Ongoing changes in global mean temperature, precipitations and continuously increasing carbon dioxide (CO₂) concentration are reshuffling conditions in which plants are growing. Indeed, atmospheric carbon dioxide concentration increases continuously due to anthropogenic emissions and currently reaches 410 ppm, while global surface temperature is already 1.09 °C higher in 2011–2020 compared to the 1850–1900 era (IPCC, 2021). Global surface temperature at the end of the century is predicted to increase by 1.0 °C to 1.8 °C (low greenhouse gas GHG emissions scenario), 2.1 °C to 3.5 °C (intermediate GHG emissions scenario) and by 3.3 °C to 5.7 °C (very high GHG emissions scenario; IPCC, 2021). Furthermore, according to the most pessimistic scenarios, rainfall variability should be amplified in the near future, and climate change is predicted to intensify the severity of wet and dry events (IPCC, 2021).

Crops are sensitive to environmental conditions, and grapevine is no exception to the rule. Berry oenological potential (i.e., its composition at harvest) is a complex trait that mainly results from genotypes (scions and rootstocks) and environmental interactions. Thus, climate change's impact on grapevine fruit composition at harvest needs to be thoroughly studied to help the wine industry adapt to future climate conditions (Duchêne et al., 2010; Schultz, 2000). Numerous studies have already characterised how grapevine and wine characteristics are impacted by elevated temperature (Luchaire et al., 2017; Sadras et al., 2013), heat stress (Lecourieux et al., 2017), water deficit (Deluc et al., 2009) or UV-B (Martínez-Lüscher et al., 2013).

Studies concerning the impact of atmospheric carbon dioxide concentration increase are scarcer and it should be noticed that studying carbon dioxide concentration effects is technically more challenging. Thus, prior studies on the impact of carbon dioxide on Vitis vinifera L. were mostly conducted in greenhouses or enclosed (tunnel or chamber) systems, using potted plants (fruiting cuttings) (Arrizabalaga-Arriazu et al., 2020; Arrizabalaga-Arriazu et al., 2021; Martínez-Lüscher et al., 2016). However, the translation of the results obtained from greenhouses to vineyard-based studies can be biased, as reported by Poorter et al. (2016). FACE(Free Air Carbon dioxide Enrichment) systems are open field setups designed to create a CO₂ enriched atmosphere around crops and represent a more realistic experimental system. Several FACE systems have been installed already, whether on various herbaceous crops such as pea (Bourgault et al., 2016), wheat and rice (Cai et al., 2016), barley and maise (Erbs et al., 2015) or trees such as poplars (Gielen and Ceulemans, 2001). According to literature reports on FACE based experiments, elevated CO₂ caused decreased stomatal conductance (Ainsworth and Rogers, 2007; Leakey et al., 2009) and increased light-saturated CO₂ uptake/net assimilation rate (Reddy et al., 2010) in C₃ plants. Consequently, the carboxylation efficiency of RuBisCo,_{compared to the oxygenation efficiency, was increased under elevated CO2. An increase in leaf area was also observed in response to an increase in CO2 atmospheric concentration (Ainsworth and Long, 2005).}

The impact of CO₂ atmospheric concentration on grapevine vegetative growth and berry composition has also been investigated by Bindi et al. (2001) using a FACE system, with different carbon dioxide concentrations (550 ppm, 700 ppm compared to ambient), on cv. Sangiovese. Vegetative growth, as well as fruit fresh and dry mass, were significantly increased by elevated CO₂ concentrations. Sugars and organic acid concentrations increased during berry development and ripening, but at maturity, these effects tended to disappear (Bindi et al., 2001). However, in these experiments, the treatment was applied instantaneously as a “shock” of CO₂, i.e. by suddenly rising local CO₂ to the desired level, which will not mimic realistic climate change scenarios, in which CO₂ increases gradually by approximately 2 ppm per year (IPCC, 2021). Therefore, such results must be discussed with caution since vines did not face a long-term acclimation towards gradual changes in carbon dioxide.

