Comparative water balance profiles of Orchidaceae seeds for epiphytic and terrestrial taxa endemic to North America

The Orchidaceae have dust-like seeds that use wind currents for long-distance dispersal. Lacking endosperm, orchid embryos consume free-living, mycorrhizal fungi as a carbon source (mycotrophy) after settling on a substrate. Few studies have investigated orchid seed morphology as it relates to ecology, but conceivably variations in seed size and testa characteristics could be linked to water loss rates aimed at maximizing germination in a particular habitat. Seeds of 2 epiphytic, 1 aquatic, and 7 terrestrial orchids native to North America were compared with respect to water balance profiles: Cleistes bifaria, Encyclia tampensis, Epidendrum nocturnum, Habenaria repens, Isotria medeoloides, Liparis elata, L. hawaiensis, Platanthera holochila, P. integrilabia, and P. leucophaea. Water content, water loss rate, activation energy, and equilibrium humidity were assessed for each species. Seeds of epiphytic orchids were smaller, lighter, more porous, and had higher water loss rates compared to terrestrials. No active mechanism for water absorption exists in seeds of either group. Water loss appears to be a species-specific phenomenon that may be linked to the ecological niches these species occupy.