## 4 Discussion
For example, it has been observed that the expression of Fe transporter, AtIRT1, was increased when Fe was deficient [41]. However the amount and localization of the protein, AtIRT1, was not dependent on the supply of Fe [42]. Additionally, the mRNA of a AtNIP5;1, an aquaporin that transports B increased its expression under B deficiency [43], but protein was reduced under the excess of B in Arabidopsis [44]. Interestingly, some nutrient transporters were not related to the availability of mineral elements when transcriptional or post-transcriptional regulation was determined. In this way, low or high Mn availability did not affect the expression of Mn transporter, OsNramp5, in rice [45]. Nevertheless, another Mn transporter, OsNramp3, was reported to have controlled Mn distribution, which changed to an open or closed state, as Mn is needed in specific tissues. [46]. In addition, it is necessary to emphasize that Fe and Mn concentrations in leaves increased in both B treatments (free and nanoencapsulated) when compared with the control, but no differences were observed between them. This could be explained by the interaction between nutrients and applied B, inducing the uptake of some nutrients such as Mn and Fe [47]. Furthermore, B sprayed as a foliar application on _Arachis hypogaea_, groundnut plants, showed an increase in the uptake of N, P, K, Fe, and Mn [48]. On the contrary, only changes in Fe and Mn, apart from B, were observed in our study. This could be explained by the activation of some routes by B accumulation in leaves, which activated divalent cation transport, as these nutrients could be mobilized by the same type of transporters. For example, Arabidopsis AtNRAMP genes can be found which are involved in the mobilization of both Fe and Mn [49]. Therefore, the homeostasis of the micronutrients is still unknown, although the mechanisms controlling the expression of transporter genes in response to the fluctuation of mineral elements, and the mechanismscontrolling the polarity of transporters, are being investigated [50]. The fact that the external B application to non-deficient B sweet potato plants changed the concentrations of Fe and Mn in leaves should provide clues about the inter-regulation of different transporters. The metabolomic analysis revealed that formic acid increased only in plants treated with encapsulated B. The formation of formiate from glyoxylate has been associated to synthase reactions of glyoxylate and succinate, and the conversion of serine to pyruvate [51]. In this way, the function of the Krebs cycle and mitochondria were analyzed, together with the significance of energy dissipation in the peroxisomes for the regulation of the cell energy balance, with the conclusion that the main function of the photorespiratory peroxisomes was associated with the integration of metabolite flows in the photosynthesizing plant cell. If formic acid is related to energy needs, it could be associated to the changes in the concentration of carbohydrates. The finding that the concentrations of fructose and glucose increased in these plants, although the concentration of sucrose decreased, implies a direct need to obtain energy, but no changes in chlorophylls and carotenoids were observed in any of the treated plants. Since it has been reported that water regulation is related to abscisic acid, which strongly up-regulates the gene expression and enzymatic activities that accelerate water soluble carbohydrates remobilization [52], this could be related to our metabolomics results, given the importance of carbohydrate remobilization in sweet potato plants [53]. Therefore, these metabolic changes could be associated with the changes in aquaporins that react directly to B supplementation. The NIPs aquaporins have been reported to play a main role in the transport of metalloids, such as B, Si, As, and Sb [54]. NIPs were initially described as glycerol transporters, but their similar conformation and charge with other metalloid compounds provided evidence of transport activities [55].