The quality of fruit harvested is the utmost concern when considering advanced phenology

Water use efficiency predictions are further complicated by the results of combination studies of elevated temperature, reduced soil water availability, and elevated CO2, which reveal synergistic effects. In an open top chamber study, combining temperature and CO2 did not result in gs being significantly reduced, contrary to results of elevated CO2 alone . When latent heat is trapped, overheating subsequently decreases the activity of RUBISCO activase, for most plants at temperatures higher than 37°C , and in grapevine between 35-40°C, varying by species . The elevated CO2 and temperature treatments showed an increase in transpiration , and the effects of drought were only temporarily delayed . Temperature and elevated CO2 had an additive effect on plant leaf area for multiple grapevine clones , highlighting that overall higher leaf area without increased WUE could be detrimental for heat stressed vines. Measurements of predawn water potential were more negative in vines at elevated CO2, indicating the demand for soil water availability of vines with increased productivity . Notedly, the production of fine roots was positively impacted by an elevated CO2 treatment, which would theoretically increase water absorption of water available . There remain inconsistent predictions of the effects of elevated CO2 on grapevine whole plant water use efficiency, which seem to be contingent upon other factors such as soil water availability, temperature, and variety of grapevine. With the evidence from these studies of elevated CO2 and combination studies of soil water availability and temperature, grapevines most likely will not benefit from a long-term increase in photosynthesis under elevated CO2. The lack of soil water available and biological temperature thresholds for RUBISCO will limit the gains in photosynthesis, pot with drainage holes and more likely the vines will struggle to release latent heat as temperatures rise.

Grapevine phenology is categorized into four life cycle stages of periodic development: budburst, flowering, veraison, and maturation. The grapevine phenological cycle is a two-year process; bud formation occurs in the first year which develop into shoots in the second year. Therefore, clusters are significantly impacted by the previous year’s climate . For grapevine grown at elevated CO2, advances in phenology compound significantly over seasons . This is likely the result of stored carbon photosynthate from the productive previous year. As a result, it can take several years to observe the effects of elevated CO2 on grapevine phenology , which leads to the question of: “To what extent does elevated CO2 impact the timing of phenological stages over the long-term?” Studies of Arabidopsis, another C3 flowering plant, provide insight to the mechanisms of phenological changes observed in grapevine. Excess carbohydrates may act similarly to phytohormones to delay the upregulation of genes involved in flowering time, as well as cell wall invertases in the meristem that down regulate photosynthesis under treatments of elevated CO2, which leads to earlier flowering . For grapevine, it is possible that excess photosynthate could trigger early flowering through the transfer of carbohydrates from leaves. One of the most robust findings to support this hypothesis is that growth under elevated CO2 results in increased carbohydrate reserves in plants . The sugars produced by photosynthesis contribute only a fraction of the source of carbon needed for rapid growth and development from budbreak to flowering and sugar accumulation in berries at veraison, the remaining needed for these growth spurts is mobilized from long-term storage of total nonstructural carbohydrates in trunks and roots . Over several growing seasons, storage of carbohydrates in the trunk will be impacted by elevated CO2 and could therefore contribute to shifts in phenology.

In a greenhouse study of fruiting cuttings where sugar accumulation in berries was measured, elevated CO2 increased the rate of ripening correlated with the photosynthetic rate . The effect of elevated CO2 on phenology was greater than the treatment of temperature elevated by 4°C . Therefore, an increase in total nonstructural carbohydrates could be a driver of advances in phenology long term, on its own, as well as with concomitant increases in growing season temperatures. Carbohydrate reserves regulate the growth and differentiation of flowers, which only occurs after the grapevine shoot is resource independent from the rest of the vine . These findings suggest that with an increase in carbon reserves stored as starch in roots, trunks and canes, second season shoots may grow faster and achieve independence earlier in the growing season. This could contribute to early flowering as a result of lifted competition for resources between vegetative and reproductive growth. In contrast, long-term studies in grapevine decreasing the leaf to fruit ratios decreased essential reserves of the TNC in the roots . The well-known viticultural technique of strategic leaf removal has been shown to delay maturation, highlighting the importance of carbon availability for phenological development . While the mechanism for phenological shifts in grapevine grown under elevated CO2 is under-studied, these shifts have been quantified using FACE experiments. The combination of elevated CO2 and temperature in OTC caused an advance in flowering time by three days and veraison by two weeks . The impact of elevated CO2 on phenological timing is greatest during the period between fruit set to veraison and this impact increases when combined with a temperature treatment . During fruit set, elevated CO2 treatments with and without temperature treatments increased total soluble solids , as well as decreased anthocyanins and malic acid concentration, which would contribute to an earlier veraison and harvest . However, the impact of high temperature may have a greater impact on this phenological period .

