Pollen and microsporangium development in Ziziphus jujuba, Z. mucronata, Paliurus spina-christi and Gouania ulmifolia (Rhamnaceae)

Abstract The aim of this paper is to investigate the ultrastructural events that occur during pollen grains development, with emphasis in pollen grain wall and tapetum ontogeny in Ziziphus jujuba, Z. mucronata, Paliurus spina-christi (Paliureae) and Gouania ulmifolia (Gouanieae). Anthers at different developmental stages were processed according to classic techniques for transmission electron microscopy. Differences in the number of endothecium layers and in the number of tapetal cell nuclei were found. Tapetal cells present an anastomosing tubular network and large vesicles with fibrillar content in the cytoplasm. Pollen grain development and ontogeny of pollen grain wall are similar in the four species. The number of endothecium layers, the number of nuclei of the tapetal cells and tapetal cells ultrastructure of the four species support the phylogenetic relationships previously published for the Rhamnaceae family. Tapetal vesicles with fibrillar or polysaccharide content seem to be an exclusive characteristic of the tribes Paliureae and Gouanieae. Some ultrastructural characters of the pollen grain wall development are common to other species of Rhamnaceae, such as the primexine matrix present at the microspore mother cell stage, the aperture entirely built up during the tetrad stage, the thick and fibrillar intine, and the granular infractectum.

摘要：本研究旨在探究花粉发育过程中的超微结构事件，重点聚焦枣（Ziziphus jujuba）、尖叶枣（Ziziphus mucronata）、铜钱树（Paliurus spina-christi，马甲子族(Paliureae)）以及咀签（Gouania ulmifolia，咀签族(Gouanieae)）的花粉壁（pollen grain wall）与绒毡层发生发育（tapetum ontogeny）。研究人员采集不同发育阶段的花药，采用透射电子显微镜（transmission electron microscopy）经典制样技术进行前处理。结果发现，受试物种的药室内壁（endothecium）层数与绒毡层细胞核数目均存在显著差异。绒毡层细胞的细胞质中具有吻合管状网络结构与含有纤维状内含物的大型囊泡。四种受试植物的花粉发育进程与花粉壁发生发育过程高度相似。四种植物的药室内壁层数、绒毡层细胞核数目以及绒毡层细胞超微结构，均支持此前已发表的鼠李科（Rhamnaceae）系统发育关系。含有纤维状或多糖类内含物的绒毡层囊泡，似乎是马甲子族与咀签族所特有的分类特征。此外，花粉壁发育的部分超微结构特征在鼠李科其他物种中也普遍存在，例如小孢子母细胞时期形成的原孢粉质基质（primexine matrix）、四分体时期完全构建的花粉萌发孔（aperture）、厚实且呈纤维状的花粉内壁（intine）以及颗粒状下覆盖层（infractectum）。