果皮和种皮微形态特征在杜鹃花属系统学研究中的应用

A systematic study of the genus Rhododendron (Ericaceae) using micromorphological characters of fruit surface and seed coat

采用扫描电子显微镜对国产杜鹃花属Rhododendron6个亚属的代表种和近缘类群杜香属Ledum杜香L.palustre的果皮和种皮微形态特征进行观察,对杜鹃花属植物果皮微形态特征进行了系统描述,并通过比较现存杜鹃花属植物和种子化石,新发现一些居间的种子类型。结果表明,果皮和种皮微形态特征具有一定的系统学意义。叶状苞亚属subgen.Therorhodion的叶状苞杜鹃R.redowskianum的果实有短而疏的刺毛,无气孔,种子为无翅类,扁平,外围轮廓长椭圆形。杜鹃亚属subgen.Rhododendron植物果皮为百合花杜鹃型,具有鳞片,气孔器散生于指状突起之间,与叶表皮的微形态特征一致,其种子为百合花杜鹃型,表面具宽而浅的沟,呈脑纹状,有别于无鳞类杜鹃花。常绿杜鹃亚属subgen.Hymenanthes果皮为云锦杜鹃型,其角质层表面不规则,无表皮毛,偶见气孔;种子为云锦杜鹃型。映山红亚属subgen.Tsutsusi果皮为岭南杜鹃型,密生长表皮毛,角质层致密;种子为南边杜鹃型和岭南杜鹃型。微形态特征与“常绿杜鹃亚属和(国产)映山红亚属均为内部支持率很高的单系类群”的分子系统发育研究结果一致。马银花亚属subgen.Azaleastrum的马银花组sect.Azaleastrum和长蕊杜鹃组sect.Choniastrum微形态区别明显,支持各自为独立的单系类群。羊踯躅亚属subgen.Pentanthera的羊踯躅R.molle果皮特征明显,可与其他亚属区分,但种子类型更接近常绿杜鹃亚属。本研究结合分子系统发育资料和叶表皮微形态特征讨论了一些近缘类群的系统发育关系;研究结果支持将腺萼马银花处理为独立的种。

Micromorphological characters from fruit surface of 24 species and seed coat of 21 species, 6 subgenera of Rhododendron from China, i.e., Azaleastrum, Hymenanthes, Pentanthera, Rhododendron, Therorhodion and Tsutsusi, were examined under the scanning electron microscope. Ledum palustre was also examined to compare with the species of Rhododendron. The micromorphological characters of fruit surface and seed coat in Rhododendron are described in this paper. Different types are distinguished here on the basis of morphological characters such as scale, trichome, stomatal apparatus of fruit surface, and ornamentation of seed coat. Compared with seeds of extant and fossil Rhododendron species, some new seed types in Rhododendron species from South China are found. R. redowskianum of subgen. Therorhodion has sparse short setas on the fruit surface without stomatal apparatus. Outline of its slightly flattened and unwinged seed is narrowly oblong. Subgen. Rhododendron, the lepidote rhododendron, with scales on fruit surface, as well as brain-line ridge on the seed coat, differs distinctly from the elepidote rhododendron. Fruit surface of the species in subgen. Hymenanthes belongs to R. fortunei-type, which is characterized as the irregular cuticular membrane without trichome, with stomatal apparatus rarely seen. In the species of subgen. Tsutsusi, the type of fruit surface belongs to R. mariae-type, which is characterized as the compact cuticular membrane with thick trichomes. As far as seed coat is concerned, only one type is found in subgen. Hymenanthes （i.e., R. fortunei-type）, whereas two types occur in subgen. Tsutsusi （i.e., R. meridionale-type and R. mariae-type）. The species of subgen. Azaleastrum show the great divergence on morphological characters. Separately, sect. Choniastrum and sect. Tsutsusi exhibit their own micromorphological characters. According to the morphological characters of fruit surface, R. molle of subgen. Pentanthera differs distinctly from the species of other subgenera. However, its R. molle-type seed is similar to R. fortunei-type seed. The above-mentioned morphological characters support the results from molecular systematic studies： except for subgen. Therorhodion, the lepidote rhododendron （subgen. Rhododendron） plus Ledum, is monophyletic; subgen. Tsutsusi from China and subgen. Hymenanthes are both monophyletic; subgenera Azaleastrum and Pentanthera are both polyphyletic; and in subgen. Azaleastrum, sect. Choniastrum and Tsutsusi are both monophyletic. Furthermore, the relationships between the closely related taxa are also inferred from micromorphological characters and molecular data. This study provides micromorphological evidence to support the separation of R. bachii from R. ovatum as a distinct species.