The stomatal traits that conserve water without compromising grapevine carbon gain depend on climate change severity and wine-growing region

Winegrapes are a valuable ($70 billion) commodity, but climate change is predicted to reduce grape yield and quality by exacerbating water and heat stress. Developing stress-resistant varieties would mitigate these impacts, but the trait values to target can be obscured by complex relationships between traits and plant performance. Stomatal traits mediate trade-offs between increasing gas exchange, to increase carbon assimilation and evaporative cooling, and reducing gas exchange, to avoid water stress. We used a functional-structural plant model to quantify the impacts of maximum stomatal conductance (gmax) and leaf water potential thresholds for 50 % stomatal closure (gs psi 50) on vine carbon gain, water stress, and temperature under historical and future conditions, assuming moderate and extreme climate change (Representative Concentration Pathways 4.5 and 8.5), for premium- and hot-climate US wine regions (Napa and the San Joaquin Valley (SJV)). Shifting from the mean trait values reported for winegrapes to water-saving values (i.e., a lower gmaxand less negative gs psi 50) reduced simulated vine transpiration and water stress below even historical levels, but the trait values that conserved water without compromising carbon gain varied between climate scenarios and regions. Extreme water-saving traits maintained carbon gain at or above historical levels in Napa under both scenarios, while intermediate water-saving traits maintained carbon gain under moderate climate change in the SJV. Vine canopy temperatures exceeded thresholds for photochemical heat damage in the SJV, regardless of the trait values. Overall, by reducing transpiration and water stress, water-saving traits would reduce irrigation demand and warming impacts on yield and quality, though more work is needed to determine whether historical carbon gain will remain adequate to support ripening, especially with heat-reducing management practices. Developing varieties with a range of water-saving trait values would provide plant material tailored to different regions and reduce the risk from uncertainty around future climate.