Bud dormancy is an important developmental stage that ensures that trees can tolerate environmental stresses in winter and bloom uniformly in the following spring. Regarding Rosaceae floral buds, exposure to chilling conditions promotes floral primordium development and the transition from endodormancy to ecodormancy. A subsequent period of warm conditions induces blooming. In Japanese apricot (Prunus mume), dormancy progression is accompanied by morphological changes that alter the bud appearance and internal structures. We used a modified BBCH scale and conducted microscopy analyses to elucidate the bud developmental stage of three cultivars with contrasting chilling requirements. The floral bud developmental period corresponding to BBCH stages 51–53 includes the transition from endodormancy to ecodormancy in all three cultivars. Male meiosis and microspore development occurred during this transition in high-chill cultivars, but were detected considerably later than the transition in the low-chill cultivar. A slow or suspended developmental phase was observed only for the high-chill cultivars upon completion of floral primordium organ differentiation, suggesting that chilling may be required to induce floral bud maturation and dormancy release only in high-chill cultivars. Possible relationships among BBCH stages, flowering-related morphological characteristics, and the dormancy phase transition in Japanese apricot are discussed.

Temperate perennial fruit tree species enter dormancy in autumn. A prolonged exposure to chilling followed by warm conditions induce dormancy release and bud break. In Rosaceae species, flowering proceeds during dormancy when bud growth is repressed. TheSHORT VEGETATIVE PHASE(SVP)‐clade MADS‐box genes such asDORMANCY‐ASSOCIATED MADS‐boxes(DAM/SVPs) encode tree bud dormancy regulators. The major effects of DAM/SVPs include growth inhibition and bud break repression through the regulation of plant hormone metabolism in vegetative and floral buds. In addition, theFLOWERING LOCUS C(FLC)‐clade MADS‐box genes were recently also proposed to encode flowering and dormancy regulators. Apple (Malus domestica) FLC may have multifaceted roles in the juvenile–adult phase transition and in the annual development‐dormancy cycle in floral buds. Specifically, in apple, we hypothesize FLC3‐like may facilitate flowering in autumn, whereas an FLC‐like (FLC2) may inhibit bud break, thereby helping floral buds to remain in a quiescent state in winter and preventing an unexpected bud break before the spring. We developed a working model in which abscisic acid,C‐repeat Binding Factor(CBF),TEOSINTE BRANCHED/CYCLOIDEA/PROLIFERATING CELL FACTOR(TCP), micro ribonucleic acid (microRNAs), and epigenetic regulation mediate processes that convert environmental signals into the transcriptional regulation ofDAMs andFLCs, thereby ensuring dormancy phase transitions and progression. Finally, possible relationships between MADS‐box genes and dormancy‐associated cellular metabolism are discussed.

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