Mycobionts, germination, and conservation genetics of federally threatened Platanthera praeclara (Orchidaceae)

Terrestrial orchids of North American prairies remain understudied with respect to ecology, mycorrhizal associations, propagation, and genetic attributes. Platanthera praeclara Sheviak and Bowles is a federally-listed perennial orchid native to midwestern prairies, and 75% of its populations have been lost in the past 130 years. Habitat destruction has been identified as the most important factor in decline of the species, and lack of information regarding its ecology, seed germination, and genetics has made difficult management of existing populations. This project was undertaken to study mycorrhizal associates, germination, and conservation genetics of P. praeclara . An extensive review of literature is presented to recount the existing information concerning terrestrial orchids and to support the need for experiments described herein. Several populations in Minnesota and Missouri were surveyed to assess the diversity of mycorrhizae associated with the orchid. The results indicate Ceratorhiza spp. as primary mycobionts of P. praeclara at various phenological stages. Some Epulorhiza strains were also recovered from protocorms and roots of the orchid. Selected isolates were used in symbiotic germination experiments for in vitro propagation of P. praeclara. Leaf-bearing seedlings developed when cold-stratified seeds of two relatively small Missouri populations were inoculated with a protocorm-derived Ceratorhiza isolate (UAMH 9847). Asymbiotic culture on synthetic media and culture in absence of mycorrhizal fungi resulted in poor germination and protocorms failed to develop beyond the non-photosynthetic stages. In situ germination experiments revealed that germination and development is very slow to occur in P. praeclara's natural habitat. Very few seeds germinated in situ even after extended incubation in field soil at various sites of extant and historic populations. Evaluation of genetic diversity in eight Minnesota populations showed a positive correlation between expected heterozygosity and population size. However, overall population differentiation was low; geographic and genetic distances were not positively correlated suggesting seed and pollen movement likely maintain gene flow among study populations of P. praeclara. Differences in allelic frequencies seem responsible for variation among populations. This observation indicates genetic drift is occurring in smaller populations, which exhibit high degree of monomorphism at allozyme loci. When all results are considered together, each P. praeclara population seems to have some unique genetic or ecological resources which require preservation and conservation. Long-term existence of the species will depend on range-wide conservation efforts which would combine in situ and ex situ techniques.