Challenges of climate change on the future grape and wine production are widely discussed in science and in the wine industry with the goal to maintain a consistent must and wine quality in the future. Therefore, the effect of elevated CO₂ (eCO₂)—as one of the relevant greenhouse gases jointly responsible for a changing climate—was investigated concerning the composition of must and wine made of two grapevine cultivars V. vinifera L. cvs. Riesling and Cabernet Sauvignon within the established VineyardFACE (Free-Air Carbon dioxide Enrichment) experiment. Must and wine analysis were conducted in three consecutive years (2014–2016) by analyzing standard must and wine parameters, e.g., total soluble solids (TSS), pH, total acidity (TA), organic acids (e.g., tartaric acid, malic acid, shikimic acid, citric acid, volatile acid and gluconic acid) or total phenolics (TP). Also, for both cultivars CIELab coordinates (L* for lightness, a* as green/red and b* as blue/yellow components) were used to test colour in young white and red wines. Additionally, total anthocyanins and monomeric indices were analyzed for young wines of the red cultivar Cabernet Sauvignon. With marginal differences between CO₂ treatments, the composition of must and young wines was not found to be negatively influenced by an eCO₂ concentration.

Aims: Microscopic bud dissection is commonly used to assess grapevine bud fertility and thereby predict yield for the following season. Grapevine yield has been shown to increase in response to elevated carbon dioxide concentration (eCO2), and this yield gain has been demonstrated under Free Air Carbon dioxide Enrichment (FACE). However, the effects of eCO2 on bud fertility and their relation to yield gain have not been investigated. Little is known about which stages of development and which yield components are affected. The aim of this study was to determine the number of inflorescence primordia (IP) per node, the cross-sectional area of the IP, and the incidence of primary bud necrosis (PBN) in grapevine compound buds grown under conditions of ambient CO2 concentration (aCO2) and eCO2, and to relate the data to yield parameters of field-grown vines at harvest.Methods and results: Plant material was collected in February 2016 and February 2017 from two Vitis vinifera L. cultivars, Riesling and Cabernet-Sauvignon, growing at the VineyardFACE experimental site of Hochschule Geisenheim University (49°59¢N, 7°57¢E) in the Rheingau wine region, Germany. Bud dissections were performed at the University of Adelaide’s Waite Research Institute, Australia. Canes were stored at 4°C until dissection at room temperature. The first eight nodes of each cane were dissected and the compound buds assessed; IP number, IP cross-sectional area (evaluated by image analysis) and PBN incidence were recorded. In Riesling, no differences were found between plants grown under the two CO2 treatments in terms of IP number per node or subsequent number of bunches per shoot. Compound buds of Riesling plants grown under eCO2 had larger IP cross-sectional area; however, this did not result in higher bunch weight or yield in response to eCO2 over the 2 years. In Cabernet-Sauvignon, IP number per node was higher in plants grown under eCO2 but no changes in bunch number per shoot were found in either season. In contrast, the larger IP cross-sectional area in Cabernet-Sauvignon in response to eCO2 translated into higher bunch weight and yield in both seasons. PBN incidence of both cultivars was highest at basal node positions along the fruiting cane, except for Cabernet-Sauvignon in 2017, where both treatments showed higher incidence at the distal ends. In both cultivars, average PBN incidence along the cane was unaffected by eCO2.Conclusions: Microscopic bud dissection can be used at an early stage of vine development to predict increased bunch weight. There was evidence of a cultivar-dependent response to eCO2 in terms of bud fruitfulness. In future, it would be interesting to investigate whether higher carbohydrate levels are responsible for the increase in IP area detectable at a very early stage of development in plants grown under eCO2.Significance and impact of the study: The findings of this study contribute to our understanding of grapevine bud fertility and yield potential, particularly under changing climatic conditions.

Carbon dioxide (CO2) as one of the main factors driving climate change is known to increase grapevine growth and yield and could, therefore, have an impact on the fruit quality of vines. This study reports the effects of elevated CO2 (eCO2) on berry development and bunch structure of two grapevine cultivars (Vitis vinifera L. cvs. Riesling and Cabernet Sauvignon) within the VineyardFACE (Free-Air Carbon Dioxide enrichment) experiment, using must analysis and non-invasive fluorescence sensor technology. Berry development was examined on five dates over three consecutive years by analyzing total soluble solids (TSS), pH, total acidity, organic acids, nutrition status, and non-invasive Multiplex measurements. Before harvest, secondary bunches were collected to examine bunch and berry parameters. Results showed that eCO2 had little impact on berry composition of Riesling and Cabernet Sauvignon during berry development, which could be related to bunch structure or single berry weight within single seasons. Elevated CO2 (eCO2) did not result in modified TSS accumulation during ripening but was directly related to the chlorophyll index SFR_R. Higher single berry weights (SBW), higher malic acid (MA), and lower tartaric acid (TAA) were examined at some stages during development of berries under eCO2 levels. Our study provides evidence that eCO2 did alter some bunch and berry parameters without a negative impact on fruit quality.

Atmospheric carbon dioxide (CO₂) concentration has continuously increased since pre-industrial times and has currently reached an average growth rate of 2.3 ppm per year. For the majority of plant species elevated CO₂ (eCO₂) improves photosynthesis and thus plant biomass production. To investigate the effects of eCO₂ on leaf physiology and morphological leaf characteristics two Vitis vinifera L. cultivars, Riesling and Cabernet Sauvignon, grown in the VineyardFACE (Free Air Carbon dioxide Enrichment) system were used. The VineyardFACE is located at Geisenheim, Rheingau comparing future atmospheric CO₂-concentrations (eCO₂, predicted for the mid-21st century) with current ambient CO₂-conditions (aCO₂). Experiments were operated under rain-fed conditions for two consecutive years (2015 and 2016). For both varieties and CO₂ treatments, leaf gas exchange measurements were performed as well as measures of epidermal flavonoid (Flav) and leaf chlorophyll (Chl) indices by using a portable leaf clip. Furthermore, leaves were sampled for spectrophotometric analysis of the leaf pigments chlorophyll a (Chl a), chlorophyll b (Chl b) and carotenoid (Car). Additionally, leaf cross-sections were produced as permanent preparations to investigate morphological characteristics of the leaf structure. Both cultivars did not differ in leaf chlorophyll meter readings or leaf pigments between the two CO₂ treatments while net assimilation was highly stimulated under elevated CO₂ for both seasons. Differences found in leaf cross-sections were detected in palisade parenchyma and epidermal thickness of Cabernet Sauvignon under eCO₂, whereas Riesling net assimilation increased by 40% under a 20% CO₂ enrichment while remaining unaffected in different leaf layer thickness. The observed results within grapevine leaf tissues provide insights to seasonal adaptation strategies of grapevines under elevated CO₂ concentrations predicted in future.