Apricot (Prunus armeniaca L.) is a species particularly prone to erratic fruit set and flower bud drop has been repeatedly reported in different cultivars and growing conditions. A number of potential causes have been explored, but a clear main cause remains elusive. However, the fact that bud drop can vary depending on the year or site suggests that the process is somehow environmentally influenced. In this work bud development and bud damage have been related with frost temperatures during

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... o and ecodormancy. Two years of experiments showed that as it might be expected, no frost damage was recorded during endo-dormancy, when the buds are fulfilling their chilling requirements. Likewise, frost damage took place, when frost occurred after bud burst. But unexpectedly, flower buds were particularly vulnerable to frost damage when they enter in ecodormancy, which occurs once chilling requirements are fulfilled but prior to bud swelling. At this time, while no external symptoms were detected in an apparent still quiescent stage, internal damage was observed as perpendicular voids characteristics of ice formation that resulted in browning of tissues in the stamens, pistil or both. Vulnerability appeared concomitantly with the establishment of vascular connections first in the anthers and later in the pistil, and differences between buds in the stage of development explain differences in bud damage for a same frost episode. The proportion of frost damaged buds clearly contributed to the proportion of subsequent bud drop that occurred later. These results provide evidence for the contribution of frost damage to flower bud drop in apricot, showing that there is a stage particularly prone to frost damage, once the buds have completed their chilling requirements and prior to bud-burst, when the first xylem connections are established. The identification of the cause of a phenomenon that is widespread in this species and that is a limiting factor in cultivar adaptation will contribute to understand and select new cultivars for new areas.