Morphological anatomy, sporogenesis and gametogenesis in flowering process of jasmine (Jasminum sambac Aiton)

Jasminum sambac Aiton is poor in seed-setting. To better understand its embryological characteristics, a detailed analysis of floral development was conducted on a popular double-petal cultivar of jasmine, including floral morphology, anatomy, sporogenesis and gametogenesis. The results showed that its ovary is biventricular with two short funiculus hemianatropous ovules, adhering symmetrically to the placenta. Normal floral buds have two tetrasporangiate stamens, which are always shorter than the pistil and surrounded tightly by the cup-shaped corolla. The two-branched style is an intermediate type between the hollow and solid types, with a solid upper one-third part and a hollow lower two-thirds part. The young anther wall is composed of epidermis, endothecium, middle layer and the glandular type tapetum, detaching from other parts of the anther during the floral development. The embryo sac is a monosporic Polygonum type which develops abnormally in most opening flowers. About 20% stopped developing into megasporocytes, 30% were arrested during the meiotic division, and 45% degraded at varying stages. Cytokinesis during microsporocyte meiosis is simultaneous, the tetrad is tetrahedral or decussate, and mature pollen is two-celled. Meiosis of microsporocytes is asynchronous, even within an anther. Development of 25% microspores is delayed due to not forming exine or undergoing mitosis. Artificial pollination and SEM observation revealed that among 15% of anthers, tetrads formed pollen massula instead of free microspores, and only one tube grew normally from the tetrad-pollens. These facts evidenced that abnormal development of embryo sacs is the main reason for the poor seed-setting, and its restricted pollen viability caused failures in fertilization. This study detailed its reproductive characteristics of jasmine and provided a reference framework for subsequent studies by aligning floral reproductive development perspective to a phenological scope.