A changing climate is threatening the sustainability of the vineyard itself as well as the quality of its outputs. More frequent heat waves result in a dissociation of primary and secondary metabolism in the grape berries due to reduced biosynthesis and thermal degradation of secondary metabolites. This decoupling between primary and secondary metabolites led to decreases in vintage quality in historical premium wine production regions. Thus, temperature mitigation strategies for wine grape vineyards in hot viticulture regions appear necessary under climate change conditions. Ultimately, artificial shading may provide a viable solution for premium grape growing regions. As this is a relatively expensive measure, utilizing the vine’s canopy volume and architecture to shade the fruit zone may also be a viable solution. However, changes in precipitation patterns resulting in more frequent droughts reduce the water supply necessary to irrigate larger vine canopies at profitable levels. Therefore, the water footprint of various vine canopy architectures must be quantified prior to adoption of larger vine canopies at wide scale in wine grape production areas as a heat mitigation strategy. The essential hypothesis for this study was that a reduction in grapevine canopy temperature achieved by decreasing the near-infrared component of incoming solar radiation would optimize berry and wine flavonoid and aroma profiles in hot wine grape production regions by reducing thermal degradation of these compounds. To test such hypothesis, partial exclusion of solar radiation was achieved using photo selective overhead shade films.
There were two specific objectives for this study, growing raspberries in pots the first being to understand the mode of action of photo selective overhead shade films on the leaf and vine physiological responses under climate change conditions. The second objective was to assess the effect of using photo selective shade films in vineyards on wine chemistry and flavonoid profile. Subsequently, this work led to investigating the water footprint of different trellises, as grapevine training systems can provide a viable alternative to overhead shade films for achieving reduced canopy temperature and shaded clusters. Therefore, the third specific objective was to quantify the grapevine’s water use efficiency and water footprint for six different trellis systems commonly used in hot viticultural areas. This study was conducted in two vineyard blocks at the Oakville Experimental Vineyard Station in Napa County, California from 2020-2022. This Ph.D. dissertation comprises three chapters. Chapter 1 describes research work conducted during 2020 and 2021 which investigated the effects of photo selective shade films on grapevine physiology and grape berry quality. Ultimately, reduction in the near infrared component of the incoming solar radiation resulted in reduced cluster temperatures, which mitigated flavonoid degradation in the grape berry. Grapes from the film-shaded trials were collected and vinified. Chapter 2 reports the chemical properties of wines resultant from the different experimental treatments, including flavonoid and aroma profiles. Wines from film-shaded treatments produced better flavonoid and aromatic profiles compared to wines obtained from uncovered vines. While shade films are effective at mitigating effects of high temperatures resulting from climate change in wine grapes, they are a costly investment for large scale grape production.
Adapting trellis systems to unconstrained and sprawling canopies is becoming more common as a method to shade clusters; however, such larger canopies have relatively higher water demands. Chapter 3 provides information from appraising the water use efficiency and water footprints of grapevine grown with six trellis systems under three regimes of applied water to provide some recommendations to growers about appropriate trellis system to adopt for long-term sustainable strategy for mitigating the adverse impacts of climate change. Grapevine is a resilient and lucrative crop with a vast global distribution . Historically, climate and cultivar associations have developed regional wine identities that are commercially and culturally valued. However, steady increases in air temperature across the world’s most famous growing regions have been observed since 1980, threatening to shift appropriate climatic growing conditions to regions located in higher latitudes and altitudes in search for cooler climates . Concern for shifting regional climates is based in the understanding that certain grape cultivars thrive in specific optimum air temperature regimes where wine quality is optimized. At the onset of global air temperature shift during the 1980s, wine quality ratings increased, presumably due to increased berry sugar concentration and riper flavors . However, during the 2010s there was a marked plateau in wine quality ratings, indicating that there may be a tipping point at which wine quality will suffer as air temperatures continually increase . Consequently, for a region to adapt to ever warming air temperatures without detrimental decreases in wine composition, mitigation strategies need to be developed. Among grape berry secondary metabolites, flavonoids play important roles in berry and wine composition.
