Mycorrhizal Structure And Function

Arbuscular mycorrhiza (AM), the so-called endomycorrhiza, are characterized by structures within root cells; these structures are called arbuscules because they grow and ramify, treelike, within the cell (see Fig. 2.4[a]). They are members of the Phycomycete fungi family. Most, but not all AM (two exceptions are known in the Endogonaceae), also have storage structures known as vesicles, which store oil-rich products. AM send out hyphae for several centimeters (a maximum of 6-10) into the surrounding soil and are instrumental in facilitating nutrient uptake, particularly phosphate ions (Allen, 1991). AM are known only as obligate mutualists (i.e., the root provides carbon, and the mycorrhiza tap an enhanced pool of mineral nutrients) and have not been cultured yet apart from their host roots. Because AM hyphae will grow out from the germinating chlamydospore toward root surfaces, responding to soluble compounds, possibly including flavonoids, they are considered to have slight saprophytic competence (Azcon-Aguilar and Barea, 1995). Some rhizosphere microorganisms seem to stimulate AM germination and mycelial growth, functioning either by detoxifying or removing inhibitors from the growth medium or by utilizing self-inhibiting compounds from the AM fungus, enabling more growth than would be possible under axenic conditions (Azcon-Aguilar and Barea, 1995).

Ectomycorrhiza (ECM) are significantly different in physiology and ecology. These are principally Basidiomycetes and proliferate between cells, not inside them as is the case for AM. An obvious morphological alteration occurs with formation of the mantle and Hartig net (a combination of epidermal cells and ECM fungal tissues) on the exterior of the root (see Fig. 2.4[b]). ECM send hyphae out several meters into the surrounding soil. The hyphae aid in nutrient uptake, including inorganic and some organic nitrogen-phosphorus compounds (Read, 1991). The hyphae constitute a significant proportion of carbon allocated to below-ground NPP in coniferous forests (Vogt et al., 1982). The reproductive structures of ECM are the often-observed mushrooms in oak or pine forests. ECM will form resting stages, or sclerotia—cordlike bundles of hyphae that can persist for years. ECM, unlike AM, often can be cultured apart from their host plants. Some ECM may have considerable decomposing capabilities and can obtain a portion of their reduced carbon from decomposing substrates (i.e., leaf litter). An innovative study by Hobbie et al. (2002) examined the flows of 13C and 14C to basidiomycete sporo-carps, needles, and litter in a western Oregon forest soil, using accelerator mass spectrometry of 1- to 2-milligram samples of soils and tissues. Mycorrhizal associations were indicated by very young (0-2 years) age of 14C, whereas the saprotrophic genera averaged 10 years in radiocarbon age (Hobbie et al., 2002). With analytical tools now at hand, Hobbie et al.

Ecosystem-Level Consequences of ECM Function

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