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

Tree species can be considered as hosts for many epiphytes. 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 for the seed germination of orchids. Dependence on symbiotic fungi for the seed germination is also a crucial factor for orchid distribution. Dendrobium aphyllum is an orchid species with restricted distribution in sub montane region of Sri Lanka. 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 seed germination of D. aphyllum. However, the effect of host tree species (Eurya ceylanica, Actinodaphne stenophylla, Eurya sp., Antidesma zeylanicum, Citrus maxima, Semecarpus nigra-viridis and two Syzygium spp.) on seed germination fluctuated with the concentration of host bark extracts. Bark physical characteristics such as bark texture, peeling behavior and bark thickness showed positive correlation with host tree specificity of D. aphyllum.. Symbiotic fungus Rhizoctonia sp. was isolated as the associate in D. aphyllum roots.