Abstract: Observing the process of flower bud differentiation, clarifying the correlation between morphology and anatomical structure of flower buds, and analyzing the dynamic changes of endogenous hormones can provide scientific basis for flowering regulation and cultivation management of Syringa microphylla. In this study, we examined flower bud differentiation and flowering characteristics of S. microphylla by external morphological observation and paraffin section technology, and measured the contents of endogenous hormones by enzyme-linked immunization (ELISA). According to the observation results of anatomical structure, flower bud differentiation of flowered S. microphylla in spring began in late May and ended in early October. The flower bud differentiation process could be divided into seven stages: undifferentiated stage, spathe primordium differentiation stage, inflorescence primordium and small floral primordium differentiation stage, sepal primordium differentiation stage, petal primordium differentiation stage, stamen primordium differentiation stage, and pistil primordium differentiation stage. There were overlaps across different differentiation stages. Indole acetic acid (IAA), abscisic acid (ABA) and zeatin riboside (ZR) changed greatly and showed a variation pattern of “down-up-down”, “up-down-up-down” and “up-down”, respectively. The content of gibberellin (GA₃) remained stable at a low level with little variation  throughout the whole process. The highest value of ABA/GA₃ appeared in the differentiation stages of spathe primordium, sepal primordium, and petal primordium. The highest value of IAA/GA₃ appeared in the undifferentiated stage, the inflorescence primordium and small floral primordium differentiation stage, and the petal primordium differentiation stage. The highest value of ABA/IAA appeared in the sepal primordium differentiation stage, while the highest values of ZR/GA₃ and ZR/IAA appeared in the differentiation stages of sepal primordium and petal primordium. The internal anatomical structure of flower bud differentiation was closely related to the external morphology. High levels of IAA, ABA, ABA/GA₃ and IAA/GA₃ were beneficial to the initiation of flower bud differentiation and the differentiation of early flower organ primordium, while the differentiation of sepal primordium and petal primordium required the accumulation of ZR, IAA, and ABA. The initiation stage of flower bud differentiation and the differentiation of sepal primordium and petal primordium may be the key periods of flower bud differentiation of S. microphylla, with an important role of the content of endogenous hormones and the balance between hormones.

Key words: Syringa microphylla, flower bud differentiation, external morphology, anatomical structure, endogenous hormone