- To compensate for the lack of chilling requirements of fruit trees grown in relatively hot climate, chemical treatments to break bud dormancy have to be applied. The success of such treatments depends on the identification of the proper stage of dormancy development. To date, no reliable means exists to indicate bud developmental stages in the orchard. We investigated the relationship between winter dormancy development and cell cycle regulation in the low chill requiring pear cultivar Spadona by a combination of morphological, histological and biochemical methods. The study was conducted in two orchards located in different climatic regions at 40 and 700 m above sea level. In both orchards, bud development was arrested during winter endodormancy. Fresh weight of mixed buds, pistil length and pedicel width increased toward the end of endodormancy. A sequence of flow cytometry analyses during endodormancy and its release in both orchards showed the existence of G2 nuclei correlated with phenotypic changes in the developing mixed buds. To find molecular markers anticipating bud break, we followed the seasonal expression of cyclin B and histone H1 kinase activity. In both orchards, mixed buds showed an increase in histone H1 kinase activity as well as in cyclin B levels just prior to the end of endodormancy. We suggest that the expression level of cyclin B and the activity of histone H1 kinase can serve as markers for early stages of mixed bud break in pear and probably in other species as well.