The sustainability of grape production worldwide faces significant challenges due to the increasing occurrences of heatwaves and droughts. Among the concerns arising from these issues are the effects of heat and drought stress on berries from two wine grape varieties, Cabernet Sauvignon and Riesling.
Grapevine serves as a crucial and versatile fruit crop worldwide, particularly in temperate regions with warm and dry summers. Unfortunately, rising temperatures and decreasing water availability due to climate change pose significant challenges for high-quality wine grape production, especially in already warm and dry regions.
High temperatures can negatively impact grape quality by reducing key chemical constituents — such as organic acids and anthocyanins — while increasing sugar levels, leading to higher alcohol content in wines. Organic acids play a crucial role in determining the organoleptic properties of wines, affecting the perception of acidity and freshness, and contributing to microbial stability during wine storage.
Water deficits caused by drought conditions can also affect grape production. Moderate water deficit can be beneficial for grape production as it promotes the accumulation of desirable metabolites, including anthocyanins and other phenolics. However, severe and prolonged heat and drought episodes can hinder growth and negatively affect yield and fruit metabolism.
Could diversification be the solution?
To address the challenges posed by climate change, experts have emphasised the need for varietal diversification and the integration of traits that improve grapevine resilience to heat and drought stress. Identifying physiological, metabolic and genetic factors contributing to abiotic stress tolerance and grape berry composition is therefore vital.
Having multiple varieties of grapes adapted to the local climate is beneficial as some grape varieties are naturally better suited to handle heat and drought than others. Already, scientists and grape growers have been working together to understand these traits and find ways to incorporate them into grapevines. These efforts include looking for grape varieties that require less water to survive or have characteristics that protect them from damage by high temperatures.
Grapevines with these resilient traits integrated can be better equipped to thrive in warm and dry regions and as such, produce more and better-quality grapes even amid tough weather conditions. This would also give grape growers more options and increase the chances of success even in challenging climates.
Understanding heat and water stress
In order to achieve the above outcomes, however, understanding the effects of heat and water stress on different wine grape varieties is crucial.
A recent study by researchers at Washington State University, Universidad Politécnica de Madrid and Texas A&M University, shed light on the significant effects of temperature and water availability, as well as their combined influence, on grape berry metabolism and composition.
The seven-day study exposed the Cabernet Sauvignon and Riesling wine grape varieties to episodes of heat stress and a combination of heat and water stress and found that heat stress led to higher berry pH, lower total acidity and overall reduction in organic acid content. Surprisingly, sugar content remained unaffected in both grape varieties.
Both alone and in combination with water stress, heat stress had a more significant impact on berry organic acid content, pH, and titratable acidity than water stress did, indicating that grape berries have a distinct, organ-specific response to environmental stresses. At the metabolic level, heat stress had a more profound impact than water stress on Cabernet Sauvignon but not on Riesling, which had a more pronounced response to water stress at the transcriptome level.
Although both grape varieties showcased similar responses at the berry metabolite level, the study's findings suggested that they may rely on somewhat different genetic mechanisms to cope with water and heat stress. These genetic variations ultimately lead to a similar physiological outcome in terms of stress tolerance.
Such findings have the potential to inform the development of climate change mitigation strategies and genetic improvement to adapt wine grape production to changing climatic conditions. By understanding how grapevines and grape ripening respond to the interaction between heat and drought stress, scientists hope to provide enhanced recommendations for irrigation management and grapevine genetic improvement. These measures will be crucial in safeguarding the future of wine grape production amid ongoing climate challenges.
Source: Scientific Reports
“Impact of heat stress, water stress, and their combined effects on the metabolism and transcriptome of grape berries”
https://doi.org/10.1038/s41598-023-36160-x
Authors: Seanna Hewitt, et al.