Study on the Anatomical Structures of Rhododendron protistum var. giganteum with an Extremely Small Population

Rhododendron protistum var.giganteum is an evergreen tree that belongs to subgenus Hymenanthes and subsection Grandia from the family Ericaceae.It is a highly endangered species in a small area in the southern part of Yunnan Province in China.This plant has been listed in the Conservation Program for Wild Plants with Extremely Small Populations in China (2012-2015 Operational Plan) due to its limited distribution and small populations.In the present study, the anatomical characteristics of the leaves, stems and roots of R.protistum var.giganteum were studied by paraffin sectioning and leaf epidermal isolation.Furthermore, seed surface, stoma and indumentum characteristics were also observed under scanning electron microscope (SEM).The paraffin sectioning results indicated that the secondary structure of the root was mainly composed of periderm and vascular cylinder.The anatomical structure of the stem consisted of the epidermis, cortex, phloem, xylem and pith.The leaf was bifacial with a cuticle on the upper epidermis.The epidermal hairs and stomata were only distributed on the lower epidermis.The stomata were anomocytic type.Palisade tissue was arranged closely and was well-developed, while the spongy tissue was arranged loosely.There were well-developed vascular bundles and bundle sheath extensions distributed in the structure of the leaf.Through SEM, it was found that the foliar trichome of R.protistum var. giganteum was composed of two layers of epidermal hairs, including a layer of cavernous villi and a layer of short lamelliform villi.Basal and apical wings were equally developed on the seeds.Seeds were flattened and exotesta longitudinal sculptures, which were conformed on the dorsal and ventral sides of the seed.The results obtained in the present study will not only help to further understand the biological characteristics of this species, but also provide a scientific basis to elucidate its systematic classification and habitat adaptation.