Pleasant esters in red wines include ethyl acetate which has a OAV threshold of 12264 µg•L-1 and is described as fruity and balsamic , as well as isoamyl acetate, described as banana aroma with a OAV threshold of 30 µg•L-1 . In 2020, ethyl acetate was reduced in C0 and D5 wines, shading and reduced cluster temperatures preserved isoamyl acetate aromas in D1, D3 and D5 wines. When compared to wines from 2021, cooler vintage conditions did not result in ester compositional changes in exposed and shaded wines. Similarly, fatty acid esters were preserved in shaded wines, while 2020 C0 wines consistently had the lowest concentration of all measured fatty acid ethyl esters and various esters, all of which are associated with fruity and candy-like aromas. Concentrations of ethyl octanoate and ethyl decanoate remained beneath the reported perception threshold, thus observed shifts in composition with shading may be undetectable in Cabernet Sauvignon wines. However, ethyl hexanoate and ethyl isovalerate have remarkably low OAV thresholds of 5 µg•L-1 and 1 µg•L-1 , respectively . In the present study, all wines were above these thresholds, indicating that reductions in fruitiness may be perceived. This overall decrease in fruity aromas with cluster exposure and excess temperatures may negatively impact the marketability of Cabernet Sauvignon wines from hot viticulture regions with increasingly more frequent heat wave events associated with climate change.
Unpleasant and rancid aromas include isobutyric acid which imparts a cheese aroma and benzaldehyde which is associated with almond aroma in red wines . In this study, growing raspberries in container isobutyric acid concentrations were only affected in 2020, with D4 having the highest isobutyric acid concentration. The detection threshold for this aroma compound is 2300µg•L-1 . Concentrations detected in the experimental wines were substantially below this threshold, indicating that this slight increase in rancid aromas in D4 wines may not negatively impact overall wine perception. Given that D4 wines also exhibited enhanced fruitiness in with improved ester composition, the trade-off of slight increases in rancid aromas may be offset by the net benefit from increased fruity aromas in the wine aroma profile. While terpenes are often critical in white wines, these compounds when present in red wines have a large effect on wine aromas as their OAV thresholds are relatively low . The OAV threshold for a-terpinene, cis-rose-oxide and linalool are 250 µg•L-1 , 0.2 µg•L-1 and 25.2 µg•L-1 , respectively . The In 2020, α-terpinene, cis-rose-oxide and linalool were all reduced in C0 wines compared D4 and D5 wines, however concentrations of these compounds did not exceed the OAV threshold. These compounds produce odors such as peach, citrus, rose, and floral aromas in red wines . Previous work indicated an increase in terpenoids, particularly linalool in wines produced from fruit under black and red shade nets .
It was demonstrated that heat treatment will down regulate genes encoding key enzymes in terpenoid metabolism in Cabernet Sauvignon grapevines . Thus, increases in terpenoid content in shade film wines in 2020 may be due to reduced cluster temperature in a growing season with frequent heat wave events. In 2021, C0 wines exhibited the highest concentration α-terpinene, while cis-rose-oxide concentrations remained low in C0, and linalool was unaffected. In 2021, a cooler growing season with fewer days above 38°C may have resulted in less variation in terpenoid composition and net accumulation of terpenoids in exposed fruit . Ultimately, climatic shifts towards more frequent heat wave events will reduce floral and citrus aromas in wines produced from overexposed clusters. However, the year-to-year weather variation will enhance the unpredictability of the development of these compounds, leading to challenges for wine producers looking to produce a consistent product. As carotenoid breakdown products, C-13 norisoprenoids like β-damascenone often described by sweet and floral aromas and has an OAV threshold of 0.05 µg•L-1 . C-13 norisoprenoids have been shown to have a positive linear relationship with sunlight exposure to the grape cluster . Under extreme light intensity and temperature conditions, there are decreases in carotenoid concentration in the berry, thus reducing C-13 norisoprenoid precursors. In the present study, β- damascenone was highest in C0, D4 and D5 wines in 2020, while β-damascenone was highest in C0 and D1 wines in 2021, contrary to previous findings in hot viticultural areas. Lee et al. reported that grape clusters without leaf removal and inner canopy clusters contained more β- damascenone than south-facing clusters exposed to solar radiation by leaf removal. Likewise, black cloth and red shade net enhanced β-damascenone concentration compared to uncovered control .