Anthocyanins are responsible for berry and wine color , while flavonols act as photoprotectants in plants, scavenging free oxygen radicals and preventing enzymatic reactive oxygen species, while also contributing to wine color through copigmentation with anthocyanins . Flavonoids are produced through the phenylpropanoid pathway , which is responsive to environmental conditions, including solar radiation. It is understood that UV-B radiation induces flavonol biosynthesis by activating MYBF1, the key transcription factor responsible for the regulation of flavonol biosynthesis enzymes including two flavonol synthase genes, VvFLS4 and VvFLS5. This occurs via a signaling cascade derived from the photoreception of UV-B radiation by Ultraviolet Resistance Locus 8 homodimers. Likewise, selectively screening out excessive UV-A and UV-B with overhead shade films would result in appropriate molecular signaling for flavonoid biosynthesis . Previous work determined that flavonol profile in red skinned grape berry was a reliable biomarker for canopy architecture. In warm climates, net accumulation of flavonols might be impeded by flavonol temperature sensitivity . Therefore, selectively removing NIR spectrum from berries would result in less flavonoid degradation due to reduced heat gain by the berry. If the grape berry was subjected to solar radiation overexposure and subsequent heat wave damage soon after sugar translocation into the berry, flavonol degradation occurred and kaempferol molar abundance in grape skins exceeded 8.6% . Kaempferol molar abundance is the ratio of the molecule to the total flavonols in berry skin. Subsequently, kaempferol molar abundance exceeded this threshold between 540-570 MJ⸳m-2 of accumulated global radiation post-veraison . Grape berry composition is derived from a balance between primary and secondary metabolites . Ultimately, in hot climate viticulture regions, the clear sky days and concomitant berry temperature gains result in decoupling of sugar and flavonoid in grape berries . Under optimal growing conditions, there is a direct relationship between sugar content and anthocyanin synthesis in grape, as some flavonoid synthesis genes such as LDOX and DFR, possess ‘sucrose boxes’ in their promoters, resulting in sugar-regulated gene expression . However, like flavonols, anthocyanins are also susceptible to chemical or enzymatic degradation at high temperatures while sugar accumulation is unaffected. Mohaved et al. described a putative peroxidase gene VviPrx31 which may be responsible for anthocyanin degradation under high temperatures. The effect of sugar and anthocyanin decoupling on berry and wine composition was investigated where ‘Cabernet Sauvignon’ berries subject to leaf removal and shoot removal treatments were harvested at 24o Brix and vinified . Compared to untreated control, plant pot with drainage wines from leaf and shoot removal treatments had reduced color stability due to less anthocyanin hydroxylation as a function of higher temperatures and solar radiation exposure. Efforts to reduce berry heat gain and through solar radiation exposure in vineyards with overhead and partial shading have been attempted but remain controversial in commercial wine grape vineyards. Cartechini and Palliotti demonstrated that average within-canopy temperatures in ‘Sangiovese’ grapevines decreased by approximately 2o C when covered with shade cloth transmitting 30% and 60% photosynthetically active radiation . Similarly, thin netting and plastic films covering ‘Italia’ grapevines reduced mid-day temperatures within the canopy at fruit height by about 6o C below air temperature. Martínez-Lüscher et al. partially excluded solar radiation with colored polyethylene shade nets. They concluded that partial shading of the canopy produced quantifiable differences in berry microclimate by reducing canopy temperature by 4o C on the SW-facing side of the canopy. The authors attributed the highest anthocyanin content in the Black-40% shade net lessened anthocyanin degradation from lower canopy temperature. However, partial shading in these experiments failed to selectively omit harmful solar radiation from the fruit, but rather reduced total solar radiation exposure by 40% of the total radiation.
The objective of this study was to selectively remove portions of solar radiation spectrum using overhead shade films in the vineyard, to mitigate the vulnerability of ‘Cabernet Sauvignon’ grape berry to solar radiation overexposure and optimize berry composition at harvest with desirable sugar accumulation and minimized flavonoid degradation. The weather conditions during the 2020 and 2021 growing seasons were compared to the long-term average for the study area over the past 10 years . Compared to the past 10 years, the 2020 growing season accumulated more growing degree days by 1 October. Conversely, 2021 was a cooler growing season with less growing degree days accumulated than the long-term average. While GDD accumulation early in the season was similar during April – June for both years, the GDD accumulation in 2020 outpaced 2021, with 1762.7°C growing degree days accumulated in 2020 compared to 1572.3°C growing degree days accumulated in 2021. The total precipitation at the experimental site from 1 March 2020 to 30 September 2020 was 84.1mm. The 2020 water year experienced 100.5 mm less precipitation than the 10-year average for the experimental site. Particularly, the 2020 growing season experienced much less precipitation during March compared to the 10-year average with 1.2mm of precipitation accumulating in March 2020. Drought conditions continued into the 2021 water year, with 66.9 mm of precipitation between 1 March 2021 and 30 September 2021. Precipitation only occurred in March and April. Precipitation in the following months of 2021 was negligible. The number of days with maximum air temperature that exceeded 34o C and 40o C in 2020 and 2021 were different . In 2020, there were 32 days that exceeded 34o C while in 2021 there were only 22. Likewise, in 2020 there were 6 days that exceeded 40o C while in 2021 there was only one day that exceeded 40o C. Cluster temperatures were affected by overhead shade films during heat wave events. During heatwave events that occurred pre-veraison , possible residual warming from the previous day resulted in warmer cluster temperatures in shaded treatments during the early morning hours . Throughout the day, 2020 preveraison cluster temperatures in shaded and control treatments did not differ until 19:00h on 11 July, with the C0 having warmer clusters than all shaded treatments. Beginning at 9:00h on 11 July, cluster temperature in both shaded and control treatments was higher than ambient temperature for the remainder of the day. The largest warming effect on clusters occurred at 13:00h, with the temperature difference between C0 cluster temperature and ambient temperature being 13.3o C. The difference in D5 cluster temperature and ambient temperature was 9.8o C at solar noon. Cluster temperature trends were similar pre-veraison in 2021 . However, differences in cluster temperature were only observed at 7:00h in 2021, again most likely residual warming effects from the previous day . The largest ΔT was 11o C between D5 and ambient temperature at 15:00h. The cooling effect of shade films on cluster temperature was more distinct during post veraison heatwave events . In the afternoon hours, cluster temperatures in shaded treatments were less than the control. In 2020, cluster temperatures under overhead shade films at 17:00h were at least 4o C cooler than clusters in C0 . At 15:00h, ΔT between D3 and ambient temperature was 14o C, the largest temperature difference observed on 18 August 2020. Similarly post-veraison C0 clusters in 2021 consistently had higher cluster temperature compared to shaded clusters in the afternoon. Reduced cluster temperatures in shaded treatments compared to C0 were first observed midday and this cooling effect of the shade films continued throughout the afternoon until 17:00h . During the warmest parts of the day , the largest ΔT was 9o C between C0 and D4. C0 was 19.8o C warmer than ambient temperature at 15:00h, the hottest hour of the day. Regardless of transmission spectra, reduced solar spectra transmission significantly decreased cluster temperatures post-veraison. In 2020, berry mass only differed between D3 and the control during post-veraison .