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

The use of endomycorrhizal fungi as an energy source (= mycotrophy) initiat es seedling development and supplements or replaces photosynthesis in all o rchids in nature. Fungus-infected and non-infected seeds of the monkey face orchid, Platanthera integrilabia, a US Federally-threatened terrestrial sp ecies, had a different set of water relations than seeds of the green fly o rchid, Epidendrum conopseum, a subtropical epiphyte. Seeds of the terrestri al species had lower water loss rates, smaller activation energies for wate r loss and absorbed water from lower relative humidities. Thus, the epiphyt e lacks the enhanced water retention capacity associated with the terrestri al species, implying that epiphytic orchids are capable of germinating quic kly given an adequately moist substrate. After germination, water content o f fungus-infected seeds was higher. These results provide first time fundam ental information related to habitat preference by analyzing seed. Germinat ion is considerably enhanced with mycorrhizal fungi that facilitate the abs orption of free water by their orchid seed hosts. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.