Ericoid Mycorrhizas

Ericoid mycorrhizas are formed between ascomycetous, and more rarely hyphomycetous fungi, and species of the Ericaceae and Epacridaceae. Members of the Ericaceae exhibit three types of mycorrhiza - ericoid, arbutoid and monotropoid. The ericoid type is the most important and is found in such genera as Calluna, Erica, Rhododendron and Vaccinium. In Australia, where the Ericaceae are replaced by the Epacridaceae, 39 species within the sub-family Styphelieae and 21 within the Epacrideae have been foud to form ericoid mycorrhizas.

The mycorrhizas formed are of the endomycorrhizal type. In Calluna vulgaris (L.) Hull, the fungus grows loosely over the lateral "hair" roots of the host plant and the septate hyphae penetrate the single layer of cortical cells, often at several points, and fill them with intracellular hyphal coils. The apex of the "hair" root is not colonised and the stele is never penetrated. The endophyte can occupy as much as 80% of the total volume of heavily colonised roots.
N.B. The "hair" roots of other ericaceous genera may be stouter and have as many as four layers of cortical cells around the stele.

Ericoid mycorrhizas have evolved in association with plants that are found in, and often dominate, some of the most climatically and edaphically stressed environments. These plants characteristically become dominant when levels of acidity become extreme. Under these very low pH conditions, mineralisation of nutrients is inhibited and metallic elements show maximum solubility. There is evidence that species of Calluna and Vaccinium have very high constitutional tolerances to some stresses. Usually, the formation of ericoid mycorrhizas increases the tolerance and there is good reason to believe that their presence is essential for survival of the plant partner.

Schmid et al. (1995) studied the roots of 11 epiphytic fern species from Costa Rica. In all species, they found an association with an ascomycete that traversed the root hairs and formed intracellular hyphal coils. A host-fungus interaction similar to ericoid mycorrhizas.

References

• Schmid, E., Oberwinkler, F. and Gomez, L.D. (1995) Light and electron microscopy of a host-fungus interaction in the roots of some epiphytic ferns from Costa Rica. Canadian Journal of Botany 73:991-996.