 The white mycelium of a mycorrhiza 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.
There are several forms of the mycorrhizal fungus, but the two main forms are Ectomycorrhizae and Endomycorrhizae.
There is very little evidence of the existance of these fungi above ground, apart from the occasional fruiting bodies that are produced by the Ectomycorrhizal forms. These can appear as puffballs or mushrooms, eg. Boletus and Amanita, all of which are usually found in close proximity to trees.
The Endomycorrhizal Fungi 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.
The mycorrhizae 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 Endo', and fruiting bodies which cast sexual sexual spores in the case of the Ecto' forms.
One 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. 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.
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.
The use of antifungal chemicals, high nitrogen fertilizers and pollution can deplete the number of these fungi in the soil.
To improve the growth or to help new plants establish more quickly, the spores can be added to the soil. They can be purchased in a granular or gel form and are usually applied at the time of planting - sprinkled over the back-filling soil. 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 for more generalsied use.
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. |
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