Despite higher recorded GDD in the present study, titratable acidity at harvest was maintained around 7 g•L-1 . Ultimately the reduction in cluster temperature imparted by the shading impeded organic acid degradation therefore maintaining berry acidity. It was identified that anthocyanin accumulation was maximized at 875 GDD and a daily mean light intensity of 220klm⸱m-2 after which anthocyanin content decreased in Cabernet Sauvignon. Previous works that used partial shading that transmitted 60% of solar radiation had also resulted in increased anthocyanin content compared to unshaded fruit in under similar growing season and climatic conditions . In 2021, shade films did not affect the anthocyanin content in berry skins at harvest, due to the cooler growing season limiting anthocyanin degradation post-veraison. The reduction in anthocyanin content observed in 2020 may result from repressed anthocyanin biosynthesis at hot temperatures via the MYB4 repressor . However, it is also highly likely that elevated temperatures in 2020 resulted in increased anthocyanin degradation in exposed fruit compared to shaded fruit, leading to shaded fruit having greater anthocyanin content. Flavonols are photoprotectants and free radical scavengers in the plant kingdom . As such, these compounds are directly responsive to light exposure of the cluster. In the phenylpropanoid pathway, MYBF1 is a transcriptional regulator ofFLS, the key gene in flavonoid synthesis . It has been shown that MYBF1 is upregulated by UV-B light, resulting in increased flavonols in grape berry skins . Thresholds for optimal sunlight exposure have been elucidated in previous solar radiation exclusion experiments, where Martínez-Lüsher et al. tracked flavonol development over the growing season under 20% and 40% shading conditions. It was determined that net flavonol biosynthesis occurs until approximately 570 MJ m-2 of accumulated global radiation which corresponds with 7.6% molar abundance of kaempferol in grape skins .

Beyond these thresholds, plastic gutter flavonols started to be degraded in the grape berries. Our study showed a similar trend for flavonol content in hot years like 2020. The control treatments in 2020 exceeded 8.6% kaempferol abundance, while shade films were effective in maintaining kaempferol abundance below this overexposure threshold. In cooler years like 2021, flavonol degradation was not observed at the global radiation threshold as a result of the cooler growing season. Rather, biosynthesis continued to increase flavonol content until harvest in 2021. Shade films effectively lengthened the period of flavonol biosynthesis and reduced the amount of time during ripening where clusters are under flavonol degrading conditions. Anthocyanins are comprised of two aromatic rings linked by three carbons in an oxygenated heterocycle . Hydroxylation and methylation of the B- ring is responsible for color and hue of each anthocyanin molecule. Increasing free hydroxyl groups on the B-ring enhances blueness while methylation of the hydroxyl groups increases redder hues in grape skins . From a winemaking perspective, 3’4’5-OH anthocyanins are more resistant to degradation during fermentation, leading to stable wine color . In this study, overhead shade films did not affect anthocyanin hydroxylation by harvest in either year of this study. However, shifts in anthocyanin hydroxylation have been previously documented: colored shade nets reducing solar radiation by 40%, showed higher anthocyanin and flavonol hydroxylation compared to unshaded treatments . Previous studies reported increases in the ratio of di-tri hydroxylated anthocyanins in grapevines under water deficits . The absence of this shift in anthocyanin hydroxylation under shade films was most likely due to similar grapevine water status among the shaded and control treatments, as the vines were not under water deficit conditions. However, shade films altered flavonol hydroxylation under hot growing conditions in 2020, with hydroxylation being the highest in the least exposed shade films . Shade films D4 and D5 transmitted 60% and 40% of UV-B radiation respectively, resulting in less flavonol hydroxylation than D1 and D3, but more hydroxylation than the control. In cooler growing conditions in 2021, all shade films had comparable levels of flavonol hydroxylation, yet hydroxylation was still greater under shade films than the control.

