Agronomy 10(2): 241; (2020)
A Conceptual Framework for Winter Dormancy in Deciduous Trees
Erica Fadón, Eduardo Fernandez, Helen Behn and Eike Luedeling

Abstract: The perennial life strategy of temperate trees relies on establishing a dormant stage during winter to survive unfavorable conditions. To overcome this dormant stage, trees require cool (i.e., chilling) temperatures as an environmental cue. Numerous approaches have tried to decipher the physiology of dormancy, but these efforts have usually remained relatively narrowly focused on particular regulatory or metabolic processes, recently integrated and linked by transcriptomic studies. This work aimed to synthesize existing knowledge on dormancy into a general conceptual framework to enhance dormancy comprehension. The proposed conceptual framework covers four physiological processes involved in dormancy progression: (i) transport at both whole-plant and cellular level, (ii) phytohormone dynamics, (iii) genetic and epigenetic regulation, and (iv) dynamics of nonstructural carbohydrates. We merged the regulatory levels into a seasonal framework integrating the environmental signals (i.e., temperature and photoperiod) that trigger each dormancy phase.

In the second half of the 20th century, many researchers focused on phenology modeling, relating temperature to phenological events, thus identifying the most effective temperature range to overcome dormancy [10]. A number of temperature-based mathematical models emerged to estimate the chilling requirement (CR) of several species and cultivars aiming to forecast bloom dates. Together with the Chilling Hours model [8], the chill models that are currently most widely used are the Utah model [11] published in 1974 and the Dynamic model [12,13,14], which appeared in 1987. Compared to the other common models, the Dynamic model is based on the most sophisticated biological concept, assuming that chill accumulation is mediated by a thermally labile precursor [1]. However, like all earlier models, even the Dynamic model uses only temperature data as input.

Also in the year 1987, Lang et al. [15] proposed a new terminology for dormancy research, distinguishing between para-, endo-, and eco-dormancy phases. According to their definition, para-dormancy refers to growth suppression that is imposed on particular organs by other tree structures (e.g., apical dominance) due to the production and/or action of inhibitory molecules. During endo-dormancy, growth is not possible even under suitable temperature conditions, as buds have not received sufficient exposure to chilling. Eco-dormancy refers to the subsequent period, in which buds on trees only need sufficient exposure to warm temperatures to resume growth [15]. Although some shortcomings have been pointed out in the nomenclature by Lang et al. [16], it is the most widely used terminology for the dormancy of temperate fruit trees.