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Mycorrhizal Fungi

picture of a mycorrhizal fungus
The white mycelium of a mycorrhizal fungus found close to the roots of a recently planted rose bush.

Mycorrhizae are very specialised organisms in the Order Glomales, which are found in close association with about 95% of all plants. (The name mycorrhiza comes from the ancient Greek for fungus and root.) It is a symbiotic relationship where both organisms derive benefit - the fungus is unable to photosynthesize and fix its own carbon so it takes some of the carbohydrates which the plant passes down to its roots. In exchange the plant receives essential nutrients and water from the fungus which has an extensive mycelium structure that reaches out further than the plant roots, so it forms a secondary root system. They also help the plant to withstand the stress of drought and prevent some of the soil-borne pathogens such as Phytophthora and Armillaria from entering the roots.
Examination of fossil plants and studies of DNA sequences, show that this relationship has existed for over 400 million years. It should not be confused with the rhizobial bacteria which have a symbiotic relationship causing root nodules that fix nitrogen. Immobile soil nutrients such as Phospherous (P), Copper and Zinc are made more available, but the level of P is important. At very low levels the fungus competes with the plant for P so it is parasitic, at high levels the plant does not need the fungus and the association will not occur.

As they move nutrients and moisture around, they are very important in 'no dig' methods of growing such as Permaculture. Also cultivating the soil damages the mycelium so reducing their effectiveness.
Some of the Slime Moulds perform a similar function.

There are several forms of the mycorrhizal fungus, but the two main ones are Ectomycorrhizae and Endomycorrhizae.

The Ectomycorrhizal Fungi (EM) have a thick network of cells that form a sheath called the Hartig Net around the root hairs of the associated plant. They do not penetrate into the cells of the plant, hence prefix 'ecto' meaning outside. There is an inward growth of the hyphae (fungal cells) between the cortical cells of the roots.
There is very little evidence of the existance of these fungi above ground, apart from the occasional fruiting bodies that are produced. These can appear as puffballs or mushrooms, eg. Boletus and Amanita, all of which are usually found in close proximity to trees. The Fly-Agaric mushroom, Amanita muscaria, which is illustrated in fairytales with its red cap and white spots, is the fruiting body of the mycorrhizal companion of pine and birch trees.
image of Fly-Agaric mushroom, <i>Amanita muscaria</i> image of Fly-Agaric mushroom, <i>Amanita muscaria</i>
Another well-known Ectomycorrhizal fungus is the truffle, an expensive culinary delicacy used for its unique flavour. It is found in association with trees and is the fruiting body which forms below ground; it depends on being consumed by an animal for dispersal of its spores so this is probably why it has such a pungent odour. There are several species of truffle in the genera Tuber, the best known is probably the Black Truffle (T. melanosporum) which grows exclusively with oak trees. The Summer or Burgundy Truffle T. aestivum, usually found with beech on chalky soil, is the only well-known truffle present in the British Isles though absent from Scotland.
Chanterelles and Porcini which produce above ground fruiting bodies, are two other Ectomycorrhizal fungi that are culinary delicacies. There are about 6000 species of Ectomycorrhizal Fungi (EM) found with about 10% of plant Families - mostly woody species, eg. Beech, Birch and Oak, though not all are edible.
The Endomycorrhizal Fungi (AM) - are a more primitive form and have hyphae which penetrate through the cell walls of the roots and into the cell membrane, but do not enter the protoplasm. The structure of the fungal growth inside the root cells can be many-branched or tree-like so this form of mycorrhiza is also known as Arbuscular Mycorrhizae (AM) - from the Latin arboreus, of a tree because of their shape, not the plants with which they associate. Fine hair-like hyphae reach far out into the soil where they take up the nutrients and water they share with the plant. Another name for this form is Vasicular-arbuscular (VA)
They have been reproducing by asexual spores for millions of years so do not produce fruiting bodies. They can also have many genetically different nuclei within the same individual fungal mass.
There are about 150 species of Endomycorrhizal Fungi which are found with about 85% of plant Families including all of the herbaceous species, grasses and some woody species, eg. Ash, Box and Horse Chestnut.

The mycorrhizal fungi are not perennial and grow for between six months and just over a year, before producing asexual spores by budding in the case of the AM, and fruiting bodies which cast sexual spores in the case of the EM forms.

Plants which are grown in a sterile medium can fail to thrive due to the lack of a beneficial fungal partner. Also the Ectomycorrhizae grow mainly in the upper layers of the soil, so cultivation can make them less available to the roots of establishing seedlings.
Our native hardy Orchids have fat roots without fine feeder roots so rely heavily on mycorrhizal fungi, and transplanted plants are difficult to establish (apart from being illegal unless self-grown and divided). Also the fine dust-like seed do not have enough stored energy to grow leaves without the extra help from the fungi. The use of antifungal chemicals, high nitrogen fertilizers and pollution can deplete the number of these fungi in the soil. Also after growing a crop which does not form mycorrhizal associations the levels will be depleted.

To improve the growth or to help new plants establish more quickly, the spores can be added. They can be purchased in a granular form and are usually applied at the time of planting - placed in the planting hole in contact with the root ball or suspended in a gel for dipping bare-rooted plants.
They are grown in a nutrient film on plant roots in controlled conditions to ensure that pathogenic organisms are excluded. There are some preparations which contain mycorrhizae which are specific for different types of tree, eg. conifers. Others contain a mixture of AM and EM fungi for more generalsied use.
Some plants have a high proportion of these beneficial fungi so their roots can be used to inoculate a new plant. An example is Ribwort Plantain which has a fibrous root ball with arbuscular mycorrhizae, so it can be chopped up and added around the roots of the new plant.
For established trees a process which combines a blast of compressed nitrogen gas with an inoculation of mycorrhizal spores is used. This reduces compaction of the soil, improving the growing conditions and the fungi encourage renewed root development, resulting in a healthier tree. On a more domestic scale the health of a poorly performing tree or shrub can be improved using some of the fungi. The ground under the canopy is loosened with a garden fork, especially at the outer edges where most of the fine feeder roots grow close to the surface, then sprinkle fungal grains over the surface and cover with a thick mulch to keep them moist. The fungi will form a mycelium which will grow down around the roots.
There is anecdotal evidence that mycorrhizal fungi applied at planting time can overcome rose replant syndrome. It has long been known that roses perform very poorly when they are planted in the same ground where others have been growing, but better results have been achieved after using the fungi.

Vegetable crops can also be more productive following inoculation of the plants and the beneficial effects can remain for subsequent crops in a rotation. However members of the Brassicaceae Family do not form associations so using the fungi would not work, but subsequent crops would benefit as levels of indigenous mycorrhizal fungi would have fallen where these crops have been growing.

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