These results may be due to the upregulation of flavonoid 3’ hydroxylase . This enzyme is responsive under sun exposure and is responsible for the generation of 3’4’ hydroxylated flavonoid precursors .It has been long recognized that the quality of wines is closely associated with the accumulation of secondary metabolites, specifically flavonoids and volatile organic compounds that have a direct effect on wine color, taste and aroma . Flavonoids in wine include anthocyanins, flavonols and flavan-3-ols. Wine color, particularly its hue and intensity, are strongly determined by anthocyanin methylation, acetylation, hydroxylation of the anthocyanin B-ring, and co-pigmentation with cofactors such as flavonols . Partial solar radiation exclusion was shown to effect anthocyanin hydroxylation. Tarara et al. demonstrated increased dihydroxylation of anthocyanins in grape berries exposed to direct solar radiation compared to shaded fruit. Likewise, Martínez-Lüscher et al. monitored anthocyanin hydroxylation under colored photo selective shade nets and found that by reducing solar radiation by 40% with black polyethylene shade nets, the ratio of tri- to dihydroxylated anthocyanins was increased compared to uncovered control fruit. Such shifts in anthocyanin hydroxylation can impact anthocyanin hue and wine antioxidant capacity. Wine aroma in both red and white wines is a matrix formed by a variety of volatile compounds. However, the composition of the matrix can be impacted by grape cultivar, vineyard conditions and fermentation conditions. Contribution of volatiles to wine flavor composition is related to its chemical structure . The most abundant class of volatile compounds found in the wine matrix are higher alcohols . These by-products of yeast nitrogen metabolism are usually described by unpleasant “solvent” or “fusel” aromas when present in concentrations greater than 400 mg/L . The more pleasant “fruity” aromas described in wine are associated with esters. Esters are often in highest concentrations in young red wines and decrease in concentration with aging . C-13 norisoprenoids and terpenes are key aromas compounds found in both red and white wines, contributing fruity and floral aromas at low olfactory concentrations . C-13 norisopenoids are understood to be derivatives of enzymatic or photochemical degradation of carotenoid pigments in the grape berry . In plants, carotenoids have photo protectant and antioxidant properties, making these pigments responsive to solar radiation in grape berries. Carotenoids in grape berries have been shown to increase in berries with increased in solar radiation pre-veraison . However, under extreme exposure to heat and solar radiation, there is a documented decrease in carotenoid concentrations during ripening . To preserve the carotenoid concentrations in the grape berry and to promote C-13 norisoprenoids in resulting wines under more frequent heat wave events and increases in air temperature, artificial shading with black polyethylene cloth has been trialed and found that shaded fruit contained more carotenoids than unshaded fruit . However, the effect of partial solar radiation exclusion on wine C-13 norisoprenoid content seems to be more nuanced. Wines produced from the shaded fruit contained more β- damascenone as well as esters compared to wines produced from unshaded fruit . Yet, there are conflicting reports showing no effect of UV exposure on β- damascenone concentrations in Shiraz wines made from clusters that underwent solar radiation exposure via varying rates of leaf removal and polycarbonate UV screens . Like C-13 norisoprenoids, final terpene concentrations in wines depends on the net accumulation in grape clusters exposed to excessive temperatures and UV radiation . The effect of photo selective overhead shade films on whole plant physiology and temporal development of berry flavonoids of Cabernet Sauvignon development over two growing seasons was previously studied in a hot region . Grape berries growing under reduced near-infrared radiation exposure in hotter than average years, resulted in a 27% increase in anthocyanin content at harvest than the exposed control due to decreases inanthocyanin degradation due to high berry temperatures . Moreover, flavonol degradation was similarly decreased, thus optimizing flavonol content in the grape berry under reduced near-infrared radiation exposure .

