This study addresses fundamental questions in bud dormancy, specifically the impact of temperature on chill accumulation and cold hardiness on dormancy release. We evaluated bud cold hardiness (CH) and time to budbreak responses of grapevines throughout chill accumulation under three treatments: constant (5°C), fluctuating (-3.5 to 6.5 °C daily), and field conditions (Madison, WI, USA), during three dormant seasons. Findings reveal that lower temperatures in chill treatments result in greater cold hardiness gains (CHfield>CHfluctuating>CHconstant). All treatments reduce time to budbreak with increased chill accumulation. However, when time to budbreak was adjusted to remove cold acclimation effects, treatment effectiveness changed. Notably, existing chill models (North Carolina, Utah, and Dynamic) fail to accurately describe adjusted budbreak responses. Our results demonstrate how chill treatment effectiveness changes when adjustments for uneven acclimation are considered. We propose a new model that includes freezing temperatures and enhances chill accumulation under fluctuating conditions.
