Nitrogen is a very important element for plant life. In importance, it comes only next to carbon, hydrogen, and oxygen as it figures into the composition of proteins, nucleic acids, enzymes, growth hormones, vitamins, etc. Thus, nitrogen metabolism in plants is a crucial process for their survival.
Leaves consist of about 16.5% of nitrogen of thor dry weight. However, a lesser percentage of this element is present in other vegetative organs. It is one of the main structural components of a living organism.
Most plants cannot absorb this element directly from the atmosphere even though it is flooded in an atmosphere. They have to depend on its supply from the soil in which they grow. There are only a few organisms which can make direct use of atmospheric nitrogen. The main sources of nitrogen are,
- Molecular nitrogen is available only to a few living organisms.
- Nitrates have to be converted into ammonia before entering the metabolism.
- Ammonia compounds in which the NH4 ions are absorbed directly by the roots of plants along with nitrate ions.
- Organic nitrogen is present in soil in the form of proteins or partially destroyed proteins which can be absorbed by plants as amino acids, ammonia or nitrates.
Definition of Nitrogen Metabolism
Nitrogen metabolism is one of the most important aspects of plant life on which animal life depends, directly or indirectly. It is defined as the series of biochemical changes taking place inside or outside which results in the construction of complex nitrogenous food from its simpler derivatives and the destruction of complex nitrogenous food into its components.
According to this definition, nitrogen metabolism in plants includes both anabolic or constructive and catabolic or destructive processes. The important anabolic processes include nitrogen fixation, amino acid synthesis and protein synthesis while the catabolic processes include proteolysis, amino acid destruction, denitrification, and nitrification.
Due to these continuous anabolic and catabolic processes happening in nature, there is a continuous supply of nitrogen to plants and other living organisms. At the same time, the percentage of nitrogen in the atmosphere is also maintained almost constant.
Stages of Nitrogen Metabolism in Plants
The series of reactions of nitrogen metabolism in plants creates a cycle. This supply of nitrogen from the atmosphere and its return to the atmosphere is called the nitrogen cycle. The nitrogen cycle involves,
- Nitrification
- Ammonification
- Denitrification
- Nitrogen fixation
- Synthesis of amino acids
Nitrogen Assimilation
Normally plants absorb nitrogen from the soil in the inorganic form either as nitrates (NO3) or as ammonium (NH4). This process of absorbing nitrogen from the soil is called nitrogen assimilation.
Plants obtain their nitrogen requirements in the following ways.
Organic nitrogen
Many plants utilize organic and inorganic nitrogen, amino acids, amides and also urea as a source of organic nitrogen for their growth. The products formed by the death and decay of plants and animals form the major source of organic nitrogen.
Soil bacteria break down proteins into their components, namely amino acids which may be directly used by the plant or amino acids may be oxidized to ammonia.
Ammoniacal nitrogen
Several plants obtain nitrogen in the form of ammonium salts. This is conditioned by factors such as the nature of the soil and the age of the plant. Plants thrive very well in lime soil supplied with ammoniacal nitrate.
An explanation of this is that the ammonia is oxidized to NH4NO3 by soil bacteria. It then reacts with lime to form CaNO3. Another explanation is that ammonia radical and ammonium are absorbed by plants and hydrogen is left behind.
Nitrate nitrogen
Most of the plants obtain nitrogen in the form of nitrates from the soil. Following are the methods of formation of nitrate in the soil.
Ammonification
Nitrogen in nitrate is present in a highly oxidized state while that in ammonium is in reduced form. Therefore, conversion of nitrate to ammonium is a reducing process. This reduction takes place in many steps. At each step, two electrons are added which are supplied by reduced Co I or NADH as well as Co II or NADPH2.
Soil contains millions of microorganisms which convert organic nitrogenous compounds into nitrates. This process takes place step by step and is done by different groups of bacteria.
- Animal excretions and dead plants and animals are first decomposed to release ammonia.
- This is oxidized to produce nitrate.
- The process of release of ammonia is called ammonification.
- Bacteria such as Bacillus mycoides, B. vulgaris etc are ammonifying bacteria which are responsible for the release of ammonia.
Nitrification
The released ammonia is further oxidized to form nitrates and this process is called nitrification. This conversion is done by nitrifying bacteria, nitrosomonas bacteria, and nitrococcus bacteria which oxidize ammonia into nitrites.
Nitrites are then oxidized to nitrates by Nitrobacter.
Denitrification
Certain microorganisms such as denitrifying bacteria like Bacillus denitrificans, present in the soil can convert the nitrites and nitrates into molecular nitrogen. This process is called denitrification. This is the final step of the nitrogen cycle after which the molecular nitrogen is available for nitrogen fixation for another cycle.
