## 4 Discussion
In our study, in four years of experiments, the TSS was always unaffected by K fertilization. We can speculate that in our experimental conditions, probably other agronomic practices, such as canopy management and crop control, might have exerted a major impact on sugar accumulation, minimizing the effect of the K fertilization. It is known, in fact, that a well-exposed foliar surface and a balanced ratio between photosynthetically active leaves and kilograms of grapes are major drivers for sugar accumulation in grape berries for Glera variety [27]. Analyzing the effect of K on the acidity and pH, [PERSON] et al. [9] found similar acidity and pH values in vines receiving 25 g K per plant and control vines with no K fertilization. On the contrary, [PERSON] et al. [44] observed that excessive K levels in grape berries decreased the free acid fraction and increased the overall pH, with detrimental effects on the wine quality. In our experiment, the 4-year average values for titratable acidity, malic and tartaric acids resulted higher in the treatment that received the highest rate of K (K60), while the lowest rates (K30, K15) were similar to the no-fertilized control. These results indicate that for the Glera variety, a reduced K supply did not positively affect the acidity. On the contrary, medium K rates (60 kg/ha) showed the best results on this grape parameter. With regards to the malic acid amount, a direct effect of K availability on the synthesis of this acid has been suggested. This might be explained by the fact that, in the berry, a K supply not completely balanced by anion uptake could determine an extra malic acid synthesis [45; 46]. [PERSON] et al. [47] found a positive relation between malic acid and K level during veraison, particularly if K in the berry juice was above the threshold of 0.8-1.0 g/L, as in our study. Moreover, a direct or indirect effect of K on the rate of malic acid metabolization during ripening was proposed, which, according to [PERSON] et al. [13], could be due to an effect of K on the tonoplast permeability. This would result in less malate available in the cytosol for use in metabolism. In support of this hypothesis, an inverse relationship between potassium and tonoplast permeability in ripening fruits was observed in other crops [48]. As observed for malic acid, tartaric acid was also higher in the treatment that received the highest rate of K (K60), followed by K30. The explanation could be linked to the pathway of synthesis of this acid. In fact, it has been reported that high amount of potassium may promote the biosynthesis of ascorbic acid [49; 50], increasing the availability of one of the precursors for the tartaric acid biosynthesis [51]. Regarding the must pH, all K treatments showed pH values comparable to those of the non-fertilized control, despite showing higher berry K content. It is commonly accepted that a high potassium concentration in grape berries leads to high pH values [7]. In this study, only a weak correlation between pH and potassium content was found. This can be related to the fact that the K rates supplied in this study induced moderate potassium accumulation in grape musts, preventing pH from increasing. Moreover, it has been reported that there are varietal differences in the relationship between juice pH and K concentration [3]; therefore, Glera response to K supply might be smaller than that observed in for other cultivars. Overall, the data in our study showed that the K fertilization did not affect the pH, as all treatments displayed comparable values in the range between 3.10-3.39, which is considered optimal for wine making.