Abstract
In order to investigate the cause of poor fruit set in 'Zuili' plums, anatomical examinations of post-bloom pistils were conducted and the dates of young fruit drop were recorded during the growing seasons of 2008 and 2009. Pistils of cv. 'Black Amber' were also examined as an abundant setting control. Two major dropping periods were detected in 'Zuili': one during the first 5 d after full bloom (DAF) and another between 10 and 17 DAF. Anatomical analyses of the pistils at the full bloom stage revealed that half of the ovules had not developed embryos, which may have caused their early drop. In most dropped pistils collected at 17 DAF, the micropyle had not been penetrated by a pollen tube, indicating that they were not fertilized. 'Zuili' ovules initiated embryo division at 10-12 DAF, although thereafter embryo development was retarded when compared to the rates observed in 'Black Amber'. Ovule fertilization failure and inactive embryo development after ovule fertilization may be the major causes of the later fruit drop observed in 'Zuili' plum trees.