Dormancy and Cold Hardiness Transitions in Winegrape Cultivars Chardonnay and Cabernet Sauvignon

Cold injury is a major cause of economic loss in winegrapes (Vitis vinifera L.) grown at high latitudes. The objective of this study was to investigate the relationship between dormancy and cold hardiness transitions in two cultivars with differing freeze tolerance (Chardonnay and Cabernet Sauvignon). Cold hardiness was measured by differential thermal analysis, and a bud forcing bioassay was used to measure the stage and depth of dormancy. Canes were sampled from field-grown grapevines in Parma, ID at periodic intervals during two consecutive winters. Both cultivars transitioned into endodormancy each year in September when day length was similar to 12.5 hr. Cold acclimation in both cultivars occurred each year in October during endodormancy and steadily increased during ecodormancy to a max hardiness in December. Effective temperatures for release from endodormancy were lower for Chardonnay (-3 degrees C) than for Cabernet Sauvignon (3 degrees C), and, each year, Chardonnay transitioned to ecodormancy earlier than Cabernet Sauvignon. From October to December, Chardonnay buds were more cold hardy than Cabernet Sauvignon buds. The number of days to budbreak under forcing conditions increased steadily during endodormancy and decreased during ecodormancy. Resistance to deacclimation during ecodormancy was inversely related to the level of bud cold hardiness and the duration of time in ecodormancy, suggesting that the mechanisms that impart hardiness may interact with those involved in resumption of growth. Results from this study show that the influence of autumn weather events on dormancy and cold hardiness transitions can affect vulnerability to subsequent cold injury and have important implications under global climate change. Differences between cultivars in dormancy and cold hardiness transitions can be used to improve cultivar and site selection.