Host tree specificity and seed germination of Dendrobium aphyllum (Roxb.) CEC Fisch in Sri Lanka

Tree species can be considered as hosts for many epiphytes and the orchids, as epiphytes, display a biased distribution among hosts. It is possible that the barks of these hosts have some chemical and physical characteristics that influence the seed germination of orchids. The dependence on symbiotic fungi for seed germination is also a crucial factor for orchid distribution. Dendrobium aphyllum is an orchid species with restricted distribution in the sub - montane region of Sri Lanka. The aims of this study were (1) to determine the host tree specificity of the epiphytic D. aphyllum using physical and chemical bark characteristics of the host, (2) to isolate and identify the symbiotic fungi in D. aphyllum roots and (3) to determine the effect of tree bark extracts on symbiotic seed germination of D. aphyllum. Methanol, dichloromethane, hexane and water were used to extract the chemical compounds in tree barks. Aqueous and organic bark extracts of non host species (Erythrina sp., Erythrina berteroana, Eurya accuminata, Semecarpus marginata, Symplocaceae cochinchinensis) inhibited the seed germination of D. aphyllum. However, the effect of host tree species (Eurya ceylanica, Actinodaphne stenophylla, Euiya sp., Antidesma zeylanicum, Citrus maxima, Semecarpus nigra-viridis and two Syzygium spp.) on seed germination fluctuated with the concentration of host bark extracts. The physical characteristics of bark such as texture, peeling behaviour and thickness showed:a positive correlation with the host tree specificity of D. aphyllum. Symbiotic fungus Rhizoctonia sp. was isolated as the associate in D. aphyllurn roots.