## 1 Introduction  
The beneficial effect of adding mineral elements to soils through fertilization has been well known for several years, and nowadays professional viticulture requires regular mineral supplementation to ensure yield and quality. Among the mineral nutrients, potassium is the most abundant cation and plays an important role in cultivated grapevines, being involved in the sugar transport to the fruits [1] and in the modulation of the acidity level in must and wine [2]. Several studies reported that high K concentrations tend to decrease the acidity level in grape berries at harvest [3]. Potassium accumulates primarily in the berry tissues during ripening. Its accumulation is a function of K availability in the soil, uptake by roots [4], extent of remobilization from the mature leaves [5] and from storage sinks within the vine woody structures [6]. The total K content in grape berries at harvest depends on different factors, such as the rootstock [7, 8], the fertilization management [9, 10] and also the climate conditions [11]. In the berry, K is involved in the neutralization of negative charges and can reduce the amount of free tartaric acid, forming potassium bitartrate and eventually affecting the final pH of the grape juice. The potassium content increases throughout berry development [12], while both tartaric and malic acid reach their maximum content before the start of ripening and then decrease until harvest. The ripening of the berry leads to an increase in volume, which dilutes the tartaric acid and jointly reduces its concentration. The malic acid decreases at a faster rate, due both to dilution and respiration processes. A positive correlation between potassium and malate has been shown during grape ripening [13] due to a probable effect of potassium on the tonoplast permeability of the vacuoles, the cellular organs that contain most of the malate. A similar positive relation between potassium and malate was observed also in peach [14]. As no relationship between malate content and the activities of enzymes involved in the metabolism of malic acid was observed, [PERSON] et al. [15] suggested that malate accumulation in the fruit could had been determined not by changes in its metabolism, but by the conditions of its transport into the vacuoles of the mesocarp cells. The current climate change context, with increasing temperatures and decrease in precipitation patterns, is imposing new challenges to vine growers. A significant earlier occurrence of grapevine developmental stages due to increasing temperatures has been reported in several wine regions [2; 16; 17], thereby placing ripening during a warmer period of the year. This results in increased sugar levels and lower acid concentrations for grapes, leading to unbalanced wines with higher alcohol content and deprived of freshness and aromatic complexity [18; 19; 20]. High temperature affects the organic acid content of berries inducing a faster consumption of malic acid as a respiratory substrate [21]. Moreover, warmer temperatures favor the accumulation of K during grape ripening, leading to an increase in the electrical neutralization of organic acids and increasing pH values in musts [7; 22]. Limited rainfall may amplify the negative effect of temperatures, contributing to increased K concentration in grapes. In fact, it has been reported that under drought stress, some key K transporters (namely shaker channels VvK1.1, VvK1.2 and VvK3.1) are up-regulated, increasing K loading into berries [23; 24]. Considering the detrimental effect of excessive K accumulation on musts, K content can represent a key element to be monitored in order to ensure grape quality under a changing environment. This is especially true for white grapes destined to the production of sparkling wines, for which titratable acidity and the malic acid content at harvest are of upmost importance [25]. The cv. Glera is a white grape variety cultivated in the northeast of Italy for the production of the renowned Prosecco sparkling wine [26].