Sarcodes sanguinea and Pterospora andromedea (Ericaceae, Monotropoideae) are nonphotosynthetic myco-heterotrophic plants. Recent studies have shown that the roots of the adult plants are always associated with closely
related but exclusive sets of Rhizopogon species (Basidiomycota, Boletales) from section Amylopogon. We have
isolated Rhizopogon species that were associated with the adult plants and used them to germinate seeds under
gnotobiotic conditions. All Rhizopogon species isolated from either plant species were capable of stimulating seed
germination in both Sarcodes and Pterospora. Under the primary conditions used, germination varied from 9 to
73% in the case of Sarcodes and 0 to 13% in that of Pterospora. The single Rhizopogon strain that failed to elicit
germination in Pterospora under these conditions did stimulate germination under slightly different conditions. By
contrast, seeds failed to germinate on all media which lacked these Rhizopogon species, or in the presence of six
other genera of basidiomycetes. Seed germination could be stimulated either through cellophane or at the edge of
fungal colonies without direct fungus–seed contact. These results suggest that a diffusible or volatile compound
that is unique to Rhizopogon stimulates germination of these plant seeds. Seed lots of Sarcodes from two successive
years had similar germination levels. Sarcodes seeds that had overwintered under natural conditions were also
stimulated to germinate. These results demonstrate the potential for long-term dormancy. We suggest that
a combination of dormancy and the use of specific germination cues might increase the opportunities of these
plants for recruitment. In addition, the specific germination response explains at least a part of the specialized
associations observed in the adult plants. Nevertheless, the seeds respond to a slightly broader range of Rhizopogon
species than has been observed to be associated with the adult plants; thus other factors must also be involved with
specificity under natural conditions.