Woody perennials in temperate climates develop cold hardiness in the fall (acclimation) and lose cold hardiness in the spring (deacclimation) to survive freezing winter temperatures. Two main factors known to regulate deacclimation responses are dormancy status and temperature. However, the progression of deacclimation responses throughout the dormant period and across a range of temperatures is not well described. More detailed descriptions of dormancy status and temperature, as factors regulating deacclimation, are necessary to understand the timing and magnitude of freeze injury risks for woody perennials in temperate climates. In this study, we modeled deacclimation responses in cold‐climate interspecific hybrid grapevine cultivars throughout the dormant period by integrating chill accumulation and temperature through the concept of deacclimation potential. We evaluated deacclimation and budbreak under multiple temperature treatments and chill unit accumulation levels using differential thermal analysis (DTA) and bud forcing assays. Deacclimation responses increased continuously following logistic trends for both increasing chill unit accumulation and increasing temperature. There are optimal temperatures where deacclimation rates increased but changes in deacclimation rates diminished below and above these temperatures. The cumulative chill unit range where deacclimation potential increased overlapped with the transition from endo‐ to ecodormancy. Therefore, deacclimation potential could provide a quantitative method for describing dormancy transitions that do not rely on the visual evaluation of budbreak. This information provides a more detailed understanding of when and how deacclimation contributes to increased risks by freezing injury. In addition, our descriptions could inform improvements to models predicting cold hardiness, dormancy transitions, and spring phenology.

: Cold climate interspecific hybrid grapevines (CCIHG) selected for their superior mid-winter cold hardiness have expanded grape production to cold climate regions. However, extreme weather events, such as polar vortexes, and high frequency of fall and spring freezes often result in yield and vine losses. The main objective of this study was to evaluate changes in bud cold hardiness of five CCIHG cultivars grown in the upper Midwest in order to identify relative risk for freeze damage throughout the dormant period, and to adapt a bud cold hardiness prediction model to CCIHG cultivars grown in cold climate regions. Bud cold hardiness was evaluated biweekly throughout the dormant period by measuring lethal temperatures for buds using differential thermal analysis (DTA). CCIHG cultivars in our study had an early acclimation response with increased levels of cold hardiness before 26 the occurrence of freezing temperatures. Maximum levels of hardiness (-28 to -30°C) were observed both years in February, however deeper levels of freezing stress resistance, probably attained by freeze

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Dormancy of buds is an important phase in the life cycle of perennial plants growing in environments where unsuitable growth conditions occur seasonally. In regions where low temperature defines these unsuitable conditions, the attainment of cold hardiness is also required to survive. The end of the dormant period culminates in budbreak and flower emergence, or spring phenology, one of the most appreciated and studied phenological events. Despite this, we have a limited physiological and molecular understanding of dormancy, which has negatively affected our ability to model budbreak. Here we highlight the importance of including cold hardiness in studies that typically only characterize time to budbreak. We show how different temperature treatments may lead to increases in cold hardiness, and by doing so also (inadvertently) increase time to budbreak. Therefore, erroneous interpretations of data may occur by not phenotyping cold hardiness. Changes in cold hardiness were very likely present in previous experiments to study dormancy, especially when those included below freezing temperature treatments. Separating the effects between chilling accumulation and cold acclimation in future studies will be essential for increasing our understanding of dormancy and spring phenology in plants.

Cold injury in plants limits distribution of perennial agricultural crops, though replacement of plants and other management practices allow for some damage to be tolerated. Grapes (Vitis spp.) represent a crop that is grown in many marginal areas due to its profitability. However, monitoring of cold hardiness or damage for deployment of cold damage mitigation and management practices is a laborious process. To that effect, a model was published in 2014 to predict cold hardiness using data from Washington state in the US. Although that model works well regionally, it has been found to not predict adequately in other regions. In part, it is known that its description of some dormancy aspects as they relate to cold hardiness are not accurate. Therefore, the objective of this work was to develop a new model that incorporate previous research on cold hardiness dynamics, such as to increase realism. Cold hardiness data from V. labruscana 'Concord', and V. vinifera 'Cabernet Sauvignon' and 'Riesling' from Geneva, NY, USA were used. Data were separated in calibration (~2/3) and validation (~1/3) datasets. The proposed model uses 3 functions to describe acclimation, and 2 functions to describe deacclimation, with a total of 9 optimized parameters. A shared response to chill between acclimation and deacclimation provides a phased integration where acclimation responses decrease over the course of winter and are overcome by deacclimation. The new model outperforms the currently available model, reducing RMSE by up to 37% depending on cultivar. The new model also tends to slightly underpredict cold hardiness, as opposed to the overprediction from the current model. The underprediction can be preferred as damage mitigation methods may be deployed prior to damage occurring with greater safety. Some optimized parameters were shared between cultivars, suggesting conserved physiological aspects of the process were captured