Mycorrhizae

Mycorrhizae denotes the symbiotic association between a fungus and the root of the plant. In symbiotic associations both the partners are benefitted. The fungus that enters in this symbiotic association is called mycorrhizal fungus. This is also known as ‘fungus root’.

It was Frank the German Botanist who discovered it in 1885. Usually the fungus associates with the roots of higher plants and thus mycorrhizal association plays a significant role for the proper absorption of minerals by the root systemand this markedly influences the general growth of higher plants.

On the basis of morphology and anatomy, mycorrhizae are classified into ectomycorrhizae, endomycorrhizae and ectendomycorrhizae. In ectomycorrhizae, the absorbing roots are almost completely enveloped by a mantle of very compact hyphae by the fungus. From this external mantle certain hyphae enter the cortex of roots and confine to the intercellular spaces of the cortical cells. The stelar portion and also the meristem at the apex are not inhabited by the fungus within the cortex of the root, the intercellular hyphae ramily extensively in the form of the reticulum to which the name ‘Harting-net’ is given. Plants like pines, cedrus, quercus, fagus etc develop ectomycorrhizae. The common fungal partners include cortinarius, amanita, rhizopogon, scleroderma etc. The fungal partners usually belong to basidiomycetes and ascomycetes.

In endomycorrhizae, the mycorrhizae is confined to the cells of the root system, causing internal infections of the cortex of root and the rhizomes of plant. The fungus is confined intracellularly. The most important role of the fungus of endomycorrhizae  is the disintegration of soil particles and the absorption and transport of these released particles into the soil. The formation of endomycorrhizae do  not alter the  morphology of the root system. Since the fungus is confined in the root cells and are not grown externally root hairs are normally formed on the surface of the roots. This is commonly seen in orchids, conifers like Tuniperus, sequoia etc.The endomycorrhizae are of 2 types depending on the nature of fungus. In the first case the fungus has separate mycelium and in the second the hyphae consist of aseptate type. The mycorrhizae formed of aseptate fungi are popularly called VAM [vesicular-arbuscular nycorrhizae]. The hyphae of this fungus branch dichotomously and produce vesicles called haustoria. These vesicles are present with in the host cells called artuscules. This kind of association is very common in bryophytes, gymnosperms and pteridophytes.

In ectendomycorrhizae, both the characters of ecto and endomycorrhizae are seen. Since these relationships are symbiotic, both the partners are equally benefitted. The root produces sugars, aminoacids etc which can be shared by the fungus. The fungus absorbs nutrients for the plants from the soil.

It is found that mostly woodt plants develop mycorrhizae for better absorption of minerals by them. The proper growth of many woody plants especially of forest trees is significantly related to mycorrhizae owing to its great capacity to absorb minerals and supply them to the roots of plants. It is seen that the seedlings of many forest trees fail to establish their growth on grasslands even if they are grown in situations of abundant supply of nutrients. If some forest soil containing the correct fungal partners is mixed with the soil around the seedlings, they are found to restore normal growth. This proves that many of the forest trees establish in the soil with the help of their fungal partner.

Also in several woody plants the mycorrhizae is not a must but it is a helping agent for better and sufficient absorption of mineral elementsfrom nutrient deficient soils. This is because several plants grow normally if they are well supplied with surplus amount of minerals especially nitrogen, phosphorous, potassium etc. even if the roots donot associate with the fungus and form mycorrhizae. But these plants show only very poor growth when mineral quantity of the soil is very low. In this situation mycorrhizae plays a significant role.

Thus it is seen that the mycorrhizae if present in root of higher plants, can increase the absorptive capacity of the root system. It is also found that fungus cannot be multiplied in mass and hence its application directly to field crops becomes very difficult. Hence the fungul propagules are applied to those plants which can be first planted in nurssery and later they are transplanted.

Article by Anju Antony, St Teresas College, Cochin.