Ectomycorrhizas

The fungi involved in the formation of ectomycorrhizas are mostly Basidiomycetes and are often species that have quite large and conspicuous fruiting bodies. Toadstools and earthballs are conspicuous examples. Fungi from other major fungal groups have also been shown to form sheathing ectomycorrhizae including two species of Endogone, zygomycetes closely related to the fungi that form vesicular-arbuscular and arbuscular mycorrhizas.

Most ectomycorrhizal (ECM) fungi can synthesize plant growth substances which cause the morphology of the root to be changed. The changes induced differ between species. At one extreme, branching is totally suppressed and, at the other, prolific branching takes place, with a single root having a dense aggregate of as many as 2000 mycorrhizal tips.

Most ectomycorrhizas are enveloped in a dense weft of hyphae while others appear smooth and seem to lack radiating hyphae. All gradations between these two forms can be found.

The first sign of mycorrhiza formation is the development of fungal mycelium around a short root. This gradually thickens over the root surface developing into a sheath (sometimes called the mantle). The sheath may be only one or two cells thick but can exceed 100µm. The entire root, including the apex, is often enclosed by the sheath so that there is no direct contact between younger roots and the soil. Root hair development is normally totally suppressed. Thus, all nutrients and water absorbed by the roots have to pass through the fungal sheath. Depending on the fungal species, various secondary structures in addition to radiating adsorptive hyphae may be found associated with the sheath.

Behind the zone of cell division, hyphae from the innerside of the sheath penetrate between the root cortical cells producing a network of hyphae called the Hartig net. The size and shape of the cells of the Hartig net differ greatly depending on the ECM fungal species as does the depth of penetration, which is never beyond the endodermis. The Hartig net is a three-dimensional branching structure that provides a large area of contact with the cortical cells allowing efficient transfer of mineral nutrients to the plant and metabolites to the endophyte. Hyphal penetration of cortical cells does not normally occur and is limited to the cell wall.

ECM hyphae may completely enclose the lateral roots and thus prevent direct acquisition of mineral nutrients by these roots.

ECM fungi can be found on about 90% of the trees in temperate forests and each tree can be associated with many fungal species concurrently. Despite the general view that ectomycorrhizas are rare in the tropics, species within at least 14 families are known to form ectomycorrhizas. This number is likely to be an underestimate since knowledge of the mycorrhizal status of angiospermous trees in the tropics is extremely limited.

In most communities where ECM fungi are present there appears a marked succession of species with increasing age of the trees and the pattern of fungal species is similar for a range of tree hosts.

Changes in the nutrient cycling in the forest floor as a result of primary succession within the plant community may account for this fungal succession. As the forest ages, herbaceous ground flora is shaded out and an increasingly greater amount of recalcitrant, tree derived litter is input to the soil sub-system, litter residence time increases and litter accumulates on the forest floor. About the time of canopy closure, the fungal species diversity is at its highest and trees reach a peak of nutrient deficiency.

It can thus be hypothesised that early mycorrhizal colonisers optimise nutrient uptake from available inorganic nutrient pools in the soil, derived from mineralisation of readily decomposable herbaceous litter. Later colonisers may not have the benefit of large inorganic nutrient pools as the rate of mineralisation of nutrients from recalcitrant tree litter is much slower and that these mycorrhizal fungi may have a degree of saprophytic ability and/or have a synergistic association with the saprophytic community.