WSU study shows different winegrape varieties react similarly to drought

Winegrape growers have long sought varieties that can better withstand dry climates and deficit irrigation. But new research from scientists at Washington State University suggests that a grapevine's variety may matter less than previously thought when it comes to drought tolerance.

"Our results go against conventional scientific dogma," said corresponding author Markus Keller, Chateau Ste. Michelle Distinguished Professor in WSU's Department of Viticulture and Enology. "The takeaway is that, when making irrigation decisions, growers should focus more on canopy size, soil type, and wine style rather than worrying about the inherent physiological differences between varieties."

The latest paper, published in the journal BMC Plant Biology, builds on two previous WSU studies examining grapevine leaf water potential and soil moisture.

Keller was inspired to pursue the research after reading a decades-old study from southern France that concluded grape varieties respond either "optimistically" or "pessimistically" to water stress.

Optimistic varieties continue to photosynthesize despite the risk of drying out, while pessimistic varieties conserve water by closing their stomata, the pores that allow water vapor to escape and carbon dioxide to enter leaves.

"There is a lot of published research about the perceived need to breed varieties that are better adapted to drought conditions," Keller said. "However, much of that research also has contradictions - sometimes within the same variety - depending on extent of water stress, time of season, what rootstocks the vines were grafted to, and who did the research."

Our results go against conventional scientific dogma. The takeaway is that, when making irrigation decisions, growers should focus more on canopy size, soil type, and wine style rather than worrying about the inherent physiological differences between varieties.

After learning that the varieties in the original French study were grown in different blocks and managed slightly differently, Keller knew he had to re-test the findings.

An opportunity to do so came in 2010, when the Washington wine industry provided funding to help create a WSU research vineyard with 30 different winegrape cultivars.

Charles Obiero, a former WSU postdoctoral researcher and co-author on all three papers, used the vineyard to test how different varieties respond to drought throughout the growing season.

For two years, Obiero measured the vines' water status at night and during the midday heat, along with their physiological responses to drought.

"The goal was to understand whether or not growers should irrigate their vines differently depending on the hydraulic strategies of the plant," Obiero said.

Instead of confirming the expected physiological differences among varieties, Obiero found that whether a variety is perceived as optimistic or pessimistic depends on how soil water is measured and how much water is available in the soil. The study showed a clear connection between soil and drought tolerance.

"Growers need to know what type of soils they're dealing with," Obiero said. "Otherwise, they could end up giving the vine water it doesn't need. If they know the grapevines' stress threshold and amount of water the soil can hold, that can help guide irrigation management for different varietal blocks."

Most previous studies had measured the plants' response without determining how much water is in the soil and available to the vine, Keller added.

"This study is unique because it recorded both," he said. "Charles measured how the amount of water in the soil changed and how the vines responded to that change. He found that all the varieties had the same critical threshold at which they begin to feel stress."

The study also highlights the importance of canopy size. Larger canopies lose more water through evaporation, causing the soil to dry out faster.

Obiero found that under identical irrigation regimes, the varieties differed markedly in canopy size, but not in how they responded physiologically to drying soil. That knowledge could help growers simplify their irrigation management practices and provide the appropriate amount of water.

"The threshold for drought stress in the vines is guided by the canopy," Obiero said. "Vines with larger canopies require more water and arrive at that stress threshold sooner than vines with smaller canopies."

Determining precisely how much water it takes to sustain grapevines while growing quality fruit will help support the Washington wine industry and Sustainable WA, a winegrape certification centered on environmental stewardship, social responsibility, and economic viability.

"This work exemplifies how WSU brings rigorous science and practical solutions together to address real-world challenges," said Raj Khosla, Cashup Davis Family Endowed Dean of the College of Agricultural, Human, and Natural Resource Sciences. "By improving our understanding of how grapevines respond to water availability, our researchers are providing growers with actionable knowledge that can support efficient water use, economic resilience, and the long-term sustainability of Washington's wine industry in a changing climate."