UC Davis

From an ecological perspective, grape production is a result of the many tiered interactions between vine, soil, pest, cultivation, and climate. Globally, vineyards are facing year-round changes in climate that impact growth and maturation, driven by an increase in atmospheric carbon dioxide from anthropogenic emissions. Carbon dioxide is not only contributing to the overall warming of the planet, for grapevine, it also fuels unwanted growth, drives an increase in pest pressure through carbon-nitrogen imbalances, and sometimes unfavorably alters the response of stomata. The potential for an increase in water use efficiency is likely transient, and concurrently limited by water availability. A major finding from the literature synthesis was the impact of advancing grapevine phenology, which is altered by both temperature and carbon dioxide. A subsequent field based phenological study using an experimental vineyard site at the University of California, Davis was used to create a model of grapevine phenology for the three major lifecycle stages: budburst, flowering, and veraison for 137 varieties over four years. The timing of the primary stages of grapevine phenology were modelled in terms of total growing degree days and used coefficient of variation as a proxy for sensitivity to climate, grouping the cultivars by genetically determined geographic origin. The estimates of each stage for every variety contributes to our understanding of alternative varieties, which can inform future selection for breeding and planting. The general intercept for these models was offset by geographic origin of the varieties, utility of the varieties as a table or wine grape, and for the stage of veraison, was also impacted by the number of days that reached temperatures of 40°C or more. Understanding that elevated carbon dioxide will likely increase drought events in major winegrape growing regions, we investigated one potential mitigation strategy via genetic transformation of grapevine for drought resistance. The CRISPR/Cas9 system was applied to functionally characterize and modify the stomatal density gene, VvEPFL9-1, in Vitis vinifera c.v. Sugraone. After successful transformation, the analysis of stomata density revealed that in edited plants the number of stomata was significantly reduced compared to the wild type control, demonstrating for the first time the role of EPFL9 in a perennial fruit crop. Two edited lines were then assessed for growth, photosynthesis, stomatal conductance, and water use efficiency in the greenhouse at both controlled ambient conditions and in a natural dry-down experiment. Intrinsic water-use efficiency was significantly impacted under both well-watered and drought conditions, confirming reduced stomatal density as a preferable trait under future drier environmental conditions. These results show the potential of manipulating stomatal density for optimizing grapevine responses under changing climate conditions. A common thread from each of these studies is that alternative varieties of grapevine are needed in the major winegrape growing areas as climate change increases the risk of high heat events and drought. Potential mitigation strategies are planting alternative varieties with more resilience to climate change and/or cultivating new material from the expansive selection of available material. We can strengthen the vineyard system by introducing more diverse cultivars, with an ideal candidate fitting the profile of heat and drought tolerant, late ripening, with strong pest resistance. We identified groups of varieties with common geographic origins that would make suitable candidates for further research based on their low sensitivity to climate, and we demonstrated that well known varieties can be modified to have increased drought resistance.