Objective  Correlation between the changes on endogenous hormones and embryo abortion of ornamental crabapple seeds was investigated.

  Method   Ornamental crabapple varieties, Malus Red Jewel with low abortion rate and M. Kelsey with high abortion rate, were compared on the endogenous hormones, including gibberellin GA₃, abscisic acid ABA, cytokinin CTK, and indoleacetic acid IAA, in the seeds during fruit growth and development. The dynamic changes were correlated to the seed embryo abortion rate.

  Results  There was significant difference on the fruit setting rate between M. Red Jewel and M. Kelsey (P＜0.05) as that of M. Kelsey declined rapidly from one to 13 d and became stable at 54 d after pollination. In the early stage of embryonic development, the contents of GA₃ and IAA and ratios of GA₃/ABA and (GA₃+IAA+CTK)/ABA in the seeds of M. Red Jewel were significantly higher than those of M. Kelsey (P＜0.05). Conversely, in the later stage, the GA₃, ABA, IAA, IAA/ABA, GA₃/ABA, and (GA₃+IAA+CTK)/ABA of M. Red Jewel became significantly lower than those of M. Kelsey (P＜0.05).

  Conclusion   The physiological fruit-dropping of M. Kelsey occurred in 1–13 d after pollination. It was the optimum time to alter the fruit-setting and improve the ornamental value of an M. tree. Embryonic abortion of both varieties of the crabapple took place within 54 d after pollination. Most of the fruits with aborted embryos did not fall off the tree and continued to grow to maturity due possibly to the increased GA₃ and (GA₃+IAA+CTK)/ABA ratio in the seeds. Consequently, maintaining a high content of GA₃ and IAA in the early stage could ensure a normal seed development. Once reached the late stage, no individual hormone, short of low level GA₃ and IAA, could warrant a normal embryonic development.