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 has 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 and growth in leaves and roots. A more negative TLP indicates the leaves and roots can undergo more severe water stress before losing turgor. 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 crops, but these traits have never been assessed for their impact on grapevine performance under water stress.
Our first year, we conducted a greenhouse experiment to evaluate whether rootstocks with a more negative root TLPdry and a larger DTLP allowed vines to maintain greater gas exchange, growth, and WUE under water stress. This year, we have reanalyzed these data and found significant correlations between a more negative TLPdry and larger root system, and between greater vine gas exchange under drought and lower values of a second trait, the root capacitance (CAPdry). CAP measures the volume of water lost from the root as the root dehydrates to TLP. A lower CAP indicates the root loses less volume, with potentially important effects on belowground water transport and root-to-shoot chemical signaling. However, TLPdry was significantly less negative in the rootstocks that field trials have classified as drought tolerant, suggesting that a less negative TLP in dry roots could benefit drought tolerance by redirecting resources to increase root growth in deeper, wetter soil. Together, these findings suggest that root TLP and CAP are promising traits to improve rootstock drought tolerance, though more work is needed to clarify their function.
Our second year, we used a vineyard experiment with 7 wine grape cultivars to test the impacts of leaf TLP and DTLP, and the biochemistry underlying variation in these traits, on scion drought tolerance, including gas exchange, vigor, and berry yield and quality, in realistic vineyard conditions. All 7 cultivars significantly adjusted TLP to improve leaf drought tolerance over the growing season, but cultivar rankings in TLP were highly consistent. However, contrary to expectation, leaf TLP was not related to gas exchange or plant water stress in this study, suggesting leaf biochemistry was responding to other stresses in these vines. Further, this study is the first to report a significant accumulation in mannitol as part of DTLP in wine grapes, which has been shown to ameliorate heat and pathogen stress in other plant species.