Despite varied reports, Lee et al. also demonstrated a linear positive relationship between norisoprenoids in the grape berry and concentrations in wine, with the berry concentration was always greater than that of the resultant wines. It may be possible that carotenoid degradation due to excessive temperatures in C0 treatments was negligible or less than the biosynthesis of C-13 norisoprenoids, resulting in similar concentrations as D4 and D5 shade film treatments. Therefore, the results of this study demonstrated that partial solar radiation exclusion with reductions in UVA, UVB and NIR radiation does not hinder norisoprenoid content in wines. Additionally, the concentrations of β- damascenone across all treatments in both years exceeded the odor active threshold for this compound, indicating that significant differences in β-damascenone concentrations between C0 and treatments may be perceivable in resultant wines.Although grapevine is one of the most resilient crops globally, the changing climate is one of the many challenges to its cultivation and ultimately wine production. According to the Intergovernmental Panel on Climate Change , global air temperatures are likely to increase 1.5-4.5o C between 2030 and 2052 . Historical records of growing degree day accumulation demonstrate that air temperatures in the world’s most notable grape growing regions have increased steadily within the last 70 years . Such increases in GDD accumulation disrupt the natural coupling and balance of primary and secondary metabolites during ripening, corresponding with a plateau in wine quality ratings . Subsequently, excessive air temperatures reduce pleasant and desirable wine aroma compounds, but also contribute to reductions in wine quality . Moreover, predicted changes in precipitation patterns and increased drought frequency threaten grapevine water status conducive to market-desired fruit yield and composition and long-term vineyard sustainability. With increasingly frequent and prolonged drought periods and less predictable rainfall, vineyards will need to adapt to changes in water availability. The evaluation and performance of drought tolerant root stock and scion cultivars has also been investigated ; however, their adoption may be limited due to historical and cultural connections to popular grape cultivars. Additionally, the effect of root stock-scion interactions on water use efficiency is poorly understood and only recently became an area of research . At the field scale, large plastic pots for plants wider row spacing to decrease vine density can decrease overall vineyard evapotranspiration by reducing the competition for water between vertically shoot-positioned vines . Pieri et al. examined the water balance of two planting densities in five viticultural regions of France and concluded that lower planting densities utilizing constrained canopies can maintain vine water status within moderate stress levels in forecasted climate change conditions. However, decreased plant density may result in lower yields at the field-scale, requiring an economic cost/benefit analysis to determine its viability as a water conserving solution. Reducing applied water through sustained and regulated deficit irrigation strategies is already commonly used to improve flavonoid composition in the grape skins . Furthermore, varied amounts of applied water have been trialed in a hot growing region such as California and demonstrated that irrigating at 50% of potential grapevine evapotranspiration was sufficient to mitigate water shortages when dormant season precipitation was limited . More deliberate interventions to adapt to heat and water shortages in the vineyard include the use of shade cloths and films to reduce canopy temperatures and vine evapotranspiration . Again however, the implementation of these shade structures is under question as they present barriers to vineyard mechanization and may be a costly and unfeasible long-term solution.
Adapting trellis systems may be another method for mitigation of climate change impacts in production vineyards. Choosing an appropriate trellis system is an important pre-planting decision during vineyard establishment. An appropriate trellis system optimizes the vine’s capacity to intercept solar radiation and produce a canopy microclimate that results in optimal berry ripening without excessive direct solar radiation overexposure to the fruit zone. A traditional and commonly used trellis system in various grapevine production areas worldwide is the vertically shoot positioned trellis. While VSP trellises were traditionally thought to improve berry ripening, the VSP trellis system maximizes light penetration and canopy porosity, producing a canopy microclimate which increases cluster vulnerability to overexposure in hot viticulture regions . More recent work has investigated the resiliency of red wine grapes in hot viticulture regions when training systems are varied from the traditional VSP trellis. In a study conducted in Napa Valley CA, USA, trellis systems with free and sprawling canopies such as a single-high wire and high-quadrilateral systems increased yield and produced berries with improved flavonoid profiles that are attributed to reduction in chemical degradation compared to traditional VSP trellis systems regardless of applied water amounts . Additionally, the interactive effect of varied applied water amounts and trellis systems has been minimally investigated. Williams and Heymann applied various fractions of estimated potential grapevine evapotranspiration to VSP and Scott-Henry trellis systems to elucidate the effect of applied water on vine productivity and fruit composition in Livermore, CA. In their study, irrigation amounts had a larger effect on vine productivity and berry quality than trellis systems due to VSP and Scott-Henry trellis systems having similar levels of overexposure to the fruit zone . Free and sprawling trellis systems which can shade the fruit zone and protect it from overexposure conditions can provide a long-term feasible heat avoidance strategy for hot viticulture regions due to promoting larger canopies. While there are demonstrated improvements to grape chemical composition with the adoption of these sprawling trellis systems, it is understood that trellis systems promoting larger leaf area indices will have a higher water demands . In regions where irrigation is required to supplement seasonal precipitation, maintaining these larger canopies may prove difficult, especially with increasingly stringent environmental regulations such as the Sustainable Groundwater Management Act in the state of California. SGMA limits groundwater extraction for agricultural irrigation . Compliance with SGMA will result in allotted water use restrictions in California’s Napa Valley, limiting growers to 120 mm of applied water each season. Therefore, there is uncertainty surrounding how growers will respond to such water restrictions in tandem with adapting to increasing temperatures in a climate change scenario. Ultimately, there is a lack of information on the water footprint of resilient trellis systems hindering their adoption in microirrigated wine grape production vineyard. Our previous works conducted with VSP trellis indicated that irrigating at different percent of potential grapevine ET affected grapevine physiology leading to different carbon allocation, water footprint, and water use efficiency in hot climates . Likewise, our previous work provided evidence that trellis systems other than VSPs provide better adaptation of wine grapes to climate change by ameliorating physiological performance and berry chemistry due their canopy architecture . Therefore, the objective of this experiment was to determine the water use efficiency of grapevine with trellis and pruning types that are commonly used in production regions characterized by hot climates. We hypothesized that the new trellises that were indicated to be more resilient to climate change would have different water needs than traditional VSP. Therefore, we applied fractions of the potential grapevine evapotranspiration estimated for VSP to different trellis systems to compare their water productivity relative to VSP under a climate change scenario.Climate change in hot viticultural regions brings two prominent challenges: higher air temperatures with unpredictable heat wave events and increased drought frequency . At the experimental site in Oakville, CA, the 2021 growing season was cooler than both the preceding and following growing seasons. Hot air temperatures may create untoward growing conditions that affect whole grapevine physiology . In hot viticultural regions like California, prolonged drought conditions are becoming more common and increasingly severe. In the current study, extreme and exceptional drought conditions were recorded across the three experimental years. Total precipitation at the experimental site during the water year within the last decade was previously reported as approximately 768mm . The total precipitation recorded during the 2019-2020 and 2020-2021 growing seasons was 234.2mm and 278.3mm, respectively and was substantially less than the long-term average.