We have usually learned about the root structure of plants grown in a greenhouse, but the roots (specially young roots) of plants growing in nature (in perhaps 97% species of plants) appear different. This is because they are infected with fungi (which is found in abundance in native soil) and form mycorrhiza.
Mycorrhiza meaning ‘fungus-root’ is a symbiotic (intimate) and mutualistic (mutually benefitted) association between the plant’s living root cells, primary cortical and epidermal cells and anon-pathogenic or weakly pathogenic fungus. They occur in young roots because the young roots are tender unlike the older parts where the epithelium and cortex are lost and a protective layer of suberin develops in cork cells. The growth of the fungus slows or ceases the growth of root hairs thus reducing the surface area of the roots but here the soil penetration is increased by the slender fungal hyphae extending from the mycorrhizae.
Two main groups of mycorrhizae are recognized: -
i.Ectomycorrhizae (Marks and Kozlowski, 1973) (example: - Pisolithus albus association with Eucalyptus camaldulensis.)- the hyphae of the fungi forms a mantle outside and inside the root in intercellular spaces of the epidermis and cortex. No intracellular penetration into epidermis or cortical cells occurs, but an extensive network of hyphae is formed between cells called ‘Hartig net’. Occurring mainly in members of Pinaceae family.
ii.Endomycorrhizae (Gerdemann, 1975; Sanders et al., 1976) (example: - orchid mycorrhizae, genus ‘Rhizoctonia’.)- consists of three subgroups most common of which are the ‘ vesicular arbuscular mycorrhizae (VAM)’. They produce an internal network of hyphae between cortical cells and extends out into the soil. They seem to penetrate directly into the cytosol of cortical cells. Occurring mainly in herbaceous angiosperms and also in gymnosperm genera example: - junipers, thuja etc.
[ although a rare group of mycorrhizae with intermediate properties, known as ‘Ectendotrophic’(example :- in Pinus banksiana by Sistotrema brinkmannii), is sometimes encountered.]
ADVANTAGES OF MYCORRHIZAE: -
They mostly occur in the young roots of plants growing in wild in soils which are mostly nonfertile. The most well documented advantage of mycorrhizae to plants is the increased absorption of phosphate, although absorption of other nutrients and water are also facilitated. Their major benefit is aiding in quicker absorption of such mineral ions that are very essential to plants but otherwise diffuse slowly towards the roots, specially phosphate, NH4+, K+, and NO3- (Chapin, 1980). Without the nutrient absorbing properties of these mycorrhizae, its hopeless to hope for better growth in plants in unfertile soils. For example: - some European pines introduced to the US grew poorly until they were inoculated with mycorrhizal fungi from their native soil.
Thus, there is scope for greater impact on agriculture and forestry through proper use of this mycorrhizae, and we can do it by understanding them better.
By: Sayan Adhikary