Fertilisation of polar nuclei and formation of early endosperms in Dendrobium catenatum: evidence for the second fertilisation in Orchidaceae

Whether the second fertilisation, i.e. fertilisation of polar nuclei, or fusion of the second sperm with polar nuclei occurs in Orchidaceae has long been controversial because of lack of evidence. In the present study, we observed fusion and fertilisation of polar nuclei and formation of early endosperms in the orchid Dendrobium catenatum Lindl., by using a resin-embedded section technique. As the product of the second fertilisation, the primary endosperm nucleus (fertilised polar nuclei) can last until the global embryo stage, indicating that initiation of endosperm development and that of embryo development were fully asynchronous. The present study demonstrated the occurrence of the second fertilisation in D. catenatum by providing lines of new evidence.