Nitrogen Fixation
Nitrogen fixation is the phenomenon of conversion of free atmospheric nitrogen into nitrogenous salts to make it available for absorption by plants. Nitrogen fixation can be either physical fixation or biological fixation. The latter is more commonly seen in nature.
Physical Nitrogen Fixation
Physical nitrogen fixation involves several steps.
- It starts when the atmospheric nitrogen and oxygen combine under the influence of electric discharge or thunderstorms, to form nitric oxide (NO).
- This nitric oxide further combines with atmospheric oxygen to form nitrogen peroxide NO2).
- When it rains, the nitrogen peroxide combines with rainwater to form nitrous and nitric acids (HNO2 and HNO3).
- These acids come down to the earth’s surface along with rainwater.
- Upon reaching the earth, these acids combine with alkali radicals such as Ca and K present in the soil to react with nitrous acid and nitric acid to form nitrites (NO2-) and nitrates (NO3-), which are soluble in water.
- These soluble forms are readily absorbed by plants through their roots.
Biological Nitrogen Fixation
Biological nitrogen fixation is enabled by living organisms such as non-symbiotic bacteria and symbiotic bacteria, yeast, some actinomycetes, blue-green algae, etc.
The most common forms of symbiotic bacteria occurring in root nodules of plants of the Leguminosae family are the different species of Rhizobium (R. leguminosarum), Bacillus radicicola, etc.
Symbiotic bacteria have been found to occur in root nodules of species of Casuarina, Cycas, Podocarpus, etc and leaf nodules of different species of Pavatta, Dioscorea, etc. Nitrogen fixation is the outcome of the symbiotic relationship between bacteria and plants.
Among free-living bacteria, Clostridium, Azotobacter, etc are the ones which fix atmospheric nitrogen under slightly acidic conditions of the soil. A large number of blue-green algae can fix atmospheric nitrogen. Eg. Nostoc, Anabaena, Cyclindrospermum.
Nitrogen Metabolism in Plants: Synthesis Of Amino Acids
Amino acids are the initial products of nitrogen metabolism in plants. They are synthesized in plant cells from the nitrogen absorbed from soil and carbohydrates.
Amino acids are a result of the reaction between an organic acid and ammonia. Due to the carboxyl group (COOH) and amino group (NH2-), these amino acids can behave as acids towards a basic compound and as a base towards an acidic compound. Thus they are said to be amphoteric compounds.
Plants mostly get their share of nitrogen in the form of nitrates. From the nitrates, ammonia is formed by a reduction process. Thus reduction of nitrates is an important step in the synthesis of amino acids.
Two major mechanisms of nitrate reduction occur in plants- reductive amination and transamination.
Reductive amination
The inorganic nitrogen in the form of ammonia (formed as a result of the reduction of nitrates) reacts with alpha-ketoglutaric acid (from the Krebs Cycle), in the presence of an enzyme named glutamic dehydrogenase and NADPH2 forms the amino acid glutamic acid.
Thus the inorganic form of nitrogen is converted into organic form. Glutamic acid plays an important role in nitrogen metabolism in plants. This conversion is accompanied by amination and reduction at the keto group or the organic acid. This is called reductive amination.
Transamination
Amino acid formation is also done by the transfer of amino groups from one kind of molecule to another. Glutamic acid formed by reductive amination plays an important role in the formation of many other amino acids with the help of the enzyme transaminase and coenzyme pyridoxal phosphate.
Amino acids such as alanine, phenylalanine, valine, leuine etc, are formed in plants by transamination. They may also be formed by transformation of amides and other nitrogenous compounds. Moreover, they have a secondary origin in the hydrolysis of proteins by proteolytic enzymes such as papain, bromelin, etc.
Amino acids thus formed are used for various metabolic activities in the cells as well as for the synthesis of proteins. Proteins are simple chains of amino acid residues connected through peptide linkage. When two amino acids combine, the amino group of one bonds with the carboxyl group of the other to form a peptide bond (CO-NH).
References
- Lu, Jianli & Zhang, Leichen & Lewis, Ramsey & Bovet, Lucien & Goepfert, Simon & Jack, Anne & Crutchfield, James & Huihua, Ji & Dewey, Ralph. (2016). Expression of a constitutively active nitrate reductase variant in tobacco reduces tobacco‐specific nitrosamine accumulation in cured leaves and cigarette smoke. Plant Biotechnology Journal. Doi: 14. 10.1111/pbi.12510
