## 4 Discussion
### Berry Development
In addition to the investigation of TSS accumulation during berry development under eCO\({}_{2}\) treatment, we confirmed that the spectroscopic index SFR_R as a chlorophyll index is a good representative of TSS (\({}^{\circ}\) Brix) for both white (R) and red (CS) grape cultivars. This was reported earlier for red cultivars Cabernet Sauvignon, Merlot, and Pinot or white cv. Vermentino under Mediterranean climate [30; 31]. This gives an additional opportunity of rapid and indirect determination of sugar content during berry ripening in addition to refractometry. In terms of a changing climate, the combination of increased malic acid and a similar sugar content of Cabernet Sauvignon berries under eCO\({}_{2}\) as pronounced in the 2015 season is not affiliated with a degradation of quality. This is especially true for the ongoing process of vinification and microbiological stability. In fact, this leads to a positive output of eCO\({}_{2}\) when no manipulation within the vine's canopy is conducted as performed in the present study. Even though vigor, yield, or bunch compactness of grapevines increased under eCO\({}_{2}\) in previous studies, there was no effect on _Botrytis cinerea_ incidence or frequency [3; 6; 7; 8; 9]. Riesling was more sensitive to degradation of acidity during berry development, showing significantly lower levels of tartaric acid or significantly higher values of malic acid depending on the growing season. Noticeable for Riesling as a white cool climate cultivar is the fact that tartaric acid under eCO\({}_{2}\) was less during the beginning of berry development in 2015, when temperatures were highest and precipitation lowest in comparison to the 2014 and 2016 seasons. The differences of 227 mm of precipitation during vegetation in 2015 compared to 441 and 371 mm for 2014 and 2016, respectively, in combination with the number of days \(>\)30 \({}^{\circ}\)C, reflect the combined effect of eCO\({}_{2}\) with other climatic parameters. This was shown in a previous study when the impact strength of eCO\({}_{2}\) on vegetative growth of Riesling was associated with annual climatic variation [9]. Getting closer to the harvest date of Riesling in 2015, the differences in tartaric acid concentration between the two treatments disappeared. Moreover, with continuing berry maturation of Riesling and simultaneous differentiation in single berry weight, malic acid degradation was lowered under eCO\({}_{2}\) at the end of 2015 and 2016. This resulted in significantly higher malic acid concentration combined with higher single berry weights under eCO\({}_{2}\) at harvest, as shown for Cabernet Sauvignon in 2015. Trials on red cultivars under eCO\({}_{2}\) regime reported that either no difference in acid concentration was detected at berry maturity [7] or an increase in total acidity was found, without differences in malic and tartaric acid [2]. So far, on white cultivars, only greenhouse experiments were conducted, showing that there was no eCO\({}_{2}\) effect on malic or tartaric acid at berry maturity [12; 13]. Interestingly, a previous study on Riesling berry size [19] demonstrated that an increase in berry size was associated with increased malic acid concentration, which could account for a direct eCO\({}_{2}\) effect on berry size, and this in turn resulted in an enhancement of malic acid under eCO\({}_{2}\). A similar achievement was made for Cabernet Sauvignon, resulting in higher malic acid and lower total acid concentration with increasing berry size classes [20]. The decelerated malic acid degradation under eCO\({}_{2}\) could also be merged to an altered microclimate within the bunch zone due to higher vigor, which was shown by various authors vice versa studying fruit zone defoliation trials on berry composition [50; 51; 52] or different temperature regimes within the bunch zone [53; 54; 55]. Shikimic acid (SA), citric acid (CA), and N-OPA provided no distinct differences between eCO\({}_{2}\) and aCO\({}_{2}\) treatment for either cultivar.