Megan Bartlett* and Rami Albasha
*University of California, Davis, One Shields Ave, Davis, CA, 95616 (mkbartlett@ucdavis.edu)

Stomatal traits determine grapevine water use, carbon supply, and water stress, which directly impact yield and berry chemistry. Breeding for stomatal traits has strong potential to improve grapevine performance under future, drier conditions, but the trait values that breeders should target are unknown. We used a functional-structural plant model developed for grapevine (HydroShoot) to determine how stomatal traits impact canopy gas exchange, water potential, and temperature under historic and future conditions in a premium and a high-production region (Napa and the Central Valley). Historic climate data (1990 to 2010) was collected from weather stations and future climate (2079 to 2099) was projected using four representative climate models for California, assuming medium- and high-emissions (RCP 4.5 and 8.5). Five trait parameters, representing mean and extreme values for maximum stomatal conductance (gmax) and leaf water potential threshold for stomatal closure (Ysc), were defined from meta-analyses. Compared to mean values, the water-spending extremes (highest gmax or most negative Ysc) had negligible benefits for carbon gain and canopy cooling, but exacerbated vine water use and stress for both sites and climate scenarios. These traits increased cumulative transpiration by 8 to 17%, changed cumulative carbon gain by -4 to 3%, and reduced minimum water potential by 10 to 18%. Conversely, the water-saving extremes (lowest gmax or least negative Ysc) strongly reduced water use and stress, but potentially compromised the carbon supply for ripening. Under RCP 8.5 conditions, these traits reduced transpiration by 22 to 35% and carbon gain by 9 to 16%, and increased minimum water potential by 20 to 28%, compared to mean values. Overall, selecting for more water-saving stomatal traits could improve water-use efficiency and avoid the detrimental effects of highly negative canopy water potentials on yield and quality, but more work is needed to evaluate whether these benefits outweigh the consequences of minor declines in carbon gain for fruit production.

Funding Support: UC Davis Department of Viticulture and Enology