Mycorrhizas as biofertilizers

Mycorrhiza (fungus roots) is a distinct morphological structure which develops as a result of mutualistic symbiosis between some specific root – inhabitating fungai and plant roots. Plants which suffer from nutrient scarcity, especially P and N, develop mycorrhiza i.e. the plants belong to all groups e.g. herbs, shrubs, trees, aquatic, xerophytes, epiphytes, hydrophytes or terrestrial ones. In most of the cases plant seedling fails to grow if the soil does not contain inoculum of mycorrhizal fungi.

In recent years, use of artificially produced inoculum of mycorrhizal fungi has increased its significance due to its multifarious role in plant growth and yield, and resistance against climatic and edaphic stresses, pathogens and pests.

Mechanism of symbiosis:
The mechanism of symbiosis is not fully understood. Bjorkman (1949) postulated the carbohydrate theory and explained the development of mycorrhizas in soils deficient in available P and N, and high light intensity. Slankis (1961) found that at high light intensity, surplus carbohydrates are formed which are exuded from roots. This in turn induces the mycorrhizal fungi of soil to infect the roots. At low light intensity, carbohydrates are not produced in surplus, therefore, plant roots fail to develop mycorrhizas.

Types of Mycorrhizas:

By earlier mycologists the mycorrhizas were divided into the following three groups:
i) Ectomycorrhiza: It is found among the gymnosperms and angiosperms. In short roots of higher plants generally root hairs are absent. Therefore, the roots are infected by mycorrhizal fungi which, in turn, replace the root hairs (if present) and form a mantle. The hyphae grow intercellularly and develop Hartig net in cortex. Thus, a bridge is established between the soil and root through the mycelia.
ii) Endomycorrhiza: The morphology if endomycorrhizal roots, after infection and establishment, remain unchanged. Root hairs develop in a normal way. The fungi are present on root surface individually. They also penetrate the cortical cells and get established intracellularly by secreting extracellular enzymes. Endomycorrhizas are found in all groups of plant kingdom.

iii) Ectendomycorrhiza: In the roots of some of the gymnosperms and angiosperms, ectotropic fungal infection occurs. Hyphae are established intracellularly in cortical cells. Thus, symbiotic relation develops similar to the ecto- and endo-mycorrhizas.

Marks (1991) classified the mycorrhizas into seven types on the basis of types of relationships with the host
(i) Vesicular-arbuscular (VA) mycorrhizas (coiled, intracellular hyphae, vesicle and arbuscules present),

(ii) Ectomycorrhizas (sheath and inter-cellular hyphae present),

(iii) Ectendomycorrhizas (sheath optional, inter and intra-cellular hyphae present).

(iv) Arbutoid mycorrhizas (sheath, inter- and intra-cellular hyphae present).

(v) Ericoid mycorrhizas (only coiled intracellular hyphae, long coiled hyphae present)

(vi) Monotropid mycorrhizas (sheath, inter-and intra-cellular hyphae and peg like haustoria present) and

(vii) Orchidaceous mycorrhizas (only coiled intracellular hyphae present).

Type (i) is present in all groups of plant kingdom; Types (ii) and (iii) are found in gymnosperms and angiosperms. Types (iv), (v) and (vi) are restricted to Ericales, Monotropaceae and Ericales respectively. Types (vii) is restricted to Orchidaceous only. Types (iv) and (v) were previously grouped under ericoid mycorrhizas.