loose association of nitrogen fixing bacteria
Certain nitrogen fixing bacteria like Azospirillum live around the roots of higher plants without developing any intimate relationship.
It is often called rhizosphere association. The bacteria obtain some plant exudate and use the same as part of their food
requirement. The bacteria fix nitrogen and exude a part of the fixed nitrogen for use by the plant. The phenomenon is termed as
associative mutualism or associative symbiosis.
Free living nitrogen fixing cyanobacteria
A number of free living cyanobacteria or blue-green algae have the property of nitrogen fixation, e.g., Anabaena, Nostoc, Aulosira,
Totypothrix, Cylindrospermum, Stigonema. Aulosira fertilissima is considered to be the most active nitrogen fixer of rice fields in
India (Aiyer et al, 1972). Cylindrospermum licheniforme grows in sugarcane and maize fields. Cyanobacteria are extremely low cost
biofertilisers.
symbiotic nitrogen fixing cyanobacteria
Nitrogen fixing cyanobacteria (blue-green algae) form symbiotic association with several plants, e.g., cycad roots, lichens, liverworts,
Azolla (fern). Azolla-Anabaena association is of great importance to agriculture. Anabaena azollae resides in the leaf cavities of the
fern Azolla where it fixes nitrogen.
mycorrhiza
It is a mutually beneficial or symbiotic association of a fungus with the root of a higher plant. The most common fungal partners
of mycorrhiza are Glomus species. Depending upon the residence of the fungus, mycorrhizae are of two types– ectomycorrhiza
and endomycorrhiza.
In ectomycorrhiza the fungal mycelium completely encloses the feeder rootlets forming sheath or mantle. The hyphae penetrate
the intercellular spaces of the root cortex to form a network, called the ‘Hartig net’. The mantle of fungal hyphae increases the
absorptive surface of roots and hence serve better intake of nutrients such as nitrogen, phosphorus, calcium and potassium from the
surrounding soil. Ectomycorrhizae are known to occur in several genera such as Pinus, Quercus, Betula, Alnus, Salix, Populus etc.
In endomycorrhiza fungus does not form an external sheath of mantle. The fungus mostly lives in the intercellular spaces as well as
intracellularly in the cortical cells of roots. Only a small portion of fungus lives outside the root. Such type of mycorrhizal association
occurs in the roots of grasses, cereals, legumes, rubber, tea, tobacco, soybean, citrus and in many orchids (Neottia, Vanilla, Galeola,
Epipogon, etc.).
One of the important type of endomycorrhizae is vesicular-arbuscular mycorrhizae (VAM). In this association, the fungal
mycelium forms some special kinds of organs, called vesicles and arbuscules, within the root cortical cells. They serve as the
food storage organs of the fungus. Endomycorrhizae stimulate the absorption of phosphorus, zinc, copper, sulphur, potassium
and various other elements by roots. They secrete antimicrobial substances to protect the plants from pathogen attack.
importance of biofertilisers
(i) They increase the yield of plants by 15–35%. (ii) Biofertilisers are effective even under semi-arid conditions.
(iii) Farmers can prepare the inoculum themselves. (iv) They improve soil texture.
(v) Biofertilisers do not allow pathogens to flourish. (vi) They produce vitamins and growth promoting biochemicals.