Using the VineyardFACE system described by Wohlfahrt et al. (2018), vegetative growth, leaf gas exchanges and yield parameters of cvs. Riesling and Cabernet-Sauvignon were compared under elevated and ambient CO₂ conditions. A significant increase in net assimilation rate, transpiration rate, stomatal conductance and water use efficiency was reported during the early years of grapevine adaptation, i.e., up to six years after planting (Wohlfahrt et al., 2018). Conversely, other results in the literature rather reported a decrease in stomatal conductance (Ainsworth and Rogers, 2007), although some authors agree with the fact that depending on weather conditions, an increase in stomatal conductance can occur under enriched CO₂ conditions (Purcell et al., 2018). Further studies showed that elevated CO₂ altered bunch parameters, increased single berry weight as well as malic acid content (Wohlfahrt et al., 2020). Moreover, minor differences in galacturonic acid for Cabernet-Sauvignon wines, pH or volatile acidity for Riesling were observed (Wohlfahrt et al., 2021). However, fruit quality at ripeness was globally not affected by elevated CO₂. Indeed, sugar concentration did not differ at harvest (Wohlfahrt et al., 2018). No negative impact of elevated CO₂ treatment was reported on must and wine composition for the years 2014 to 2016, and no difference occurred in total anthocyanin concentration in young wines of Cabernet-Sauvignon (Wohlfahrt et al., 2021).

In previous studies of the VineyardFACE, young grapevines adapted to the fumigation from an early stage of their development, and vegetative growth, as well as berry composition, was extensively studied. However, the long-term effect of elevated CO₂ has hardly been studied yet, as recently highlighted by Clemens et al. (2022). In VineyardFACE, vines have been well established since 2014, and the duration of the fumigation allows us to study grape berry parameters and composition of well-adapted vines using Cabernet-Sauvignon under near future (i.e. mid-century, 2050) forecasted scenarios.

This study is unique since it has been performed in a vineyard located FACE setup (VineyardFACE) where vines have been treated by elevated CO₂ concentration for almost one decade from shortly after planting to mature vines. Hence, the results of the investigations can be interpreted as long-term acclimation responses of the vines. The net assimilation rate was higher under elevated CO₂ treatment for both the 2019 and 2020 seasons (Table 1). Similar physiological responses were reported in previous seasons (Wohlfahrt et al., 2018), resulting in enhanced vegetative growth.

Regarding reproductive plant biomass, berry volume did not differ significantly between ambient and elevated CO₂ treatment. However, when combining the years 2020 and 2021, there was a significant increase in berry volume at 25 % véraison and a trend of higher berry volume under elevated CO₂. Moreover, single berry weight demonstrated a clear trend of an increase under elevated CO₂ treatment for both seasons 2020 and 2021, a trend that was alleviated at maturity. Biomass increase in yield under eCO₂ for red cultivars such as Sangiovese was reported by Bindi et al. (2005), assuming a higher berry weight under eCO₂. For Cabernet-Sauvignon, single berry weight increased under elevated CO₂ on the same VineyardFACE experimental setup (Wohlfahrt et al., 2020) when the vines were younger. Berry size is reported to be related to berry and wine quality, particularly at véraison, which coincides with a simultaneous sugar accumulation and organic acid degradation (Chen et al., 2018). Sugars accumulated in berries are mostly glucose and fructose (Kliewer, 1966). Mature berries display the same amount of glucose and fructose, whereas, at the early stages, glucose is predominant. In this study, no difference was noticeable between the two treatments, neither for TSS in must nor in total sugars from frozen pulp powder, as it was shown for TSS at the beginning of VineyardFACE experiments (Wohlfahrt et al., 2020).

Organic acids start to accumulate in the early stages of grape berry development and are mainly represented by malic and tartaric acid (Kliewer, 1966). During ripening, malic acid is degraded by increased respiration rate (Conde et al., 2007), whereas tartaric acid concentrations decrease by dilution effect due to berry volume increase. Our results suggest no significant effect of elevated CO₂ on both malic and tartaric acids. The effect of temperature on organic acid and especially malic acid degradation has already been reviewed (Etienne et al., 2013). Using potted plants in greenhouses, Arrizabalaga-Arriazu et al. (2020) found that malic acid in berry decreased from mid-véraison onwards under elevated temperature, whereas under elevated CO₂ (700 ppm), malic acid increased at véraison but was significantly reduced at maturity. In VineyardFACE, malic acid degradation was slowed down under elevated CO₂ concentration for Cabernet-Sauvignon (Wohlfahrt et al., 2020). Our results suggest that this effect is alleviated by long-term acclimation of the vines.

