Post-embryonic development and seedling morphogenesis of Dendrobium moniliforme (L.) Sw. under asymbiotic culture conditions

Dendrobium moniliforme is an endangered orchid species in China. It is important to understand the seed germination and plant morphogenesis for the protection of D. moniliforme germplasm resources. Using asymbiotic culture, morphological and anatomical method, we found the polar spherical embryo formed, and seed germination was completed after 6-9 days culture. It took about 27 days that the polar spherical embryo continued cell division and tissue differentiation, including the apical meristem formed, the rhizoid on the base of embryo developed. The mature embryo is consisted of cotyledon, the apex growth cone, the axial structure, and the rhizoid, but there is no differentiation of radicle primordia. After 57-60 days culture, the adventitious root primordia could be observed on the adjacent to the apical meristem zone, and also the first true leaf has developed, which meant the mature embryo developed into protocorm. With the development of the second true leaf, the stem axis extended significantly, the adventitious roots were growing out laterally as well, and the seedling morphogenesis was completed after 63-67 days culture. From a seed to the seedling, it should be divided into four stages: seed germination, post-embryonic development, protocorm formation and seedling morphogenesis. Not only the post-embryonic development is the key step in the mature embryo formation, but also the protocorm is a unique structure to be different from the embryo in morphogenesis process of D. moniliforme. Our research work resolves the controversy over the embryogenesis and development of Dendrobium and provides basic data for plant biological theory.