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Dual organs of absorption that are formed when symbiotic fungi inhabit healthy absorbing organs (roots, rhizomes, or thalli) of most terrestrial plants and many aquatics and epiphytes.
Mycorrhizae appear in the earliest fossil record of terrestrial plant roots. Roughly 80% of the nearly 10,000 plant species that have been examined are mycorrhizal. Present-day plants that normally lack mycorrhizae are generally evolutionarily advanced. It has been inferred that primitive plants evolved with a symbiosis between fungi and rhizoids or roots as a means to extract nutrients and water from soil. The degree of dependence varies between species or groups of plants. In absolute dependence, characteristic of perennial, terrestrial plants, the host requires mycorrhizae to survive. Some plants are facultative; they may form mycorrhizae but do not always require them. This group includes many of the world's more troublesome weeds. A minority of plant species characteristically lack mycorrhizae, so far as is known, including many aquatics, epiphytes, and annual weeds.
The three major types of mycorrhizae differ in structural details but have many functions in common. The fungus colonizes the cortex of the host root and grows its filaments (hyphae) into surrounding soil from a few centimeters to a meter or more. The hyphae absorb nutrients and water and transport them to host roots. The fungi thus tap far greater volumes of soil at a relatively lower energy cost than the roots could on their own. Moreover, many, if not all, mycorrhizal fungi produce extracellular enzymes and organic acids that release immobile elements such as phosphorus and zinc from clay particles, or phosphorus and nitrogen bound in organic matter. The fungi are far more physiologically capable in extracting or recycling nutrients in this way than the rootlets themselves.
Mycorrhizal fungi are relatively poorly competent in extracting carbon from organic matter. They derive energy from host-photosynthesized carbohydrates. Hosts also provide vitamins and other growth regulators that the fungi need.
The major types are ectomycorrhizae, vesicular-arbuscular mycorrhizae, and ericoid mycorrhizae. Ectomycorrhizae are the most readily observed type. Ectomycorrhizal hosts strongly depend on mycorrhizae to survive. Relatively few in number of species, they nonetheless dominate most forests outside the tropics. Vesicular-arbuscular mycorrhizae (sometimes simply termed arbuscular mycorrhizae) form with the great majority of terrestrial herbaceous plant species plus nearly all woody perennials that are not ectomycorrhizal. Vesicular-arbuscular mycorrhizal hosts range from strongly mycorrhiza-dependent, especially the woody perennials, to faculative, as are many grasses.
Ericoid mycorrhizae are restricted to the Ericales, the heath order. The hosts are strongly mycorrhiza-dependent. Though relatively few in number, heath species dominate large areas around the world and are common understory plants in many forests. Other mycorrhiza types include those special for the Orchidaceae (orchids) and Gentianaceae (gentians). See Ascomycota
The succession of plants from pioneering through seral to climax communities is governed by availability of mycorrhizal propagules. When catastrophic fire, erosion, or clearcutting reduce the availability of mycorrhizal fungi in the soil, plants dependent on those fungi will have difficulty becoming established. Each mycorrhizal fungus has its own array of physiological characteristics. Some are especially proficient at releasing nutrients bound in organic matter, some produce more effective antibiotics or growth regulators than others, and some are more active in cool, hot, wet, or dry times of year than others. Healthy plant communities or crops typically harbor diverse populations of mycorrhizal fungal species. This diversity, evolved over a great expanse of time, is a hallmark of thriving ecosystems. Factors that reduce this diversity also reduce the resilience of ecosystems.
Mycorrhizal inoculation of plants in nurseries, orchards, and fields has succeeded in many circumstances, resulting in improved survival and productivity of the inoculated plants. Inoculation with selected fungi is especially important for restoring degraded sites or introducing exotics. Because ectomycorrhizal fungi include many premier edibles such as truffles, seedlings can also be inoculated to establish orchards for production of edible fungi. See Fungi