Understanding the impact of climate change on anthocyanin concentrations in Napa Valley Cabernet Sauvignon

Abstract

Context and purpose of the study

Climate change is having a significant impact on the wine industry through more regular drought conditions, fires, and heat events, leading to crop loss. Furthermore, these events can reduce overall quality of the fruit, even when crop yields are not impacted. Anthocyanins are considered one of the most important classes of compounds for red wine production and are known to be sensitive to vine water status and heat events.  Therefore, they are likely to be impacted by environmental conditions across broad spatial scales. In order to evaluate this, a large-scale project was undertaken in 2015 to look at the differences in berry secondary metabolite chemistry across sub-appellations within the Napa Valley, where there is an exceptional amount of climatic diversity. This study was expanded in the 2021 and 2022 growing seasons, to evaluate impacts of agronomic practices and environmental conditions on crop yields and berry chemistry.  The goal of the project is to evaluate the impact of intra- and inter-annual variation in climate on secondary metabolites in Cabernet Sauvignon across a climatically diverse and economically important wine growing region in California; with the aim of increasing the industry’s awareness of potential climate change impacts on important grape secondary metabolites.

Material and methods

In 2015, 66 Cabernet Sauvignon commercial blocks were monitored across 10 sub-appellations within Napa Valley. In 2021 and 2022, 50 commercial blocks were monitored across 13 sub-appellations. Berries were collected from veraison until commercial harvest, with each site having 120 berries being collected every 14 days, totaling 3 to 5 sampling dates per site depending on harvest timing. Berry skins were separated from pulp and seed to extract the phenolics in skins. Extractions were conducted using 2:1 acetone:water (v/v) for 24 hours. Acetone was removed under reduced pressure and samples were then analyzed for high and low molecular weight polyphenols by RP-HPLC. Weather stations collecting hourly temperature, precipitation, humidity and radiation data were present in all blocks, and irrigation records, phenology data, and agronomic practices were collated for all sites. 

Results

Across all years, trends indicate that warmer sub-appellations had lower overall anthocyanin concentrations when compared to cooler sites, yet there was a substantial amount of variation driven by other factors.  Although work has been done within the context of greenhouses looking into the impact of heat on anthocyanin degradation, this work provides a large-scale analysis of the impacts of climate on anthocyanin production across the season in Cabernet Sauvignon.