## 3 Results
### Berry Development
The Cabernet Sauvignon single berry weight response was higher for all years compared to Riesling with a 14% increase under eCO\({}_{2}\) (Figure 2b). Response of eCO\({}_{2}\) in single years was 1% in 2014 (\(y=1.0085x-0.0019\), \(R^{2}=0.96297\)), 28% in 2015 (\(y=1.2794x-0.1173\), \(R^{2}=0.96510\)), and 14% in 2016 (\(y=1.1381x-0.0194\), \(R^{2}=0.9667\)) (Figure S1b, Supplementary Materials). Both cultivars showed the highest eCO\({}_{2}\) stimulation of single berry weight in 2015 and the lowest in 2014. It is shown that, during berry development, the SFR_R ratio decreased when berry color changed from green to yellow for Riesling (Figure 3) or green to red for Cabernet Sauvignon (Figure 4), while total soluble solids (TSS) increased. No differences occurred for SFR_R for both cultivars and for TSS in Cabernet Sauvignon between treatments and over the three years. Highest TSS values were found in 2015 close to harvest for both cultivars with 19.8\({}^{\circ}\) and 20.5\({}^{\circ}\) Brix for aCO\({}_{2}\) and eCO\({}_{2}\) in Riesling (Figure 3) and 19.5\({}^{\circ}\) and 20.3\({}^{\circ}\) Brix in Cabernet Sauvignon (Figure 4), respectively. Probabilities of the Bayesian generalized linear mixed model analyses for SFR_R and TSS were found to be quite variable (Figures 8 and 9). Hence, the trends for slightly lower TSS in eCO\({}_{2}\) compared to aCO\({}_{2}\) at the end of the season in 2015 and 2016 cannot be attested as \"significantly different\". This  
Figure 3: Chlorophyll index (SFR_R) and sugar accumulation (TSS) in aCO\({}_{2}\) and eCO\({}_{2}\) Riesling berries over the three seasons 2014 (**a,d**), 2015 (**b,e**), and 2016 (**c,f**). Letters indicate seasonal development stages veraison (V) and harvest (H). Mean values per treatment \(\pm\) SD of the three FACE rings. was indicated by low probabilities for a positive difference--below 50% for the last sampling dates in 2015 and 2016 (18%-38%)--across both cultivars, with Riesling a little more affected in 2016 than Cabernet Sauvignon (Figures 8 and 9). Surprisingly, TSS probabilities for Riesling at the sampling date closest to veraison showed a significant positive difference in 2015 (92%) and a significant negative difference in 2016 (9%), as shown in Figure 8.  
The relationship between SFR_R and TSS (\({}^{\circ}\) Brix) in Riesling (Figure 5a) and Cabernet Sauvignon (Figure 5b) berries during ripening showed a better inverse correlation for Riesling (\(R^{2}\) = 0.94186) than for Cabernet Sauvignon (\(R^{2}\) = 0.68841). When excluding the pre-veraison data with TSS values \(<\)7\({}^{\circ}\) Brix for Cabernet Sauvignon, the correlation increased with \(R^{2}\) = 0.76096. Riesling showed, in general, higher SFR_R values at similar Brix values compared to Cabernet Sauvignon. Total acidity (TA) and amino-acid concentration (N-OPA) during berry ripening are shown in Figures 6 and 7, respectively. N-OPA presented the highest variation in values for both cultivars and years. Therefore, it was not surprising that there was no distinct difference between eCO\({}_{2}\) and aCO\({}_{2}\) detected, and, due to the high variability within treatments, this led to less precise predictions. Figure 4: Chlorophyll index (SFR_R) and sugar accumulation (TSS) in aCO\({}_{2}\) and eCO\({}_{2}\) Cabernet Sauvignon berries over the three seasons 2014 (**a**,**d**), 2015 (**b**,**e**), and 2016 (**c**,**f**). Letters indicate seasonal development stages veraison (V) and harvest (H). Mean values per treatment \(\pm\) SD of the three FACE rings. Figure 5: Relationship between SFR_R and total soluble solids (TSS) in Riesling (**a**) and Cabernet Sauvignon (**b**) berries of the two treatments aCO\({}_{2}\) and eCO\({}_{2}\). Figure 6: Total acidity degradation during berry ripening in 2014, 2015, and 2016 for Riesling (**a–c**) and Cabernet Sauvignon (**d–f**) and the two treatments aCO\({}_{2}\) and eCO\({}_{2}\). Letters indicate seasonal development stages veraison (V) and harvest (H). Total acidity is expressed as tartaric acid in g/L.