Megan Bartlett,* Gabriela Sinclair, Gabriela Fontanesi, Thorsten Knipfer, Andrew Walker, and Andrew McElrone
*University of California, Davis, One Shields Ave, Davis, CA 95616 (mkbartlett@ucdavis.edu)

Breeding more drought-tolerant rootstocks is a key strategy to reduce irrigation demand and mitigate the impacts of climate change on grape yield and ripening. Identifying traits that confer drought tolerance would accelerate rootstock improvement by providing clear targets for breeding. The living cells in the root are a significant bottleneck for water uptake from dry soil, but the traits that capture the responses of these cells to water stress have never been evaluated as potential drivers of rootstock drought tolerance. We used a greenhouse drought experiment to test the relation- ships between these traits and vine gas exchange, water transport, and vigor across eight commercial rootstocks that field trials have classified as drought-tolerant (110R, 140-Ru, 1103P, and Ramsey) or sensitive (101-14, 420A, 5C, and Riparia Gloire) grafted to Chardonnay. Root capacitance (CAP) and turgor loss point (TLP) were significantly different across rootstocks and shifted to significantly lower values in water-stressed plants, which we would expect to improve root drought tolerance. The rootstocks with a lower CAP, which indicates the roots retained more water as root water potentials decline, maintained significantly greater stomatal conductance and photosynthesis under water stress (r2 = 0.31 and 0.45, both p < 0.001). The rootstocks classified as drought-tolerant had significantly lower root CAP and TLP values under well-watered conditions, but exhibited significantly smaller shifts in TLP under water stress, indicating these rootstocks would undergo cell collapse at less negative root water potentials, contrary to our expectations. These findings suggest breeding rootstocks for a lower CAP would improve vine water uptake and gas exchange under dry soil conditions, but more work is needed to understand the function of these traits in the deep, highly heterogenous rooting environments in the field, where these rootstocks vary widely in rooting depth and access to deeper, wetter soil layers.

Funding Support: American Vineyard Foundation