## 4 Discussion  
### Berry Development  
Berry development of grapevines plays an important role in the formation of fruit quality parameters that are substantial for efficient wine making and the following wine profile. According to previous work, vines exposed to eCO\({}_{2}\) conditions predominantly affect total soluble solid accumulation and acid degradation [2; 5; 7; 11]. These two main components are well known to be sensitive to several abiotic factors such as soil, temperature, precipitation, or sunlight [44; 45; 46; 47; 48; 49], which all count as climate-driven relevant markers. Nevertheless, all studies conducted on field-grown grapevines under eCO\({}_{2}\) conditions embraced that there is no positive or negative repercussion of eCO\({}_{2}\) on grape or wine quality [2; 3; 7]. On the other hand, eCO\({}_{2}\) was shown to affect grapevine vigor and growth, especially bunch and berry weight or berry number [3; 6; 9], and it could, therefore, lead to quality changes by altered berry sizes [14; 15; 16; 17; 18; 19; 20; 21]. Single berry weight was significantly affected for both cultivars by eCO\({}_{2}\) in 2015 and 2016, even though Riesling showed a clear response at full maturity in 2016. As published in a previous paper due to the onset of the experiment [9], the 2014 season did not yet show differences between CO\({}_{2}\) treatments in terms of yield, and this can be confirmed in the present study for single berry weight during berry development. That CO\({}_{2}\) enrichment started in 2014 and generative growth is attributed to i.a. infloresence initiation in the season prior, the lack in CO\({}_{2}\) effect was already linked to that [9]. Nevertheless, the relation of single berry weight between aCO\({}_{2}\) and eCO\({}_{2}\) showed an overall eCO\({}_{2}\) response of a 7% gain of berry weight for Riesling and a 14% gain for Cabernet Sauvignon over all three years. This result was in agreement with previous field studies of [PERSON] et al. [3] and [PERSON] et al. [6] who reported an increased cluster weight and an increased 100-berry weight for two  
Figure 12: Posterior predicted probabilities for differences in berry size categories between eCO\({}_{2}\) and aCO\({}_{2}\) for each year from Bayesian mixed effects adjacent category models with category specific effects for both cultivars, Cabernet Sauvignon and Riesling. Point estimates represent the median difference for each year and variety. different red cultivars under eCO\({}_{2}\). In addition, fruit weight, volume, and diameter of pear, as another example of a perennial crop, were also found to be enhanced under eCO\({}_{2}\)[22; 23]. Regarding the berry composition during ripening over the three years, both cultivars, Riesling and Cabernet Sauvignon, did not differ in TSS between the two treatments. Even though values for TSS during berry development were predominantly lower under eCO\({}_{2}\) and both cultivars, there was no significant difference detected except for one sampling date in 2015 and 2016 for Riesling, showing higher and lower TSS, respectively, at the beginning of berry development. The sugar content of must at harvest date is not influenced by eCO\({}_{2}\)[2; 7; 9]; however, due to higher net assimilation and yield under eCO\({}_{2}\), an average of 7% and 10% increases in sugar yield occurred for Riesling and Cabernet Sauvignon, respectively [9]. Interestingly, [PERSON] et al. [7] demonstrated that TSS of Sangiovese was stimulated by eCO\({}_{2}\) during ripening, but the positive effect disappeared when grapes reached maturity. [PERSON] et al. [2] also confirmed this for Touriga Franca, whereas only [PERSON] et al. [5] indicated that eCO\({}_{2}\) resulted in a higher TSS accumulation for Shiraz at harvest. Berries of both cultivars reached a higher sugar concentration for both treatments at maturity in 2015 compared to the other seasons. This disparity could be due to the climatic condition in the 2015 growing season with the highest number of heat days allowing higher sugar accumulation.