Nitrogen is the most essential element for living being next to water. It constitutes 78% of earth atmosphere and present at about 10-15% in living protoplasm. These are present in form of proteins, vitamins, chlorophylls, cytochromes, nucleic acid, hormones and functional group of enzymes in plant cell. Above all nitrogen is the fundamental constituent of nucleic acids which play viral role in regulating metabolism, growth reproduction and heredity.
Therefore plants require nitrogen in larger amount and are act as a principal macro nutrient. Nitrogen is indispensible for plants and is taken up by the plant from external source. Majority of plants cannot utilize nitrogen directly in elemental form. Thus higher plants unable to up take atmospheric nitrogen as such.
Source of nitrogen to plants:
There are different sources of nitrogen from which plant can directly or indirectly in take or assimilate nitrogen.
a) Atmospheric nitrogen :
Atmospheric gases contain nitrogen in form of nitrogen gas and oxide of nitrogen majority of plants can not assimilate it directly. But some bacteria, blue green algae and leguminous plants can fix the atmospheric nitrogen into available form and assimilate.
b) Nitrate, nitrites, ammonia in soil (Inorganic nitrogen) :
Is soil nitrogen present in from of ammonium salts. Well nitrates, nitrires and etc as inorganic nitrogen. Out of these nitrate form can be easily assimilated by higher plants.
c) Amino acids in soil (organic from) :
Many soil micro organisms utilize nitrogen in amino acid form which available in soil by decomposition of dead bodies.
d) Organic nitrogenous compounds (Proteins & amino acids, nucleic Acids) in bodies of insects :
Insectivorous plants fulfill their nitrogen requirement by trapping and digesting insects. These plants includes Picher plants, (nepenthes), sundew plant (drosera), Bladderwort (Utricularia), Venus fly trap (Diones) etc.
Mechanism of nitrogen assimilation in plants:
The process by which nitrogen is taken up by plants for the synthesis of proteins is called nitrogen assimilation. Higher plant absorbs nitrogen preferably in form nitrate.
Therefore nitrogen at what ever form it is present should be transformed in to available nitrate form. Then the nitrates after entering into the root converted or reduced into among and utilized in the tissues making various organic compounds.
Nitrogen assimilation in plants takes place through following steps:
I. Nitrogen fixation:
First atmospheric nitrogen either non-symbiotically or symbiotically converted into nitrates and make it available in soil or root nodules. Some tree living bacteria like Azatobacter, clostridium, chlorobium and some symbiotic bacteria like rhizobium blue green algae like nostoc help in nitrogen fixation.
II. Nitrate reduction :
After fixation in to nitrates, these are enzymatic-ally reduced to nitrites and ammonia and then can be assimilated. Stepwise reduction of nitrate takes place by addition of two electrons and ultimately reduced to ammonia. The electrons supplied to this system by reduces co-enzyme NADH and NADPH.
(a) Reduction of nitrate to nitrite:
Reduction of nitrate to nitrite takes place in presence of the enzyme nitrate reductase which require reduced co-enzyme NADH or NSDPH for its activation.
This enzyme which is molyboflavo protein with an operative syllahydryl group was first isolated by Evans and nason (1953) from neuro-spora and soyabean leaves. The enzyme isolated from beurospora could utilize only reduced co-enzyme isolated from beurospora could utilize only reduced co-enzyme NADPH 2while the enzyme isolated from soyabean leaves could utilize both the reduced coenzyme NADH and NADPH.
The enzyme contain FAD as its prosthetic group with which is associated molybdenum actual reduction of takes place as shown.
Electron are transferred from reduced coenzyme to FAD which become reduced FADH2 from reduced FADH2 the electrons are finally transferred to NO3 through molybdenum so that NO3 and H2O are formed.
This nitrate reduction takes place chiefly in green leaves and roots.
(b) Reduction of nitrite to ammonia:
It takes place in the presence the enzyme nitrite reeducates which requires NADH and NADH2. This reaction occurs in leaves in the presence of light more rapidly and in dark at slow rate.
Manganese seems to be associated with the prosthetic group of this enzyme which was first isolated by Nason et al from Neurospore and soyabean leaves.
Previously it was thought that the reduction of nitrite to ammonia involved the formation of two intermediate compounds like hyponitrites and hydroxylamine but it is uncertain due to –
Hyponitrite is quite unstable
Hydroxylamine is toxic
Moreover, they have never been observed in Free State in the cells.
More over it is believed now that, hyponitrite and hydroxylamine may at the best formed at the surface of enzymes and readily leave the surface of enzymes when they completely reduced in to ammonia or other intermediates.
After ammonia is formed this ammonia enters into the biosynthetic path ways of plant cell, like reductive amination, transpiration etc to produce different amino acids. Usually ammonia reacts with the intermediates of kreb’s cycle in presence of NADPH2. Amino-acids in cells take part in the synthesis of protein and other nitrogenous compounds that help in body building.