Previous works show berry sunlight exposure to alter the composition of anthocyanins, such as the proportion of acetylated and coumarylated forms . Modulation of acylated, methylated, and hydroxylated forms of anthocyanins result from the synergistic effect of solar radiation exposure and the coupled increases in berry temperature . Generally, high berry temperatures resulting from increased solar exposure results in increased acylated anthocyanins in the grape berry, particularly coumarylated forms . Also, high temperatures result in accumulation of highly methylated anthocyanins such as malvidin derivatives, as these compounds are less likely to degrade than their counterparts . In 2020, D1 and D5 wines demonstrated highest concentrations of acetylates, coumarylates, and methylated anthocyanins compared to C0 wines. While D1 and D5 treatments demonstrated cluster temperatures less than those from C0 treatments , the concomitant thermal degradation of total anthocyanins in C0 treatments proved to negate any modulation towards acylated or methylated forms in resultant wines. Similarly in 2021, C0, D1 and D5 wines exhibited reduced acylation compared to D4 wines. Again, while D4 consistently exhibited less intense cluster temperatures, square plastic plant pots the thermal degradation in more exposed treatments eclipsed any identifiable acylation modulation from hot growing conditions. Acylated anthocyanins are more stable compounds and provide color stability and increase blueness in wine . However, an increase in methylated anthocyanins will lead to redder hues in wine .
Therefore, the improvement in acylated and methylated anthocyanin content due to partial solar radiation exclusion may enhance color perception in young red wines through color stabilization and alteration of wine hue. Likewise, anthocyanin hydroxylation is also directly influenced by temperature and solar radiation exposure. Previous studies on berry exposure utilizing UV selective shade nets as well as leaf removal, demonstrated anthocyanin tri-hydroxylation increases with increasing berry temperature . Increases in trihydroxylation are driven by accumulation of malvidin derivatives and the temperature sensitivity of F3’H, the catalyzing enzyme for 3’- hydroxylated anthocyanin biosynthesis . The highest ratio of tri- to di-hydroxylated anthocyanins in 2020 C0 wines were driven by higher concentrations of 3-pcoumaroyl-glucoside derivatives of delphinidin, petunidin and malvidin, despite the ratio of tri- to di-hydroxylated anthocyanins being unaffected at harvest in the grape berry in 2020 . Among shade film treatments in 2020, the reduction of UV light exposure, was the determining factor in anthocyanin hydroxylation patterns rather than berry temperature. Previous shade net studies at the experimental site showed a reduction in UV radiation with black-40% and blue-40% shade nets led to higher anthocyanin tri-hydroxylation in the grape berry compared to control vines at harvest . With the reduction of UVB and UVC radiation in D4 and D5 vines, anthocyanin trihydroxylation was reduced, regardless of temperature. Ultimately, the upregulation of F3’H from sun exposure could be negated by the reduced catalytic activity of this enzyme under high temperatures experienced in 2020.
