Water requirements of terrestrial and epiphytic orchid seeds and seedlings, and evidence for water uptake by means of mycotrophy

Abstract

The use of endomycorrhizal fungi as an energy source (=mycotrophy) initiates seedling development and supplements or replaces photosynthesis in all orchids in nature. Fungus-infected and non-infected seeds of the monkey face orchid, Platanthera integrilabia, a US Federally-threatened terrestrial species, had a different set of water relations than seeds of the green fly orchid, Epidendrum conopseum, a subtropical epiphyte. Seeds of the terrestrial species had lower water loss rates, smaller activation energies for water loss and absorbed water from lower relative humidities. Thus, the epiphyte lacks the enhanced water retention capacity associated with the terrestrial species, implying that epiphytic orchids are capable of germinating quickly given an adequately moist substrate. After germination, water content of fungus-infected seeds was higher. These results provide first time fundamental information related to habitat preference by analyzing seed. Germination is considerably enhanced with mycorrhizal fungi that facilitate the absorption of free water by their orchid seed hosts.