The objectives of this study aimed to determine the extent to which the impact of photo selective overhead shade films on flavonoid development transfer to wine and the cascading effects of partial solar radiation exclusion had on aroma composition of resultant wines. The experiment was conducted in Oakville, CA, USA during two consecutive growing seasons at the University of California Davis, Oakville Experimental Vineyard. The vineyard was planted with “Cabernet Sauvignon” clone FPS08 grafted onto 110 Richter rootstock. The grapevines were planted at 2.0 m × 2.4m and oriented NW to SE. The grapevines were trained to bilateral cordons, blueberry container vertically shoot positioned, and pruned to 30-single bud spurs. Irrigation was applied uniformly from fruit set to harvest at 25% evapotranspiration as described elsewhere . The photo selective shade film treatments were previously described in Marigliano et al. and their properties presented in Figure 1. Shade films were designed to target portions of the electromagnetic spectrum previously observed and measured at the experimental site . Briefly, four photo selective shade films and an untreated control were installed in 3 adjacent rows on 12 September 2019. The shade films remained suspended over the vineyard until 20 October 2021. The shade films were 2 m wide and 11m long and were secured on trellising approximately 2.5 m above the vineyard floor. Each experimental unit consisted of 15 grapevines in 3 adjacent rows. Grape clusters were harvested by hand from each experimental unit when berry total soluble solids reached 25o Brix on 9 September 2020, and 7 September 2021, respectively. Vinification was conducted in 2020 and 2021 at the UC Davis Teaching and Research Winery. Upon arrival at the winery, grapes were destemmed and crushed mechanically. Must from each field experimental unit was divided into three technical fermentation replicates . K2S2O2 was added to each treatment- replicate and must was allowed to cold-soak overnight at 5o C in jacketed stainless-steel tanks controlled by an integrated fermentation control system . The following day each treatment-replicate was inoculated with EC- 1118 yeast to initiate fermentation. Musts were fermented at 25°C and two volumes of must were pumped over twice per day by the integrated fermentation control system. During the winemaking process, TSS was monitored daily using a densitometer and fermentations were considered complete once residual sugar contents were less than 3 g⸱L-1 . Wines were then mechanically pressed using a screw-type basket press. Following pressing, wine samples were collected for flavonoid analysis. Malolactic fermentation was initiated with the addition of Viniflora® Oenococcus oeni . Malolactic fermentation was carried out at 20o C. Upon completion of MLF, free SO2 levels were then adjusted to 35 mg⸱L-1 and wines were bottled. Using a spectrophotometer , color intensity , hue, total polyphenolic index and % of polymeric anthocyanins was determined following procedures described by Ribéreau-Gayon, Glories, Maujean, and Dubourdieu . Wine samples were diluted in water and absorbance readings were taken at 280, 420, 520, and 620nm. The absorbance at 740 nm was subtracted from all absorbance readings to eliminate turbidity. CI was calculated as the sum of absorbance at 420, 520 and 620nm. Hue was calculated as the ratio between the absorbance at 420 and 520nm. The percentage of polymeric anthocyanins was determined via absorbance measurements at 520nm after anthocyanin bleaching with a sodium bisulfite solution . TPI was determined by diluting wines with water and recording absorbance at 280nm. Volatile compounds in wine samples were analyzed following procedures described previously . Briefly, 10-mL of each wine sample was transferred to a 20-mL amber glass vial . Each vial also contained 3 g of NaCl and 50μg of an internal standard solution of 2-undecanone . After agitating at 500 rpm for 5 mins at 30o C, samples were exposed to 1 cm polydimethylsiloxane/divinylbenzene/Carboxen , 23-gauge SPME fiber for 45 mins. Helium was used as a carrier gas at a flow rate of 0.8636 mL/min in a DB-Wax 231 ETR capillary column  with constant pressure and temperature at 5.5311 psi and 40o C, respectively. The oven temperature was kept at 40o C for 5 mins and then incrementally increased by 3o C/min until reaching 180o C. Oven temperature was then increased by 30o C/min until reaching 260o C, at which temperature was maintained for 7.67min. The SPME fiber was desorbed split mode with a 10:1 split for wine samples and held in the inlet for 10min to prevent carryover effects. The method was retention time-locked to the 2-undecanone internal standard.