3.4.1  Main types of mycorrhizas

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Mycorrhizal associations are classified according to the way in which fungi interact with a host plant root, in particular the nature of the interface that forms between host plant and fungus. This classification leads to a number of distinct types of mycorrhizal association; however, only three of these are widely distributed in the plant kingdom: arbuscular mycorrhizas, ectomycorrhizas and ericoid mycorrhizas. Mycorrhizal types generally form with a characteristic group of plant species but there are occasional examples of overlap such as eucalypts which have both arbuscular mycorrhizal and ectomycorrhizal types. Arbuscular mycorrhizas occur in a vast array of herbaceous genera (in fact, some 80% of all plant species), while ectomycorrhizas are most common in tree species (including the families Betulaceae, Pinaceae, Fagaceae, Dipterocarpaceae, Leguminaceae and Myrtaceae). Ericoid mycorrhizas are confined to genera within the Ericales, including Ericaceae and Vacciniodeae in the northern hemisphere and Epacridaceae in the southern hemisphere.

Not only do the three types differ in host preference and in the structures they form during association with the host root, they also differ in the ways by which they enhance host plant growth. Indeed, each type appears to have evolved to suit a particular soil habitat, with arbuscular mycorrhizal infection most common in vegetation that is native to regions of relatively high mean annual temperatures and rates of evapotranspiration, where mineralisation of soil organic matter to inorganic nitrogen is rapid. By contrast, ectomycorrhizal associations are largely confined to trees in habitats where temperatures and evapotranspiration are lower, leading to slower rates of decomposition and accumulation of plant litter in soil. In heathlands of polar and alpine regions, where temperatures and rates of evapotranspiration are further reduced (and in some Mediterranean-type heaths such as those found in Australia), ericoid mycorrhizas predominate.

In gross terms, key nutritional differences between these three environments are as follows. In upland soils dominated by ericoid mycorrhizas, the processes of ammonification and nitrification (conversion of organic nitrogen to NH4+ and NO3, respectively) are severely inhibited, leading to an accumulation of organic nitrogen and rendering nitrogen the major growth-limiting nutrient. In the context of Australian heaths, nitrogen may be equally limiting in the sandy heathland soils, where its availability will rely on the (often moisture-limited) decomposition of plant debris. In forests where ectomycorrhizas dominate, ammonification occurs more readily, but nitrification remains slow and nitrogen availability is the major nutritional limitation to plant growth. By contrast, nitrification generally occurs freely in soils that favour arbuscular mycorrhizal associations. The mobility of NO3 in soil means that nitrogen is available relatively freely to plant roots so that phosphorus availability often becomes limiting to growth.

Each type of mycorrhizal association has thus evolved distinctive symbiotic forms to enhance host plant growth and survival (Read 1991). While such strategies endow largely nutritional benefits upon the host, mycorrhizal infection is also known to enhance plant water status, confer protection against root pathogens, contribute to soil structure via hyphal binding of soil particles and render plants less susceptible to toxic elements in some circumstances. This section will, however, focus solely upon nutritional benefits to the host plant.