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  Section: General Biotechnology / Plant Biotechnology
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Biological Nitrogen Fixation

Factors Affecting Nodule Development
There are many factors which affect formation and longevity of nodules in roots of leguminous plants. These are (i) concentration of inorganic nutrients, (ii) soil temperature (optimum temperature between 25 and 30°C favors nodulation; it is inhibited at cooler and warmer extremes), (iii) light and shading (high light increases nodule numbers, whereas shading depresses nodule weight), (iv) CO2 concentration (high CO2 concentration increases nodule numbers), (v) addition of nitrogen (on addition of nitrogen both numbers and weight of nodules are reduced), and (vi) rhizosphere microorganisms. 

Moreover, carbohydrate storage in plant is increased in abundance of light and CO2 concentration as a result of which the numbers and weight of nodules are increased. Nitrogen retards carbohydrate concentration; therefore, carbohydrate : nitrogen ratio (CNR) theory may be applicable to support nodulation.


Non-Symbiotic N2 fixation




Ecology of diazotrophs


Special features of diazotrophs



Sites of N2 fixation



Nitrogenase and reductants



Presence of hydrogenase



Self regulatory systems


Mechanism of N2 fixation

Symbiotic N2 fixation


Establishment of symbiosis



Host specificity and root hair curling



Infection of root hairs



Nodule development



Nodule development and maintenance


Factors affecting nodule development


Mechanism of N2 fixation in root nodules

Genetics of diazotrophs


Nod genes


Nif genes



Nif gene cloning


Hup genes


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