Identifying weak links in the hydraulic chain of grapevines: insights into the impacts of water stress on water uptake and transport physiology Dr. Andrew McElrone USDA-ARS, UC Davis, Department of Viticulture and Enology, USA

Water scarcity threatens the economic viability of viticulture in dry growing regions of the western US, and changing climatic conditions could exacerbate this situation. Growers commonly employ conservation techniques like deficit irrigation, where less water is applied than is needed to match vineyard evapotranspiration (ET) demands. This practice depletes soil water over time and induces water stress in vines. Water stress in grapevines results in decreased growth and fruit yield, but is often used to control vigor and improve fruit quality. Many physiological processes in grapevines are disrupted by water stress, but the severity and timing of disruption vary between roots, stems and leaves. Some literature suggests that grapevines are hyper-susceptible to drought-induced hydraulic dysfunction. Here I will summarize a compilation of our hydraulic physiology, X-ray microCT, neutron radiography, sap flow, and modelling efforts on various Vitis genotypes that demonstrate most hydraulic dysfunction in grapevines occurs outside the xylem (i.e. in fine root cortical and epidermal cells, where water is initially absorbed, and in leaf mesophyll cells, where water evaporates before exiting the stomata). A thorough understanding of the mechanisms impacted by water deficits is needed to effectively implement precision irrigation strategies tailored to a given genotype and to maximize vineyard water use efficiency. Such information would enable growers to better approach stress thresholds of vines thereby limiting detrimental effects such as reduced fruit ripening or future bud fruitfulness, and could also accelerate germplasm screening to identify improved plant material that better tolerates water stress.