Platanthera chapmanii: culture, population augmentation, and mycorrhizal associations

Much is unknown concerning the molecular and reproductive ecology of temperate terrestrial orchid species. The rare species Platanthera chapmanii was used in this study because of its wide geographic range, relatively quick growth and development, and its known ability to be asymbiotically propagated. Seed dormancy is a survival mechanism that can be evolutionarily advantageous for many temperate orchid species. Many temperate terrestrial species are known to require pre-germination treatments (e.g. cold-moist stratification) to induce germination. Based on an Analysis of Variance and Fisher’s Least Significant Difference test, P. chapmanii seeds that were treated with cold-moist stratification conditions at 5°C for 8 or 12 weeks had a higher rate of germination than seeds that are not cold-moist stratified. After germination, however, development up to nine months was independent of the pre-germination treatment. This is most likely because of dormancy mechanisms the species has adapted to prevent germination at inopportune times. Many species of temperate terrestrial orchids exhibit extremely low germination rates (≤5%) when they do not undergo cold-moist stratification (Bowles et al. 2002). The southern distribution of P. chapmanii may be the reason for a relatively high germination rate without the cold-moist stratification treatment (25%). This capacity to germinate in the absence of stratification could be a useful adaptation as climate continues to change. Rare terrestrial orchids are thought to be sensitive to commercial fertilizers, and the nutrient supplementation study described above was designed with this in mind (Silvertown et al. 1994). There were no significant differences in plant height between the three nutrient treatments (0.00x, 0.25x, and 0.50x). It is possible that significant differences would exist if other response variables including chlorosis and tip necrosis were measured, or if the concentration of fertilizer was increased. Differences in shoot height was highly variable among treatments, and hence an increase in replications could also add more confidence in the data. From the population augmentation studies, strong conclusions cannot yet be drawn. In this research, Platanthera chapmanii was documented to form mycorrhizal pelotons with fungi from the families Tulasnellaceae and Ceratobasidiaceae. Of the 122 high quality fungal nuclear ribosomal internal transcribed spacer (nrITS) sequences, 121 were of Tulasnellaceae. The only Ceratobasidiaceae sequence was from a naturally occurring P. chapmanii individual that was sampled in spring 2015. Mean pairwise distance of the sequences from greenhouse sources were smaller than the mean pairwise distance of sequences from naturally occurring plants, but OTU richness was the same. In addition, a majority of the OTUs (13) clustered together on the same clade of the maximum likelihood tree independent of treatment. Based on the results, it is suggested that the taxon is specific towards associations with narrow clades of the Tulasnellaceae.