Dormancy remains a poorly understood process in temperate woody perennial plants. These plants require cumulative exposure to low temperatures (chilling accumulation) during winter to respond to warm temperatures in spring (forcing) and properly break bud. For successful establishment of temperate woody perennial fruit crops, it is important to understand chilling accumulation and dormancy requirements of species and cultivars. Our recent work indicates cold hardiness is an important co-variate in the analyses related to timing of budbreak, and thus chilling accumulation models and dormancy progression studies. Here we set out to understand aspects of chilling accumulation in different conditions by evaluating two measures of dormancy progression, (i) a classic forcing assay, where time to budbreak is evaluated; and (ii) a newer phenotyping of cold hardiness deacclimation rates using grapevine (Vitis spp.). For a comprehensive analysis, we used grapevine canes from V. vinifera cvs. Cabernet Sauvignon and Riesling, and V. hybrid cvs. Concord, Frontenac, Itasca, Marquette, and Petite Pearl. Canes were collected in several states across the continental United States (CO, IA, MN, NY, PA, SD, TX, WI), and in two locations for two states (NY, WI), representing approximately eight different USDA Cold Hardiness Zones (4a-7b), over the course of two winter seasons (2023-2025). Collections occurred in December, January, February, and March of each season. Upon collection or receipt of shipments, initial cold hardiness of buds was measured using differential thermal analysis (DTA). Following, canes were prepared into single node cuttings, and placed in cups of water and in a growth chamber for forcing (22ºC, 16h day/8h night). A subsample of 15 cuttings was used to evaluate time to budbreak, while the remaining cuttings were used for cold hardiness measurements in semi-regular intervals. We used simple linear regression with cold hardiness measurements to determine deacclimation rates (loss of cold hardiness over time; ºC/day). In general, buds from warmer locations (IA, TX, and Long Island, NY) had less initial cold hardiness (field cold hardiness) than colder locations. Dormancy progression was faster in colder locations than warmer locations, observed in both budbreak assays and evaluation of deacclimation rates. Based on our data, time to budbreak is a function of initial cold hardiness and deacclimation rate. Future work will examine the response of deacclimation rates to chilling accumulation models to determine chilling models that best describe dormancy responses across climates, which will then be incorporated into models that predict field cold hardiness and field budbreak.
