WSU study finds a shared drought-stress threshold across 30 winegrape cultivars
A two-year experiment in Prosser, Washington, found that 30 winegrape cultivars began experiencing drought stress at a similar critical point once soil-water availability and canopy size were taken into account. The researchers argue that irrigation decisions should focus more on measurable soil and vine conditions than on presumed differences in varietal drought tolerance.
A common threshold challenges a familiar model
Research conducted at Washington State University’s irrigated agriculture research center in Prosser examined 30 winegrape cultivars and found that they reached drought stress at a similar threshold regardless of variety. The result suggests growers may improve irrigation planning by considering soil conditions and canopy size instead of assuming that individual cultivars have fundamentally different drought tolerances.1
The paper, published in BMC Plant Biology, challenges the practice of describing varieties as either “optimistic” or “pessimistic” under water stress. Under that model, optimistic vines are expected to continue photosynthesis and sugar production as soil dries, while pessimistic vines are thought to conserve water earlier by closing their stomata, the leaf pores governing water loss and carbon-dioxide uptake.1
Corresponding author Markus Keller, a professor in WSU’s Department of Viticulture and Enology, said this framework has influenced decisions about planting in dry regions and the use of deficit irrigation. That technique is widely employed in winegrape production to restrict water consumption while managing vine growth and fruit quality.1
Why earlier findings differed
Keller noted that previous research produced conflicting classifications, sometimes even for the same cultivar. Results could vary with the season, degree of water stress, rootstock and experimental methods. He was partly motivated by a landmark French study conducted decades earlier, but said its varieties had been planted in separate blocks and managed somewhat differently.1
The Washington wine industry helped fund a research vineyard established in 2010, allowing WSU to study all 30 cultivars in one location. The latest paper follows two earlier WSU studies in BMC Plant Biology: a 2023 investigation into whether leaf water potential could serve as a proxy for soil moisture, and a 2025 analysis of the relationship between leaf water potential and soil moisture across the same cultivars.1
Two years of controlled drying and recovery
Charles Obiero, a former WSU postdoctoral researcher and a co-author of the study, monitored the vines as water availability declined during the growing season. Researchers fully irrigated the plants, allowed the soil to dry gradually, irrigated them again and then imposed a second drying period.1
Across two years, Obiero measured vine water status at night, when the plants were resting, and during the midday heat, when demand was greater. He also recorded physiological responses, including stomatal behavior, and measured the water remaining in the soil. The design paired direct observations of the soil-water supply with measurements of the vines’ response, unlike experiments that assessed plant behavior without determining how much soil water was available.1
Canopy size can create apparent varietal differences
The researchers concluded that apparent optimistic or pessimistic behavior depends partly on the method used to measure soil water and on how much water the soil makes available. After accounting for those factors, all 30 cultivars displayed the same critical threshold for the onset of drought stress.1
Although the stress threshold was shared, canopy size varied substantially among cultivars receiving equal irrigation. Larger canopies lose more water through evaporation and transpiration, causing soil to dry more quickly. One cultivar can therefore reach the stress point sooner because it carries more leaf area, rather than because its underlying drought physiology is fundamentally different.1
Implications for vineyard irrigation
Obiero said irrigation decisions should consequently give central importance to soil type and canopy management. A block with greater water-holding capacity can delay stress longer than one with coarse soil, while a large-canopy vine requires more water than a smaller vine under identical weather. Knowing both the soil’s water-holding capacity and the vines’ stress threshold could enable growers to schedule irrigation more accurately.1
For wineries and the wider beverage sector, more precise irrigation could reduce water use and costs where supplies are constrained. The findings also call for reconsideration of decisions that match cultivars with drought-prone sites chiefly on the basis of varietal identity rather than measurements of soil and canopy conditions.1
The possible benefits remain conditional on the results being reproduced at other sites and under other conditions. If they are, growers facing increasing heat and water scarcity may be able to base deficit-irrigation choices more heavily on directly measurable field conditions while still balancing water savings against vine health and fruit quality.1
Potential role in Washington sustainability standards
WSU said the research could support Sustainable WA, Washington’s winegrape certification program covering environmental stewardship, social responsibility and economic viability. A clearer understanding of vine responses to declining soil water could help growers use irrigation more efficiently while sustaining production under a changing climate.1
Sources
Written by the Dionysus editors from the sources above.
