differences between treatments according to [PERSON]’s least significant difference test (\(p\leq 0.05\)).  
\end{table} 
Table 7: 2,2-azino-bis (3-ethylbenzthiaazoline-6-sulfonic acid) (ABTS) antioxidant capacity (\(\mu\)M Trolox equivalents per gram of fresh weight) in tomato fruits at different storage times at room temperature (24 \(\pm\) 1 \({}^{\circ}\)C) and refrigeration (4 \(\pm\) 1 \({}^{\circ}\)C and relative humidity at 80%). On the other hand, a correlation between antioxidant activity and bioactive compounds exist only for some specific antioxidants. For example, in tomato fruits, antioxidant activity DPPH increased when phenolic and lycopene contents increased, but flavonoid and \(\beta\)-carotene decreased [34]. In tomato fruits under drought stress, the antioxidant activity ABTS, lycopene, total phenolics, and flavonoids increased, but the increase between this compounds was not in the same magnitude [3].