Concerning berry composition, it was firstly reported that vines in FACE systems under elevated CO₂ were displaying a change in sugar and organic acid concentration, but this CO₂ effect did disappear at maturity (Bindi et al., 2005). In temperature gradient greenhouses, a mitigating role of elevated CO₂ was demonstrated on grapevine vegetative growth and yield when combined with elevated temperature and drought (Kizildeniz et al., 2015).

No effect of elevated CO₂ was found at maturity on total amino acid concentration, although CO₂ treatment reduced alpha-ketoglutarate derivatives in later stages. Moreover, phenylalanine content was significantly increased at véraison, and it was found that elevated CO₂ treatment decreased amino acid concentration at véraison and two weeks after mid-véraison (Arrizabalaga-Arriazu et al., 2020).

Anthocyanins are considered important metabolites in fruit, which are usually altered during wine ageing to form polymeric pigments. Elevated temperature decreased anthocyanins concentration (Spayd et al., 2002) and accumulation. Furthermore, genes for anthocyanins biosynthesis were down-regulated under high-temperature conditions (Mori et al., 2007). Indeed, elevated temperature conditions decreased anthocyanins concentration (Arrizabalaga-Arriazu et al., 2021). However, the effect of elevated CO₂ on anthocyanins remains less evident in the literature. Berry characteristics were described to be unaffected by elevated CO₂ in Open Top Chambers on cv. Touriga Franca, however, total anthocyanins and polyphenol concentrations in red wine were reported to decrease under elevated CO₂ treatment (Gonçalves et al., 2009). When elevated temperature and elevated CO₂ were combined, it was reported that total anthocyanins and malic acid declined (Salazar Parra et al., 2010). Studies applying different UV-B doses and two temperature/CO₂ regimes to grapevine fruit cuttings cv. Tempranillo demonstrated that anthocyanins concentration differed during berry development between the treatments. Indeed, their concentration was higher under elevated CO₂ and elevated temperature combined two weeks after véraison, but at maturity, the trend reversed (Martínez-Lüscher et al., 2016). When studying the two parameters independently, Arrizabalaga-Arriazu et al. (2020) found that elevated CO₂ treatment did increase anthocyanin concentrations at the onset of véraison and mid-véraison, whereas after mid-véraison, their concentration decreased, in a clone genotype-dependent manner._{In the case of the current study, the decreased anthocyanin concentration could be due to increased single berry weight. Indeed, an increase in berry size could lead to a lower skin surface to berry volume ratio (Ojeda et al., 2002). Moreover, it was found that under carbon limitation induced by variation in source-to-sink ratio using Cabernet-Sauvignon the proportion of di-hydroxylated anthocyanins decreased (Bobeica et al., 2015; Wang et al., 2021). In our study, the di- to tri-hydroxylated anthocyanins ratio seemed to be higher, although not significantly, under elevated CO2 treatment. To gain more knowledge, in the future experiment, the degradation rate of monomeric anthocyanins under eCO2 conditions forming polymeric pigments will need to be investigated to better follow the ageing potential of the red wines.}

In a recent study, the parameters of young wine, such as total anthocyanins and organic acids, were more affected by the vintage effect than the CO₂ treatment (Wohlfahrt et al., 2021). These results confirm that, even if photosynthesis is still enhanced by elevated CO₂ treatment, primary and secondary metabolites content of berries may not differ under scenarios of near-future atmospheric CO₂ conditions.

This study aims to evaluate the effects of 2050 atmospheric carbon dioxide_{conditions on the berry composition of cv. Cabernet-Sauvignon that has been grown under elevated CO2 concentration for almost one decade. No major differences in primary metabolites were found under elevated carbon dioxidetreatment in two recent seasons, although it was shown that photosynthesis and the net assimilation rate are still enhanced by elevated carbon dioxide. However, from the berry quality point of view, elevated carbon dioxide has little effect on berry ripening dynamics and fruit composition. Concerning at least atmospheric carbon dioxideconcentration, the tipping point for grapevine seems to be not already crossed. However, climate change results in a combination of factors such as elevated temperature, drought in certain regions, and of course, elevated CO2. Further investigations focusing on combined environmental factors on primary metabolism intermediates, as well as aroma compounds, are part of ongoing studies within the VineyardFACE.}

We acknowledge the technical staff of the Department of General and Organic Viticulture for operating vineyard management in the VineyardFACE and the UMR 1287 EGFV laboratory team for their help in berry composition analysis. This work was supported by a PhD grant from the German-French University to C. Kahn (Grant # DGESIP/A1-3 No2019-0203).