Summary: The goal of this multi-year project is to identify leaf and root traits that improve drought tolerance and water-use efficiency (WUE) in grape cultivars and rootstocks. Growers will need more drought tolerant and water-efficient vines to avoid declines in berry yield and quality under the hotter, drier conditions expected for California’s wine regions. This project is focused on leaf and root values for the drought tolerance traits the turgor loss point, measured in waterstressed conditions (TLPdry), and the adjustment in turgor loss point between well-watered and water-stressed conditions (DTLP, or TLPwet – TLPdry). TLP measures the water potential threshold that causes cell collapse, which impedes water transport through the leaves and roots. A more negative TLP indicates the leaves and roots can undergo more severe water stress before experiencing cell collapse. Plants can also change their cellular biochemistry to make TLP more negative under water stress, with a greater adjustment indicated as a larger DTLP. Previous studies found an important role for these traits in drought tolerance in other crop species, but these traits have never been assessed for their impact on grapevine performance under water stress. In the first year of this project, we evaluated whether rootstocks with a more negative root TLPdry and a larger DTLP allowed vines to maintain greater gas exchange (i.e., stomatal conductance, photosynthesis, and transpiration), canopy growth, and WUE under water stress. We also tested the ability of a rapid method previously developed for leaves to measure root TLP, to make it feasible to screen for this trait across the many genotypes developed in rootstock breeding efforts.

We subjected 8 commercially important rootstock varieties (420A, 5C, 101-14, Ramsey, Riparia Gloire, 1103, 110R, 140Ru), grafted onto the same scion (Chardonnay), to a greenhouse drought experiment, to evaluate the impacts of these traits on gas exchange, growth, and WUE. We found these rootstocks to vary significantly in TLP and DTLP. All of the rootstocks adjusted TLP to become significantly more negative under water stress, and rootstock differences in DTLP reversed their rankings in TLP between wet and dry conditions. The rapid method successfully captured the TLP variation measured by the more time-consuming standard method. Root TLPdry was significantly correlated with vine water use under water stress, in terms of stomatal conductance and whole-plant transpiration, but in the opposite direction than we expected. Instead, a less TLPdry was associated with greater water use, and there was no relationship with DTLP. Further, the rootstocks that growers identified as most drought tolerant in vineyard conditions also exhibited a less negative TLPdry. In contrast, photosynthesis was reduced by water stress, but was not related to any of the drought tolerance traits. Overall, these findings suggest that TLP plays an important role in rootstock drought tolerance, but differently than expected. Our future work will test whether cell collapse potentially benefits root growth by redistributing water and carbohydrates to support growth into wetter soil, and evaluate the impact of scion differences in leaf TLP and DTLP on vine drought tolerance.