## 3 Results
### Evolution of Proanthocyanidins during Ripening Period  
Proanthocyanidins or condensed tannins accumulate mainly in berry skins and seeds before veraison. Different authors have found that the greatest synthesis of proanthocyanidins takes place after the fruit sec, which occurs a few weeks before veraison, which is when the highest concentration of these compounds is reached [42]. From this moment on, the skin proanthocyanidins decrease slightly or remain stable [42]. In addition, these compounds play an important role in the final red wine quality as they are responsible for different properties such as bitterness, astringency, structure or colour. In this paper, we will only show the relationship of the results with the skin parameters because it is known that these are considered of higher quality than those from the seeds. Regarding total proanthocyanidins in skins (Figure 5A, numerical data can be seen in ST4 of the Supplementary Materials), in general, a decrease in their total concentration was observed until the fifth sampling. From this moment until harvest, this decrease was more attenuated. Regarding treatments, different behaviours were observed throughout their evolution during maturation. In this case, the grapes treated with MeJA obtained the lowest concentrations of tannins in all the samplings, except at the time of harvest, when the values were equal to those obtained in the control grapes. In contrast, [PERSON] et al. [26] found that MeJA-treated grapes had a higher total concentration of proanthocyanidins in the skin than control grapes throughout the ripening period. In addition, [PERSON] et al. [24] showed an increase in skin proanthocyanidin content in MeJA-treated Monastrell grapes during two consecutive seasons, but in Tempranillo grapes this treatment only increased the tannin concentration during the second season. Therefore, it is evident that factors such as season and variety will also influence the obtained results. Regarding the treatments with nanoparticles, the grapes treated with Nano-MeJA showed lower concentrations of tannins than the control grapes until the fifth sampling. From this moment until the harvest, these grapes obtained the highest concentrations of tannins. Finally, the grapes treated with Ap-MeJA, in general, obtained similar values to those obtained by the control grapes, except in the sixth sampling, which surpassed the control concentration, and at the time of harvest, were they obtained the lowest concentrations of tannins. As in the case with anthocyanins and flavonoids, it seems that the different morphology and structure between ACP and Ap nanoparticles induced the grapes treated with Ap-MeJA to obtain the lowest concentrations of proanthocyanidins in the skin at the end of the ripening process. This effect could be associated with the low % MeJA content of Ap-MeJA and the need for higher Ap-MeJA nanoparticle uptake by the plant, as previously discussed. Other authors have addressed the use of chitosan as a conventional dissolution or as nanoparticles in varieties such as Sousao, and showed a decrease in the concentration of tannins in the skin when both were applied [57]. Other parameters such as mDP (mean degree polymerisation), % epigallocatechin and % galloylation were also measured. Regarding mDP (Figure 5C), this parameter was always higher in the skins than in the seeds and is related to the number of monomeric units that are formed by proanthocyanidins, which in the case of skins can be up to 30 [58]. We were able to observe a great variability in the values obtained from the control and treated grapes during the ripening period. Despite that, it is remarkable that the highest values obtained for this parameter were in the second and sixth sampling for the control grapes and the highest value reached by Ap-MeJA was in the third sampling. On the contrary, treated and untreated grapes equalled their values at harvest time, although Ap-MeJA treated grapes slightly increased their value compared to control grapes.