Suraj Kar, Ricky W. Clark, Joseph B. DeShields, Ian Ivey, and Alexander D. Levin*
*Oregon State University, Southern Oregon Research & Extension Center, 569 Hanley Road, Central Point, OR, 97502 (alexander.levin@oregonstate.edu)

Drought is an existential issue for growers across most American grape acreage and in grapegrowing regions throughout the world. Adaptation to drought therefore has always been a crucial consideration for growers. One strategy to manage grapevines under water-limiting conditions is to use drought-tolerant rootstocks. However, the grapevine rootstocks that are in use today were bred primarily for Phylloxera tolerance, so their drought responses have not been sufficiently examined. Plants, under drought stress, tend to discriminate against the lighter carbon isotope (C¹²) for the heavier one (C¹³) during photosynthetic carbon assimilation. This physiological trait – carbon isotope discrimination (δ¹³C) – has been used as a proxy for water status in grapevines. Therefore, this three-year-long research project was initiated to evaluate drought tolerance of 10 commonly-used rootstocks using δ³C, vine phenology, physiology (stomatal conductance [gsw] and stem water potential [Ψ_stem]), yield, and fruit quality (juice total soluble solids, pH, and titratable acidity [TA]) in a young, replicated field trial at the Southern Oregon Research and Extension Center in the Rogue Valley AVA. In the pilot year of the study (2023), all rootstocks were irrigated uniformly and baseline data were recorded. A significant rootstock effect was observed on phenology (budbreak, berry set, and veraison), gsw measured at veraison, vine yield, cluster weight, berries per cluster, and TA in the juice. In subsequent years of this trial, irrigation will be withheld from half of each experimental plot (scion-rootstock-block) postbloom. Under drought stress conditions, differences between rootstocks may be magnified and can be capitalized to highlight specific strengths and weaknesses of each genotype. Therefore, the main aim of this project, identification of drought-tolerant rootstocks and establishing selection and management criteria, will be satisfied at the end of this three-year-long field trial.

Funding Support: Oregon Wine Board (OWB), California Grape Rootstock Improvement Commission (CGRIC)