Abstract:

Objective: Plants of genus Eurya are dominant species in the tropical and subtropical evergreen broad-leaved forest, which is traditionally considered as a strictly dioecious plant. However, bisexual plants exist in Eurya obtusifolia trees at Jinyun mountain in Chongqing in addition to the typical female and male plants. The morphology and structure of flower buds at different stages of differentiation and the differences and similarities of the differentiation process between bisexual flower buds and typical female and male flower buds were investigated to understand the whole process of flower buds differentiation in different sexes and their morphological characteristics in each differentiation period, to identify the critical period of sex differentiation of flower buds, and further to provide important morphological evidences for exploring the relevant mechanism of sex differentiation. Method: Typical flower buds of female, male and bisexual trees of E. obtusifolia were used as experimental materials. The changes of external morphology and tissue structure during flower bud differentiation were observed and analyzed using conventional paraffin section. Result: 1) The E. obtusifolia has 1-4 flower buds, located in the axils of new or two-year-old branches. 2) The differentiation of flower buds began in early August and basically completed differentiation in the middle and late December, lasting about 120 days. After that, the flower buds were dormant, and entered the initial flowering period from February to March of the next year. The flower bud differentiation time of bisexual flower buds was later than that of male and female flower buds. 3) The process of flower bud differentiation can be divided into 5 stages:bract differentiation, sepal differentiation, petal differentiation, stamen and pistil differentiation, and stamen and pistil formation. 4) In the process of flower bud development, the primordia of both male and bisexual flower buds appeared at the same time. The primordia of stamens in male flowers developed normally while the primordia of pistil stopped developing, and both stamen and pistil primordia in the bisexual flower developed normally. Only pistil primordium was found in female flower bud, but not stamen primordium. 5) In the early stage of the development of bisexual flowers, the development speed of pistil primordium was slightly faster than that of the stamen primordium. The development of pistil was consistent with that of the female flower, the base of carpel primordium healed and enlarged, the middle depression formed ovary and the top healed and extended upward to form style. The development of stamens was consistent with that of the male flower. The upper end of stamen primordium expanded to form anthers and the lower end formed short filaments. At the mature stage of pistil and stamen, the floral organs continued to grow and develop gradually. 6) The length-width ratio of flower buds of the three types of sexes showed an increase first and followed by a decrease during the whole process of differentiation. The male flower buds reached the highest value in the differentiation stage of the bracts, while the female flower buds and the bisexual buds peaked in the stamen and pistil differentiation stages. The dynamic changes of the external morphological characteristics(shape, color) of flower buds in the five differentiation stages were conical (green)→elliptical or approximately circular shape (green faded, deep purple) →round and fat (royal purple)→round, the top of male flower bud was round and blunt, the top of female flower bud and bisexual flower bud was gradually sharp (purple red gradually faded and green deepened)→oval (purple red completely faded, from tender green to yellowish green or brownish green). Conclusion: The flower bud morphology and internal tissue structure of three sexual expression plants of E. obtusifolia are consistent during bract differentiation, sepal differentiation and petal differentiation, but there is a significant difference in the pistil and stamen differentiation stage. The differentiation of bisexual flower bud is similar to that of male flower bud, both with pistil and stamen primordium appearing. It is clear that the key stage of sex differentiation is pistil and stamen differentiation, and there is a corresponding relationship between the external morphology of flower buds and the differentiation stages.

Key words: Eurya obtusifolia, flower bud differentiation, sexual variation, morphology