## 3 Results and Discussion  
### Physicochemical Parameters  
The evolution of the basic quality parameters during the on-vine late-harvest dehydration of Vidal grapes is depicted in Figure 1. It should be noticed that the sugar and titrated acid content are represented in two ways, namely, as 'g/L' for juice and as 'g/berry' for the total amount per single berry. The substantial reductions in berry weight (BW) and berry volume (BV) throughout the sampling period suggested that typical natural dehydration occurred during this process (Figure 1e). At T5, the BW was only 78.2% that at T1, and the BV was only 77.0% that at T1. A berry weight loss (BWL) of 21.8% was caused by on-vine dehydration, which directly reflected the level of berry water loss. In our opinion, it is not appropriate to express BWL (%) as berry water loss (%), as in some articles [19; 23], because water loss (%) is the percentage of lost water in the net water content in grape berries, while BWL (%) is the percentage of lost water in the whole weight of the grape berry. The increasing trend in BWL (Figure 1f) indicated that the water loss proceeded more effectively in the later stages of the sampling period, due to the drier weather and more wind in this period (see the meteorological data shown in Figure S1). Adequate sunshine and a windy, cool, dry climate not only ensured the slow and continuous on-vine dehydration of the grapes, but  
Figure 1: Physicochemical parameters during the on-vine dehydration process of Vidal grapes. T1, T2, T3, T4, and T5 represent the sampling points at different dehydration degrees. Values are the means of three replicates. The error bars represent the standard deviation. (**a**) Total Soluble Solid (TSS); (**b**) pH; (**c**) Total Sugar and Sugar Content; (**d**) Titratable Acid and Acid Content; (**e**) Berry Weight and Berry Volume; (**f**) Berry Weight Loss, Juice Yield, and Juice Volume per 100 Berries. also protected them from fungal infections. Some authors have proposed that the rate of water loss and dehydration temperature have an important influence on the phenolic and volatile compounds in the grape berries and even in the final wines [10; 14; 24]. Juice yield (%) (calculated in mass percent) and juice volume per 100 berries (JV100) are shown in Figure 1f. After two months of natural on-vine dehydration, the juice yield (%) of Vidal grapes reduced by 54.4%, from 53.5% to 24.4%. The JV100 reduced from 76.6 mL to 25.6 mL after this process, indicating a 66.6% reduction in final wine production. There is no doubt that lower yields lead to higher quality. The total sugar concentration (g/L) and TSS (\({}^{\circ}\)Brix) of the juice were enhanced dramatically by the on-vine water-loss process (Figure 1a,c). The sharp increase in sugar concentration in grape juice between T4 and T5 provides further evidence that the water-loss rate was much higher in the later stage of dehydration. However, even though the pH value increased continuously, the titratable acid concentration in the grape juice showed different patterns during the dehydration process, presenting fluctuating changes (Figure 1b,d). The low pH of Vidal grapes at technical maturation in the Weixi region has led to a series of winemaking problems, an essential reason why this variety is used to make late-harvest dessert wines in this region. On-vine dehydration provides a good solution to the low pH problem in this area, as can be seen from our results. The enhancement of the sugar content and the fluctuation in acidity were consistent with the results of previous studies on icewine grapes [19] and postharvest off-vine drying grapes [11]. [PERSON] et al. (2015) [25] have attributed the reduction in must acidity during icewine grape late-harvest to the precipitation of potassium tartrate caused by on-vine freeze-thaw cycles. However, the temperature data in Figure S1a indicate that the Vidal grapes in our study did not suffer from freeze-thaw cycles.