In this article we will discuss about:- 1. Nitrogen Cycle in Plants 2. Biological Nitrogen Fixation of Plants 3. Synthesis of Amino Acids in Plants 4. Protein Synthesis in Plants 5. Soil Less Culture (Hydroponics) 6. Application of Fertilizers.
Contents:
- Nitrogen Cycle in Plants
- Biological Nitrogen Fixation of Plants
- Synthesis of Amino Acids in Plants
- Protein Synthesis in Plants
- Soil Less Culture (Hydroponics)
- Application of Fertilizers
1. Nitrogen Cycle in Plants:
Although nitrogen occurs to the extent of about seventy eight percent of air by volume, it is not as a rule utilized by plants in its free state. However, nitrogen occurs in the dry substances of the plant to the extent of 1-3 percent only.
Nevertheless, it is indispensable to the life of the plant, as it is an essential constituent of proteins, chlorophyll and protoplasm. Moreover it is essential for growth, particularly of the leaves. An excess of nitrogen causes vigorous growth of vegetative parts, specially the leaves, but delays reproduction activity.
One of the fundamental biological requirements for life to persist is that the nitrogen cycle should continue to function. During this process, the atomospheric nitrogen is fixed into organic combinations, such as amino-acids, proteins, nucleic acids, etc., in living organisms via inorganic forms as NH4+ (ammonia).
As living organisms die and decay, inorganic nitrogen is liberated.
Ammonification:
The dead remains of animals and plants are decomposed through microbial activities to produce ammonia (NH3).
Nitrification:
Here, ammonia is rapidly converted first to nitrites (NO2), and then to nitrates (NO3). The conversion of ammonia to nitrite is carried out by bacteria Nitrosomonas, and of nitrite to Nitrate by Nitrobacter. Now, nitrate is available to the plant.
Denitrification:
Where nitrate (NO3) may convert into N2 gas by bacteria Pseudomonas. This nitrogen gas may be again fixed in the form of NH4+ through the process of biological nitrogen fixation.
Nitrogen of the soil:
The amount of nitrogen in the soil varies from 0.096 to 0.21 per cent. However, the soil makes the main source of nitrogen for the plant. In the plant the nitrogen exists as inorganic and organic compounds. The chief forms of inorganic compounds are the nitrates and nitrites of potassium and calcium, and also ammonia and its compounds; while the organic compounds are chiefly the proteins.
Normally the ammonium compounds found in the soil are made available for the use of the green plants after conversion into nitrate by the action of certain nitrifying bacteria living in the soil. This process is known as nitrification.
During this process the ammonium compounds are oxidized into nitrate in two stages:
(i) These are acted upon by the nitrite-bacteria (Nitrosomonas) and oxidized into (-NO2) and
(ii) The nitrite thus formed is again acted upon by the nitrate-bacteria (Nitrobacter) and further oxidized into nitrate (-NO3). The nitrate, thus produced is readily absorbed by the green plants.
2. Biological Nitrogen Fixation of Plants:
Biological nitrogen fixation is carried out by both free-living and symbiotic bacteria.
The free-living nitrogen fixing bacteria are:
Cyanobacteria, Azotobacter and Clostridium.
Symbiotic N2 Fixation:
Sometimes the ammonium compounds (NH4) are made available to the plants by the nitrogen fixers. The best known nitrogen-fixing symbiotic bacterium is Rhizobium (R. leguminosarum). This bacterium lives in soil to form root nodules in plants of the family Leguminosae such as gram, pea, groundnut, beans, etc.
Root nodules are little outgrowths on roots. When a section of the fresh root nodule is examined, it looks pinkish in colour due to the presence of a pigment called leghaemoglobin. This pigment is closely related to haemoglobin, the red pigment of human blood.
Like haemoglobin, legha emoglobin is an oxygen scavenger. The enzyme-nitrogenase which catalyses the fixation of nitrogen function under anaerobic conditions. Leghaemoglobin combines with oxygen and protects nitrogenase.
Nodule acts as a site for N2 fixation. It contains all the necessary biochemical compounds, such as nitrogenase and leghaemoglobin. The enzyme nitrogenase is a Mo-Fe protein and catalyses the conversion of atmospheric N2 to NH3. This enzyme is extremely sensitive to oxygen to protect it from oxygen; nodules contain an oxygen scavenger, called leghaemoglobin.
Formation of root nodules in leguminous plant:
When a root hair of a leguminous plant comes in contact with Rhizobium (a bacterium) it is curled or deformed. Certain specific chemical substances secreted by bacteria (Rhizobium) are responsible for curling.
At the site of curling of root hair, rhizobia (bacteria) invade the root tissue and proliferate within the root hair.
Some of the bacteria enlarge to become membrane bound structures called bacteroids. The bacteroids cannot divide, and therefore, some bacteria remain untransformed, and they allow infection to spread.
An infection thread made up of plasma membrane is formed, which grows inward from the infected cell of the plant, that separates the infected tissue from rest of the plant.
Cell division is stimulated in the infected tissue and more bacteria invade the newly formed tissues.
It is believed that a combination of cytokinin produced by the invading bacteria and auxin produced by plant cells, promotes cell division and extension, which leads to molecule formation.
The nodule thus formed, is responsible for direct vascular connection with the host for exchange of nutrients.
