Is there a specific stage to rest? Morphological changes in flower primordia in relation to endodormancy in sweet cherry (Prunus avium L.)

KEY MESSAGE: Comparison of five sweet cherry cultivars—over 3 years—shows an asynchronous early and late flower development, but a consistent, specific flower developmental stage for dormancy. In temperate woody deciduous perennials, dormancy is a survival strategy to persist winter temperatures; but chilling is also required for the release of flower bud dormancy and for the completion of flower development. This was noticed over 100 years ago, but the biological mechanisms underlying cold regulated dormancy and its release remain poorly understood. That chilling is required for the completion of flower development led us to hypothesize that a particular stage of flower development may be consistently associated with the dormant phase of flower bud development. Flower development of five sweet cherry cultivars was examined weekly under stereoscopic and optical microscopes over 3 years. Chilling requirements for each cultivar were determined by placing weekly shoots in forcing conditions. The establishment of a flower developmental scale showed that early and late flower development, in the autumn and spring, were asynchronous among cultivars and years. However, in all circumstances, dormancy occurred at the same stage of flower development, characterized by the presence of all flower whorls, with the anthers clearly differentiated in the four locules, and the pistil showing an incipient ovary, style and stigma. The length of time flower buds remained at this stage differed between cultivars and was related to their chilling requirements and date of flowering. The observation that a particular stage of flower development, common to the five cultivars examined, exists during the combined rest period provides a framework for further studies on the physiology and cellular biology of dormancy.