锦绣杜鹃花药发育及散粉孔形成的形态与解剖学研究

Anther development and formation of dehiscence pore in Rhododendron × pulchrum

关键词:  杜鹃属, 锦绣杜鹃, 花药发育, 孔裂, 药室内壁, 薄壁组织

Key words:  Rhododendron, Rhododendron &#215  pulchrum, anther development, poricidal dehiscence, endothecium, parenchyma

摘要:

被子植物中部分类群的花药开裂方式为孔裂,人们对此类花药的发育与散粉孔的形成及散粉机制了解甚少。杜鹃属(Rhododendron)植物的花药普遍为顶孔开裂,为探究其花药的发育和散粉孔的形成及散粉机制,该研究对锦绣杜鹃(Rhododendron × pulchrum)进行了解剖观察和石蜡切片。结果表明:(1)锦绣杜鹃花药顶端成孔区与花药主体具有不同的组织构成,成孔区由薄壁组织构成,起源于雄蕊原基顶端的分生组织,花粉成熟时薄壁细胞分解成为散粉孔; 花药的主体由孢原细胞发育而来,孢原细胞经多次分裂分化成为具多层花药壁的花粉囊。(2)锦绣杜鹃的花药壁在小孢子母细胞至小孢子四分体时期发育完全,有6～7层细胞,包括1层表皮、2～3层药室内壁、1～2层中层和1层绒毡层,中层在小孢子四分体形成后即分解,绒毡层在花粉完全成熟前分解消失,花药壁在花粉成熟时有表皮和2～3层药室内壁。(3)与纵裂型花药不同,锦绣杜鹃的药室内壁在花粉成熟时不发生纤维化,但因积累多糖而增厚,具有韧性和弹性。(4)锦绣杜鹃花粉发育过程中小孢子母细胞产生的4个小孢子不分离,成熟花粉为四合花粉,花粉间具有粘丝。推测锦绣杜鹃具多层药室内壁且加厚可使花粉囊空间缩小,从而将上部花粉“挤出”散粉孔,而花粉间的粘丝使昆虫传粉时可将花粉成团带出,其药室内壁多层且积累多糖是与顶孔开裂相适应的特征。

Abstract:

Flowers of Rhododendron have unique poricidal dehiscence anther, but the formation of the dehiscence pore and the mechanism of pollen releasing are not fully known. The purpose of this research was to study the anther development of Rhododendon × pulchrum through microdissection and paraffin section to find out how the dehiscence pore develops and what tissues are involved in its development. The results were as follows:(1)The tissue forming the apical pore and the main body of the anther diverged from the very beginning of the stamen development. The dehiscence pore is formed by breaking down of parenchyma tissue, which is derived from the apical meristem of stamen primordium. The body part of the anther comes from the archesporial cells and they develop into regular pollen sacs with multi-layered anther walls.(2)The anther wall is fully differentiated at the stages from microspore mother cell to microspore tetrad, with 6-7 layers of cells, including 1 layer of epidermis, 2-3 layers of endothecia, 1-2 layers of middle layers and 1 layer of tapetum. The middle layers soon break down after completion of microspore tetrads and the tapetum disappears before the pollen tetrads are fully mature. The epidermis and 2-3 layers of endothecia remain to the end.(3)Unlike those of longitudinal dehiscent anthers, the endothecia of Rhododendon× pulchrum are not fibrous at maturation. Instead, they are persistent and somewhat elastic through accumulation of polysaccharide granules in cells.(4)The four microspores produced by one microspore mother cell do not separate from each other and the pollens are released as tetrads, with viscous threads between pollens and among tetrads. The authors assume that the thickened multi-layer endothecia reduced the inner volume of the pollen sacs, therefore the pollen tetrads at the top are “squeezed out” from the dehiscence pore, and the viscous threads allow the pollen tetrads to be pulled out in clusters by pollinators.