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
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