However, N2 fixation, occurs under the control of plant nod genes and bacterial nod, nif and fix gene cluster.
Reduction of Atmospheric N2:
Here, atmospheric nitrogen is reduced by the addition of hydrogen atoms.
The three bonds between two nitrogen atoms (N°N) are broken and ammonia is formed.
Three main components are required for nitrogen fixation, they are:
(i) A strong reducing agent.
(ii) ATP to transfer hydrogen atoms to dinitrogen, and
(iii) The enzyme systems.
The reducing agent FAD and ATP are provided by photosynthesis and respiration respectively.
Thus formed ammonia is utilised for the synthesis of amino-acids.
These amino-acids are translocated to other parts of the plant, which act as building blocks for the synthesis of various proteins.
3. Synthesis of Amino Acids in Plants:
Amino-acids are supposed to be initial products of nitrogen assimilation.
Each amino-acid consists of at least one carboxyl (-COOH) group, and one or several amino (-NH2) groups.
Synthesis of amino-acids takes place by two main methods.
They are as follows:
(i) Reductive amination, and
(ii) Transamination.
i. Reductive Amination:
Here, ammonia reacts with α-ketoglutaric acid, which results in the formation of glutamic acid.
The reaction is as follows:
Enzyme responsible for this reaction is glutamate dehydrogenase.
ii. Transamination:
This process involves transfer of amino (-NH2) group from one amino acid to the keto group of keto acid.
Glutamic acid is main from which other seventeen amino acids are formed through transamination.
The enzyme responsible for this reaction is called transaminase.
Amides:
There are two most important amides found in plants:
They are:
(a) Asparagine, and
(b) Glutamine.
They are formed from two amino acids, called:
(a) Glutamic acid, and
(b) Aspartic acid.
During this process hydroxyl (-OH) part of the acid is replaced by another (-NH2) radicle.
The enzymes responsible for such reaction may be glutamine synthetase or asparagine synthetase.
Amides have more nitrogen than amino acids, and make structural part of most proteins.
4. Protein Synthesis in Plants:
Proteins consist of one or more polypeptide chains. Each such chain consists of hundreds of amino acids. The number of amino acids varies greatly among proteins, thus, molecular weight of proteins also varies.
The linkage of amino acids and amides in the polypeptide chain occurs through peptide bond, which involves the carboxyl (-COOH) group of one amino acid and the amino (-NH,) group of the next.
The proteins are highly specific because of the sequence in which the amino acids are present in the protein.
Proteins have significant role in the structural and functional organisations of the cell.
Structural proteins constitute various cellular components and some extracellular parts, such as cuticle and fibres.
Functional (enzymatic and hormonal) proteins control almost all metabolic, biosynthetic, bioenergetic, growth regulating, sensory and reproductive activities of the cell.
All the proteins that are required by the cell for its different purposes are synthesized by the cell itself intracellularly.
5. Soil Less Culture (Hydroponics):
In general, soil supplies the mineral nutrients for plant growth. Since the minerals required by plants for their growth are easily absorbed in solution and therefore, it is possible to grow plants in water containing the required amount of mineral salts taking care that aerial parts of the plants are exposed to air and light.
Thus, cultivation of plants by placing the roots in the nutrient solution is called hydroponics. It is necessary to aerate the solution to provide roots with adequate oxygen supply.
The earliest experiment using a culture solution was done by Sachs (1860) and he showed the essentiality of nitrogen for plant growth. Later Knop (1865) gave the formula of nutrient solution.
His prescription for preparing a nutrient solution was used for a long time. About seventy years later Arnon and Hoagland developed a formulation to study the micronutrients as well. Iron was supplied as ferrous sulphate and often it precipitated out.
This problem has now been solved by dissolving the ferrous sulphate along with a chelating agent Na-EDTA (i.e., disodium salt of ethylene-diaminetetra acetic acid.)
In water culture experiments, the seedlings are grown in water containing the known nutrients in a particular proportion. The culture solutions may contain all essential elements except that whose importance has to be studied.
By excluding a particular element in a culture solution characteristic deficiency symptoms can be observed. Deficiency symptoms may vary from species to species. The result obtained from soil-less culture may then be used to determine deficiencies under field conditions.
6. Application of Fertilizers:
Soil fertility is defined as the ability of the soil to provide all essential plant nutrients in appropriate form and in a suitable balance. A soil can be highly fertile, i.e., it has a ready supply of nutrients in suitable form. A fertile soil may be highly saline or alkaline which may not be good for agriculture. The soil poor in fertility may be made fertile by adding fertilizers in suitable dosages.
Soils generally contain sufficient quantities of essential minerals. However three important elements (i.e., NPK-nitrogen, phosphorus and potassium) need to be replenished in crop fields as they are depleted by repeated cultivation.
The common sources of N, P and K in this country are – sodium nitrate, ammonium sulphate, ammonium nitrate, ammonium chloride, urea calcium ammonium nitrate, superphosphate, bonemeal, rock phosphate and calcium magnesium phosphate.
The bags of NPK fertilizers are labelled 15-15-15 or 17-18-9 which refer to the percentage by weight of nitrogen, phosphorus and water soluble potassium. These fertilizers are used in different dosages according to the climatic conditions, soil and crop.