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

1,2WANG Yu-Guo 2LI Guang-Zhao 1ZHANG Wen-Ju 1YOU Jia 1CHEN Jia-Kuan* 1(Ministry of Education Key Laboratory for Biodiversity Science and Ecological Engineering, Institute of Biodiversity Science, Fudan University, Shanghai 200433, China) 2(Guangxi Institute of Botany, Guangxi Zhuang Autonomous Region and the Chinese Academy of Sciences, Guilin 541006, China)  
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 frommolecular 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.
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