In vitro germination of nonphotosynthetic, myco-heterotrophic plants stimulated by fungi isolated from the adult plants

Sarcodes sanguinea and Pterospora andromedea (Ericaceae, Monotropoideae) ar e nonphotosynthetic mycoheterotrophic plants. Recent studies have shown tha t the roots of the adult plants are always associated with closely related but exclusive sets of Rhizopogon species (Basidiomycota, Boletales) from se ction Amylopogon. We have isolated Rhizopogon species that were associated with the adult plants and used them to germinate seeds under gnotobiotic co nditions. All Rhizopogon species isolated from either plant species were ca pable of stimulating seed germination in both Sarcodes and Pterospora. Unde r the primary conditions used, germination varied from 9 to 73%, in the cas e of Sarcodes and 0 to 13% in that of Pterospora. The single Rhizopogon str ain 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 speci es, or in the presence of six other genera of basidiomycetes. Seed germinat ion could be stimulated either through cellophane or at the edge of fungal colonies without direct fungus-seed contact. These results suggest that a d iffusible or volatile compound that is unique to Rhizopogon stimulates germ ination of these plant seeds. Seed lots of Sarcodes from two successive yea rs had similar germination levels. Sarcodes seeds that had overwintered und er natural conditions were also stimulated to germinate. These results demo nstrate the potential for long-term dormancy. We suggest that a combination of dormancy and the use of specific germination cues might increase the op portunities of these plants for recruitment. In addition, the specific germ ination response explains at least a part of the specialized associations o bserved 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 speci ficity under natural conditions.