*3.8. Antioxidants*

The GSH content in the leaf of sweet potato increased with the rise of V treatment, and the maximum concentration was observed at 75 mg L−<sup>1</sup> V treatment (Figure 7A). The GSH content at 10 and 25 mg L−<sup>1</sup> V treatment was insignificantly increased (16.4 and 28.5%); however, a significant increase was observed at 50 and 75 mg L−<sup>1</sup> V treatment, which were 134% and 324% higher than the control, respectively. Total polyphenols and flavonoid concentrations decreased significantly with the rise of V treatment (*p* < 0.05). Interestingly, total polyphenols and flavonoids drastically reduced at 10 and 25 mg L−<sup>1</sup> V treatment, then again increased at 50 and 75 mg L−<sup>1</sup> V treatment, however still significantly lower than in the control plants (Figure 7B,C).

**Figure 7.** The influence of various V treatments on the antioxidants in the leaves of sweet potato. (**A**) GSH content, (**B**) total polyphenols (TPC), and (**C**) total flavonoid (TFC) content. Duncan's test indicates a significant difference (*p* < 0.05) between the means of the five treatments indicated by different alphabetical letters.

#### *3.9. Antioxidant Enzymes*

Antioxidant enzyme activities were significantly influenced by V treatment (*p* < 0.05). We found a significant decrease in APX and POD activities with the rise of V concentration (*p* < 0.05; Figure 8A,D), and maximum reduction was detected at 75 mg L−<sup>1</sup> of V treatment. As compared to the control, a 62.1% reduction in APX and a 57.5% in POD was detected at 75 mg L−<sup>1</sup> V treatment. Conversely, the CAT and SOD were positively influenced by V treatment, and a significant increase was observed with the rise in V concentration (*p* < 0.05; Figure 8B,C). Furthermore, the highest activities of CAT and SOD were observed at the 75 mg L−<sup>1</sup> V treatment, and a 1085% increase in CAT and a 164.7% increase in SOD were observed compared with the control.