In the cooler 2021 vintage, the ratio of tri- to dihydroxylated anthocyanins was unaffected, due to non-significant effect of shade films on acetylated anthocyanins. Ultimately, increased tri-hydroxylation in young red wines will also impact wine hue, resulting in more purple wines . Flavonols in the grape berry skin act as a photoprotectant and are strongly induced by ultraviolet radiation . Flavonol composition in the grape berry can be used to determine overexposure, specifically by quantifying the molar abundance of kaempferol. C0 berries in this study were shown to be overexposed by surpassing the previously described threshold of approximately 7% molar abundance of kaempferol . In both years of the study, flavonol composition in grape berries was maximized in C0 fruit, but D4 and D5 fruit contained the most flavonols across the shade films with minimal thermal degradation of the compounds on the vine . Likewise in both wine vintages, flavonol concentration was modulated by UV radiation exposure, proportional to the amount of UV radiation transmitted to the grapevine. Of the wines produced from shade films treatments, D4 allowed for the most UV transmission while subsequently reducing near infrared transmission by approximately 15%. These light conditions ultimately optimized flavonol content in D4 wines compared to the other shade treatments from both wine vintages. As such, this demonstrated the transmissibility of berry composition under shade treatments to directly improve wine flavonoid profiles. For hot viticulture regions, photoselective solar radiation exclusion provides a strategy to improve not only flavonoid profile but also wine color intensity through copigmentation with anthocyanins.These compounds are derived from microbial mediated cleavage of the C-C double bonds in linoleic and linolenic acids, by lipoxygenase and alcohol dehydrogenate enzymes in yeast . Compounds such as 1-hexanol and -2-hexen-1-ol are associated with aromas such as cut grass, green, fat and herbaceous aromas and their OAV thresholds are 8000 and 400 ug/ L, respectively . The effect of shade films on C6-alcohols was evident in both years; however, there was a yearly effect on which alcohol was altered by the treatment. In 2020, -2- hexen-1-ol was the lowest in D4. In 2021, -2-hexen-1-ol was unaffected by shade films, while 1-hexanol was highest in C0, D4 and D5. Although there was a statistical difference in C6 alcohols, the differences were not large enough between C0 and treatments to crossthe OAV thresholds for these compounds. Increases of C6-alcohols in C0, D4, and D5 wines may be explained by solar radiation overexposure in the treated clusters. L. He et al. reported higher linoleic and linolenic acid biosynthesis with leaf removal at veraison. Subsequently, fruit exposed to increased solar radiation had elevated precursors for C6- alcohol production during yeast metabolism. Additionally, L. He et al. showed higher initial concentration of C6-alcohols in grape berries from leaf removal treatments due to modulation of the volatile compound metabolome and transcriptome in grape berries exposed to sunlight under dry-hot conditions. Therefore, in our experiment which has similar climatic conditions to L. He et al. , fruit from shade films with higher percentages of UV radiation may have both an increase in linoleic and linolenic acids to act as C6-alcohols aromas precursors and increased C6-alcohols in the exposed grape berries. Ultimately, overexposure of the grape berry led to more green and grassy aromas in wine, which may lead to an unripe perception of these wines. Higher alcohols are also produced during fermentation from yeast metabolism of amino acids. These compounds are generally pleasant aromas including mushroom, roses, honey, candy and fruity notes. Of these compounds, shade treatments increased isoamyl alcohol concentration in 2020 and benzyl alcohol concentration in wines from both vintages. Isoamyl alcohol is associated with solvent and cheese aromas and, while benzyl alcohol is characterized as being citrusy and sweet . The odor active thresholds for these compounds are 30000 mg/L and 10000 mg/L, respectively . In 2020, C0 had the lowest concentration of isoamyl alcohol in wines. The effect of shading on the concentration of isoamyl alcohol in wines varies in literature . In hot growing regions, 75% of total solar radiation exclusion with black polyethylene canopy side shade nets resulted in wines with reduced isoamyl alcohol compared to the uncovered control vines .
However, this experimental site was in a region that received approximately 704.5° C less growing degree days than the present experimental site in the hotter 2020 season, and 514.1°C growing degree days less than the cooler 2021 season. In the study by Lu et al., 2021, reduced solar radiation exposure in a cooler growing region may have resulted in reduced isoamyl alcohol in shaded fruit. When cluster temperatures exceed 42°C in exposed vines, there is a reduction in isoamyl alcohol in resultant wines compared to wines produced from fruit under red and black shade nets . With cluster temperatures of C0 fruit exceeding 42°C, excessive cluster temperatures may be prompting the reduction in isoamyl alcohol and overall wine fruitiness from those produced from overexposed clusters. However, square pot plastic while there was a statistical difference in isoamyl alcohol concentrations between C0 and treatment wines, the effect was not large enough to exceed the OAV threshold for this compound . Shade films affected the ester composition predominantly in 2020 wines. 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, 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, aterpinene, 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 aterpinene, 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 bdamascenone 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, b-damascenone was highest in C0, D4 and D5 wines in 2020, while b-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 b-damascenone than south-facing clusters exposed to solar radiation by leaf removal. Likewise, black cloth and red shade net enhanced b-damascenone concentration compared to uncovered control .