## 4 Discussion  
This study has investigated the effect of potassium fertilization on yield and grape composition of Glera cv., a white grape variety used to produce the renowned Prosecco sparkling wine. Given the enenological destination of this variety, maintaining grape acidity under warmer conditions has become a major challenge for Glera cultivation. In this context, potassium fertilization may be of great interest given the impact this technique may have on the acidity and pH values of musts. This trial was carried out in a sub-area of the DOC Prosecco where soils were characterized by good potassium availability (see Table 2). We tested three different K rates: 15, 30 and 60 kg/ha, the latter being comparable to the conventional K rate used in this specific area. Considering the negative effects caused by excessive K accumulation on musts, our goal was to assess if reduced K fertilization may influence and improve grape yield and quality, with specific attention for the acidity and pH parameters. Our results indicate that K had more effects on grape composition than on yield components. Previous studies investigating the impacts of potassium fertilization on grapevine production showed discordant results. [PERSON] et al. [37] reported that a potassium fertilization with 300 g per vine increased yield and cluster number in the grapevine cv. Bidaneh Qermez compared with a non-fertilized control. On the contrary, [PERSON] et al. [38] analyzed the effect of potassium supply on the yield of \"Riesling\", \"Chardonnay\" and \"Cabernet Sauvignon\" under controlled conditions and reported that increased K fertilization (60 kg K/ha) did not affect the fruit yield. In another greenhouse trial, potted Cabernet Sauvignon receiving 25 g K per vine were compared to control vines fed with 0 g K (no K added) and authors found that total yield, cluster number per vine, cluster and berry weight were unaffected by K supply [9]. A significant reduction in the yield of Concord grape variety was observed in another study, where extremely high levels of potassium fertilizer (225 to 900 kg/ha) were supplied using KCl as a source. In this case, the probable cause of yield loss was the Cl accumulation in soils due to the prolonged use of KCl [39]. In our experiment, moderate or low amounts of potassium were applied, similarly to what was performed by [9; 38], and this may explain why the fertilization impact on yield components was smaller than that observed in other studies. It must also be outlined that data obtained from different studies are not subject to easy comparison, due to differences in the experimental conditions and in the plant material used for the study. The relationship between the K concentration in grape berries and wine quality is well documented; however, there is still some controversy over the effects of K fertilization on sugar and acid contents in grapes [40]. K supply plays a key role in the sugar/acid ratio of wine, which is crucial to ensure a balanced taste. [PERSON] et al. [41] reported no significant variation in the sugar content of cv. Riesling italic after five foliar applications of a 5% solution of K\({}_{2}\)SO\({}_{4}\) in comparison to an untreated control. In a similar experiment [42], the sugar content of cv. Zweeigelt treated with potassium applied foliarly was lower than the untreated control. In the greenhouse trial mentioned above [9], the sugar content of potted Cabernet Sauvignon vines receiving 25 g K per plant did not show differences in comparison to the control vines. Conversely, other studies argue that K fertilization causes a significant increase in the sugar/acid ratio. [PERSON] et al. [43] found that increasing the amount of K fertilization caused an increase in the sugar/acid ratio due to a reduction in acidity. The authors postulated that higher potassium levels (120 g K\({}_{2}\)O per vine, corresponding to 266 kg K\({}_{2}\)O/ha) caused an excessive migration of K cations into the fruit, leading to the production of potassium bitartrate from tartaric acid.