Time to budbreak is not enough: cold hardiness evaluation is necessary in dormancy and spring phenology studies

Summary 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.