The differences among treatments for berry mass observed post-veraison were non-significant by mid-ripening and remained as such until harvest. TSS, pH and TA were also monitored throughout the growing season in both years. Overhead shade films did not affect TSS in must at any sampling point throughout the 2020 season . TA was only significantly higher in D3 compared to the control, while pH was only significantly higher for the control when compared to D1. As berries developed, there was no significant effect of shade films on pH and TA compared to C0 from veraison until harvest. Differences in berry mass occurred later in the season in 2021 compared to 2020 . At mid-ripening, berry mass of D3, D4, D5 and C0 were similar and greater than that of D1. At harvest, shade films did not have any effect on berry mass. Unlike 2020, differences in TSS were observed at veraison and mid-ripening . At veraison and mid-ripening, D3, D4, D5 and C0 had similar TSS. In 2021, D1 consistently differed from D5 at these sampling points for TSS. However, it had similar TSS as D4 and C0 at mid-ripening. There were no differences in TSS in between shade film and control fruit at harvest. Early season differences in pH were observed, with C0 having similar pH to D4 and D5 . When compared together, the shade films had similar pH at the green berry stage. There were no further differences in pH between treatments and control as ripening progressed. The TA only differed at mid-ripening with D1, D3 and C0 having the highest titratable acidities.

D4 and D5 had similar TA, growing blueberries in pots which was significantly less than D1 . Titratable acidity did not differ between shaded and control fruit at harvest. Compared to the control, grape berries grown under shade film had higher skin anthocyanins at both mid-ripening and harvest in 2020. In all treatments, total skin anthocyanin content peaked at mid-ripening and then decreased from mid-ripening to harvest, with D5 showing the smallest decrease in total skin anthocyanin content . However, the shade treatment films resulted in 27% greater anthocyanin content than C0 at harvest. The proportion of tri-hydroxylated anthocyanins increased throughout berry development in all treatments . However, shade films did not affect anthocyanin proportion of hydroxylation compared to the control in this year . In 2020, total skin flavonol content increased in both shaded treatments and the unshaded control until the veraison . However, C0 consistently had higher flavonol content compared to shaded treatments. Between the shaded treatments, D4 and D5 produced fruits with significantly more flavonol content per berry compared to D1 and D3 at each sampling time point, except at immediate pre-veraison, where flavonol content in D4 was not significantly different compared to D1 and D3. At mid-ripening flavonol content decreased in both shaded and unshaded fruits. At harvest, there was no significant difference in flavonol content between C0, D4, and D5. Shade films D1 and D3 had less total skin flavonols than C0, D4 and D5, containing approximately 0.06 mg/berry. The proportion of tri– to di-hydroxylated flavonols was affected by the overhead shade films . At mid-veraison, there was a greater proportion of tri-hydroxylated flavonols with D1 and D3 compared to D4, D5, and C0. The differences between treatments were pronounced at harvest in 2020 with C0 resulting with the least amount of tri-hydroxylated flavonols in 2020. In 2021, differences in total skin anthocyanin content were evident at veraison and midripening . At veraison, total skin anthocyanin content was higher in D5 compared to D1. Shade films C0, D3, and D4 had similar total skin anthocyanin content to D1 and D5 at veraison. At mid-ripening, D5 has significantly higher total skin anthocyanin content to C0, with D1, D3 and D4 having similar anthocyanin content. At harvest, overhead shade films did not have an impact on total skin anthocyanin content. However, anthocyanin content increased from mid-ripening to harvest in D1, D3 and D4, while they appeared to reach a plateau in anthocyanin content in D5 and C0.

The effects of overhead shade films on anthocyanin hydroxylation were only observed at mid-ripening with D1 having higher proportions of 3’,4’,5’ to 3’,4’- hydroxylated anthocyanins than D4 and D5, and C0 along with D3 did not differentiate with other treatments . In 2021, the accumulation trend of skin flavonol content differed compared to that of 2020. At the first sampling point, total skin flavonols were the highest in C0 while D1 had the lowest flavonol content . The flavonol content continued to increase as ripening progressed. From mid-ripening to harvest, C0, D5 and D4 had the highest flavonol content compared to D1 and D3. In 2021, total skin flavonols did not decrease prior to harvest. The seasonal trend of di- to tri-hydroxylated flavonols differed in 2021 compared to 2020. Early in the season, D1 and D3 had more tri-hydroxylated flavonols . From veraison to harvest D1, D3 and D5 had more tri-hydroxylated flavonols compared C0. Similar to 2020, C0 consistently had the lowest ratio of tri- to di-hydroxylated flavonols at every sampling point with the difference at harvest. In 2020, molar abundance of kaempferol peaked at mid-ripening . C0 had the highest molar abundance of kaempferol. The molar abundance of kaempferol in D5 was significantly higher compared to D1 and D3. A decrease in kaempferol molar abundance was observed from mid-ripening to harvest. Nevertheless, at harvest, molar abundance of kaempferol remained the greatest in C0 compared to the other overhead shade films, and D1 had the lowest kaempferol molar abundance. In 2021, the molar abundance of kaempferol increased until midripening and then appeared to either level off or decrease from mid-ripening to harvest in all treatments . Differences in molar abundance of kaempferol were observed at veraison and mid-ripening but not at harvest. At veraison and mid-ripening, C0 had more kaempferol than D1 and D3. Similar molar abundance of kaempferol was observed betweenD5 and other treatments at veraison, and D4 and other treatments at mid-ripening. The weather in 2020 and 2021 varied considerably leading to year-to-year variation in the study. In 2020, the air temperatures were higher than the long-term 20-year average for Oakville, CA.

