New methods to improve symbiotic propagation of temperate terrestrial orchid seedlings from axenic culture to soil

This research aimed to improve the success of soil transfer of terrestrial orchid seedlings after symbiotic germination in the laboratory. Three native Western Australian terrestrial orchids (Caladenia arenicola Hopper & A. P. Brown, Diuris magnifica D. L. Jones and Thelymitra crinita Lindley) were used in this study. The key to improved seedling survival on transfer to soil was found to be the use of an intermediate stage between the Petri dish and soil where larger seedlings were grown in an axenic environment with controlled humidity. There was no apparent benefit of pre-inoculating potting medium with appropriate strains of mycorrhizal fungi for subsequent growth of symbiotic seedlings under glasshouse conditions. Initial survival of seedlings in the glasshouse was high. However, some seedlings failed to produce tubers (from modified roots or droppers) necessary for plant survival through the summer dormancy period, and this caused survival to decrease to 40-60% of the glasshouse-grown seedlings in the first year. The initiation of tubers on droppers by C. arenicola was inversely correlated with leaf size, with smaller plants more likely to form tubers. This suggests that leaves and tubers were competing for resources. However, larger seedlings that did tuberise had larger tubers that were more likely to survive summer dormancy. There was no correlation between leaf size and root tuber size in D. magnifica, but the number of tubers produced was greatest in larger plants. As with C. arenicola, plants of D. magnifica and T. crinita with larger tubers were more likely to survive summer dormancy. Methods developed in this study enable the production of both actively growing symbiotic seedlings and dormant tubers which improve the success of translocation of laboratory-grown terrestrial orchids to field sites.