Grapevines vulnerable to frost damage will suffer from early budburst, with subsequent losses in yield . One consequence of increased shoot vigor at elevated CO2 is the expected increase in bud fertility, which will likely increase the number of flowers per vine . Changes in cluster density and phenological timing impact the carefully articulated annual harvest. Unbalanced sugar/acid ratios resulting from early harvest decrease the quality of grapes and wine produced, discussed further in the “Berry and Wine Chemistry” section below . Shifting the lifecycle of grapevine will have a global impact on winegrape production.Fruit composition is a major area of concern for growers and winemakers alike, especially aromatic compounds. The changes in pest interactions, physiology, and timing of veraison in response to elevated CO2 will collectively impact the resulting grape and wine quality . For successful wines, in the grape berry there is a balance of acid and sugar at harvest. Increasing atmospheric carbon available impacts the balance as ripeningadvances and sugar accumulation is accelerated . Flavonoids and anthocyanins are important for the flavor, color, and mouthfeel of wine. The molecular analysis from the original Italian FACE experiments showed increases in total flavonoids, total anthocyanins, and total non-anthocyanin flavonoids in the wine produced with carbon enriched grapes grown at 700 mg/L , which typically would affect the color and mouthfeel of wine. Interestingly, a subsequent experiment using 500 mg/L CO2 open top chambers determined there were significant increases in ethyl 2-methylbutyrate , isoamyl acetate , ethyl hexanoate , ethyl octanoate , butyric acid , and isovaleric acid concentrations and a significant decrease in ethyl acetate concentration in wines produced from enriched CO2 grapes after one year , which contribute to the balance of floral and fruity characteristics in wines . In the second year they found lower methionol , 1-octanol , and 4- ethylguaiacol , and they found higher ethyl lactate and linalool concentrations, large pot with drainage although these changes in berry chemistry did not appear to significantly affect the quality of wine produced . These results agree with early studies led by Bindi et al. that did not find significant effects on the quality of wine produced from grapes grown at elevated CO2 . Although the changes observed in compounds contributing to flavor have been noted as so far insignificant for quality, a major concern for winemakers is the increase in alcohol content resulting from an increase in sugar concentrations in berries, as a result of higher CO2 concentrations . In the past, winemakers have added sugar to the fermentation to increase the final alcohol percentage , depending on legal regulations for winemaking.However, in recent years winemakers have begun removing sugar through processes like reverse osmosis in order to prevent alcohol levels from rising . Overall, elevated CO2 is altering the balance of sugar accumulation, the levels tartaric and malic acids in berries and wine, and the impact on wine quality continues to be investigated . The most recent FACE studies on grapes continue to evaluate the berry chemistry and quality developing over years of exposure to elevated CO2. The VineyardFACE in Germany analyzed must from grapes after pressing and did not find a significant increase in sugar content from conditions of carbon enrichment . The Gonçalves et al. study also concluded that changes in water availability and heat stress could change their predictions in wine quality.

We should expect that with the shifts in phenology and physiological changes to berries, early harvest will impact the quality of grapes in terms of reaching maturation too quickly . Viticulturists could also anticipate altered physiological demands to have long-term impacts on berry quality . In contrast to the ecological pressures discussed above, the rates of some fungal infections may be reduced in elevated CO2 scenarios. With higher carbon allocation to roots, grapevine mycorrhizal colonization may be promoted by elevated CO2 , which has been shown to protect grapevine against the nematode Xiphinema index by stimulating defense gene response . A study of elevated CO2 on several varieties of grapevine seedlings showed a reduced severity of the infection of Xanthomonas campestris pvviticola, a vector of bacterial canker in immature grapevine . This may be the result of lower stomatal conductance ; with stomatal aperture reduced, there is less opportunity for bacteria to invade the leaf pores . Also, researchers recorded a reduced instance and severity of powdery mildew infection in cv Barbera, at elevated CO2 . The Geisenheim VineyardFACE site recorded changes in the bunch architecture but did not see an increase in the frequency of B. cinera, botrytis bunch rot, a necrotrophic fungus, occurrence . Changes in leaf chemistry phenotype, specifically carbon content, , will increase the pressure of grapevine pests in future climates. Increasing available carbon dioxide, without a concomitant increase in nutrient levels in the soil, leads to an increase in C:N ratios in leaves . Insects consume at higher rates when nitrogen has been diluted to meet their nitrogen intake needs and chewing insect pests will generally eat more leaf tissue in elevated carbon dioxide scenarios . Elevated CO2 increased individual survival rates and increased the fecundity of female mealybugs, which eat phloem of grapevine damaging the temporal and perennial plant tissue . The European grapevine moth, Lobesia botrana, is a major problem for European vineyards, affecting both the berries and flowers of grapevines; and has already invaded North and South American vineyards . L. botrana is also responsible for spreading Ochratoxin A-producing Aspergillus fungi, which typically spikes in occurrence during hotter and drier years . At higher temperatures simulating future climate conditions, L. botrana female growth rate and pupal mass increased , while researchers found a down regulation of expression of ethyleneresponsive factors, which suggests grapevines can become more vulnerable to herbivory or abiotic stress under future climate change as these are the major stress and defense response factors . A comprehensive study of soil and elevated CO2 showed the decomposition pathway is altered by the carbon-, nitrogen-, and phosphorus-acquiring enzymes in the soil with a significant increase in nematode density . More than 4,000 plant-parasitic nematodes exist, posing a well-known global issue for grapevine, reducing total crop production by 8.8- 14.6%, and one of the worst threats from the nematode Xiphinema index is GLRV . Under elevated CO2 conditions, if ethylene is suppressed and salicylic acid is increased, it is likely that grapevine will struggle with an increase in pest and disease vectors such as nematodes and fungi . Grapevines largely rely on human intervention for defense against pests and diseases , and this reliance could increase in future climates. Consider the grapevine “immune system” as weakened in terms of chemical defense, but some altered carbon dynamics under elevated CO2 may be beneficial for reducing severity of pest pressure.