In previous studies at this experimental site, similar heat wave events were recorded. In 2017 there were 7 days above 40o C and 64 days above 30o C . Conversely, 2021 was a cooler growing season than the 20-year average and recent past years. Compared to precipitation trends of the past 20 years, 2020 and 2021 were severe drought years. The yearly variation in temperatures and precipitation in this study helps to exemplify the unpredictability of growing conditions forecasted with climate change. The application of solar radiation exclusion may become increasingly necessary for wine grape production in hot climates to maintain optimal berry and wine chemistry. Ponce de León and Bailey quantified berry temperature in a VSP trellis system using thermocouples and subsequently modelled berry temperature temporally and spatially. In an uncovered VSP trellis system, black grape berries in direct sunlight can reach temperatures over 10o C above ambient temperatures with the hottest hours being from 15:00h to 17:00h, while naturally shaded fruits followed ambient temperature . Similarly, Martínez-Lüscher et al. found that sun exposed grape berries reached temperatures approximately 15o C warmer than ambient air in the afternoon. We observed a temporal shift in the efficacy of overhead shade films. Prior to veraison, overhead shade films did not reduce cluster temperatures, as green berries do not absorb as much heat as black berries after veraison. However, drainage gutter shaded berries were still warmer than ambient temperature which conflicted with the assumptions from the model presented by Ponce de León and Bailey. After veraison, the cooling effect of shading film was evident as black berries absorbed heat. Shade films in 2020 exceeded the performance of black shade netting with 40% shade factor used by Martínez-Lüscher et al . Partial shading with black shade netting reduced cluster temperature of cluster temperature by 3.7o C, while overhead shade films reduced cluster temperature by at least 4o C compared to uncovered control vines. During a heatwave post-veraison in 2021, berry temperatures reached a maximum temperature of 58o C in C0, which was the highest recorded berry temperature in both years. At this temperature extreme, shade films were effective in reducing berry temperature. Even when the berry temperatures did not reach this extreme temperature, overhead shade films performed with a similar cooling effect. The cooling effect on clusters results from the shielding of grapes from NIR, which minimized the heat load on the clusters in the afternoon hours. While D4 was the most effective at reducing cluster temperature when maximum temperatures were reached, D5 optimized flavonoid development by balancing heat reduction and solar radiation exclusion. This balance was achieved with the reduction of NIR transmission by approximately 27%. Grapevine physiological responses to reduced photosynthetically active radiation via shading in hot climates have been reported. Previous work with partial shading via colored shade nets reduced total solar radiation by 40%, without selecting specifically for PAR reduction and found no differences in net carbon assimilation, stomatal conductance, leaf water potential and most importantly, yield. When calculated as season-long integrals, overhead shade films had no effect on photosynthetic parameters. This may be attributed to the transmission spectra of the polyethylene shade films. Each shade film reduced PAR transmission by approximately 20% from full transmission. The photosynthetic capacity of grapevines is optimized between 800 and 1200 μmol•m-2 s-1 of solar radiation, despite 2000 μmol•m-2 s-1 of solar radiation provided under control conditions. Since leaf area was maintained across treatments and PAR was only reduced by 20%, the photosynthetic capacity of the grapevines was unaltered under the shade films. Negligible differences in canopy size and the replacement of 25% ETcresulted in no significant effect on Ψstem or gs integrals between treatments within a given year. However, C0 and D4 in both years were trending towards more negative Ψstem values, which may be due to larger transmittance of NIR radiation and increased evaporative demand. Similar effects on plant water status and gas exchanges were observed by shading via shade nets when canopy size was maintained across treatments. By maintaining aspects such as canopy size and plant water status required for adequate ripening across the treatments, the effects of shading on berry composition were most likely related to the fruit zone microclimate, specifically reduction of temperature.Plant organ development relies on a balance of carbon and water availability . At low doses, ultraviolet light reduces cell division and expansion. However, previous studies indicated that berry size is unaffected by changes in solar radiation, alterations in amounts of specific wave bands, or temperature. Rather, berry size is a function of cluster compactivity and the amount of irrigation . As our applied water amounts and cluster count were constant in both shaded and control treatments, berry size was unaffected. Consequently, yield was unaffected by overhead shade films as well. In our experiment, temperature and solar radiation were coupled. However, changes to temperature caused by the overhead shade films were not enough to result in changes in berry TSS accumulation in both years. Regardless of shading, grape berries reached the commercial winemaking standard of 25 o Brix. While this desired sugar concentration is often attained in hot vineyard climates, the decoupling of sugar and anthocyanin development driven by heat waves may cause issues with achieving commercial wine expectations, leading to higher alcoholic wines with immature flavonoid composition. Tartaric and malic acids are present in the grape berry at all developmental stages. As the berry ripens, malic acid accumulates until a metabolic shift at veraison. After verasion, the berry loses malic acid to cellular processes such as respiration and gluconeogenesis. Elevated temperature has been shown to reduce must acidity.. Ultimately the loss of malic acid from increased temperature is demonstrated to be due to increased degradation rather than reduced pre-veraison biosynthesis. . Must acidity values as low as 4.66 g•L-1 have been reported in a hot climate region as the San Joaquin Valley, California in Merlot grapes under pre-bloom mechanical leaf removal . In this study, must acidity and pH at harvest were not affected by overhead shade films. Rather, titratable acidity and pH at harvest in 2020 were maintained at previously reported levels from the experimental site, despite a warmer than average growing season, where approximately 500 more GDDs accumulated in 2020 than those previously reported by Martínez-Lüscher et al. .