MUTUALISM AND ANTAGONISM IN THE MYCORRHIZAL SYMBIOSIS, WITH SPECIAL REFERENCE TO IMPACTS ON PLANT COMMUNITY STRUCTURE

Examination of the roots of land plants has revealed the occurrence of mycorrhiza in the majority of species, over 70% of which are hosts to zygomycetous fungi that form vesicular-arbuscular (VA) associations. On the basis of experiments with a small number of host species showin g enhancement of growth following colonization, it is widely assumed t hat wherever mycorrhizas are observed, the symbiosis is of the mutuali stic type. The value of definitions based on structural rather than fu nctional attributes is here brought into question by experiments simul ating the ecologically realistic circumstance in which seeds germinate in soil in the presence or absence of established VA mycelium. These reveal a spectrum of fungal impacts in which some species respond mutu alistically, while others, putative hosts or nonhosts, are antagonised , showing reduction of yield and survivorship and, hence, a loss of fi tness relative to plants grown without VA fungi. Antagonised species n ormally grow in disturbed, open habitats and fail to establish in clos ed communities. It is hypothesised that their turf incompatibility ari ses from a sensitivity to interference by VA fungi, which consigns the m to ruderal habitats. Mycorrhizal fungi, thus, play a role in definin g the ecological niches occupied by plants and in determining of plant community composition.