Nitrogen's Role in Plant Growth

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Questions and Answers

What is the chief form of nitrogen that plants can utilize for growth?

Nitric acid

Nitrate (correct)

Nitrites

Ammonia

Which of the following substances is NOT a constituent element of chlorophyll?

Oxygen

Carbon

Nitrogen

Phosphorus (correct)

Which process involves the conversion of atmospheric nitrogen into nitrogenous salts?

Biological fixation (correct)

Nitrogen metabolism

Nitrogen respiration

Organic synthesis

Which types of nitrogen-fixing bacteria are categorized as free-living anaerobic?

Clostridium Signup and view all the answers

What role do root hairs play in the infection process of Rhizobium?

They facilitate the deformation and entry of bacteria. Signup and view all the answers

During which natural phenomenon does non-biological nitrogen fixation primarily occur?

Lightning Signup and view all the answers

Which substance is secreted by legume roots to aid in recognizing Rhizobium?

Homo serine Signup and view all the answers

What type of microorganisms are primarily involved in biological nitrogen fixation?

Bacteria Signup and view all the answers

What structure is formed within the root hair cell during the infection by Rhizobium?

Infection thread Signup and view all the answers

Which of the following is an example of non-symbiotic nitrogen fixation?

Azotobacter Signup and view all the answers

Which nitrogenous compound is produced by the oxidation of nitric oxide during non-biological fixation?

Nitric acid Signup and view all the answers

What is the initial condition of free-living Rhizobium bacteria in relation to nitrogen fixation?

They require a host for nitrogen fixation. Signup and view all the answers

What components are found in the tubular infection structure formed inside the root hair?

Mucopolysaccharides and bacteria Signup and view all the answers

Which type of algae includes Gloeothece and Synechococcus?

Unicellular Signup and view all the answers

In which category does the formation of root nodules in leguminous plants fall?

Symbiotic Signup and view all the answers

What is the role of Rhizobium spp. in the context of leguminous plants?

Supply fixed nitrogen to the plant Signup and view all the answers

Which plant is associated with the genus Frankia for nodule formation?

Causuarina equisetifolia Signup and view all the answers

What are the dimensions of root nodules typically like?

From pinhead to one centimeter in size Signup and view all the answers

Which species of Rhizobium is commonly associated with soybeans?

Rhizobium japonicum Signup and view all the answers

Which of the following is an example of non-nodulation in nitrogen fixation?

Lichens Signup and view all the answers

What distinguishes Bradyrhizobium from other Rhizobium species?

It is slow-growing Signup and view all the answers

What is the primary function of reduced ferrodoxin in the electron transport process?

To act as an electron carrier Signup and view all the answers

Which enzymes are primarily involved in the reduction of nitrate to ammonium in plants?

Nitrate reductase and Nitrite reductase Signup and view all the answers

What is the outcome of ammonification in the nitrogen cycle?

Release of ammonia from organic matter Signup and view all the answers

Which compounds are primarily required by plants in larger quantities for nitrogen assimilation?

Nitrates and Ammonium Signup and view all the answers

After the complete reduction of nitrogen gas to ammonia, what happens to the associated enzyme?

It is liberated and can be used again Signup and view all the answers

What is the primary role of the nitrogenase enzyme in nitrogen fixation?

To reduce nitrogen gas into ammonia Signup and view all the answers

Which of the following elements is NOT mentioned as a cofactor necessary for nitrogen fixation?

Copper Signup and view all the answers

What is the function of leghaemoglobin in the host cells?

To maintain a stable oxygen concentration Signup and view all the answers

Which molecule acts as the electron donor in the pathway of nitrogen fixation within root nodules?

Glucose-6-phosphate Signup and view all the answers

What is the primary function of hydrogenase in the nitrogen fixation process?

To catalyze the production of protons and electrons from hydrogen Signup and view all the answers

Which of the following compounds is synthesized in the leaves and transported to the roots for use in nitrogen fixation?

Sucrose Signup and view all the answers

What type of environment is nitrogenase active in?

Anaerobic conditions Signup and view all the answers

Study Notes

Role of Nitrogen in Plants

Plays a vital role in growth, reproduction, and heredity
Essential building block for Chlorophyll, cytochromes, alkaloids and vitamins
Second most important component of plants after water

Sources of Nitrogen

Plants receive Nitrogen from various sources:
- Atmospheric Nitrogen: Makes up 78% of the atmosphere but is unavailable to plants in its free form
- Nitrates, Nitrites and Ammonia: Nitrate is the primary source
- Amino Acids in the soil: Utilized by soil organisms and absorbed by plants
- Organic Nitrogenous Compounds: Absorbed by insectivorous plants from insects

Nitrogen Fixation

The conversion of free nitrogen to nitrogenous salts, making it usable for plants
Two types of nitrogen fixation: Biological and Non-Biological

Non-Biological Nitrogen Fixation

Occurs primarily through lightning during the rainy season
A series of chemical reactions convert atmospheric nitrogen into nitric acid and eventually into usable forms like calcium nitrate and ammonium nitrate

Biological Nitrogen Fixation

Involves micro-organisms converting free nitrogen into nitrogenous salts
Two prominent types: Symbiotic and Non-symbiotic

Non-Symbiotic Nitrogen Fixation

Free-living micro-organisms carry out the process
Includes bacteria (aerobic, anaerobic, photosynthetic and chemosynthetic), blue-green algae, and fungi
Examples of bacteria: Azotobacter, Beijerenckia, Clostridium, Chlorobium, Rhodopseudomonas, Desulfovibrio, Thiobacillus
Examples of blue-green algae: Gloeothece, Synechococcus, Oscillatoria, Tolypothrix, Nostoc, Anabaena
Examples of fungi: yeasts and Pillularia

Symbiotic Nitrogen Fixation

Occurs through a partnership between micro-organisms and plants
Micro-organisms fix nitrogen within the plant
Three categories: Nodule formation in leguminous plants, nodule formation in non-leguminous plants and non-nodulation

Nodule Formation in Leguminous Plants

Occurs in around 2500 species of the Leguminosae family (like Cicer arientium, Pisum, Cajanus, Arachis)
Rhizobium bacteria residing within specialized root nodules fix nitrogen.
The symbiotic relationship provides food and shelter for the bacteria and in return, the bacteria supply fixed nitrogen to the plant.
Even after harvesting, nodules buried in the soil can continue fixing Nitrogen.

Nodule Formation in Non-leguminous Plants

Some plants, apart from legumes, also form root nodules for symbiotic nitrogen fixation.
Examples include: Causuarina equisetifolia (with Frankia), Alnus (with Frankia), Myrica gale (with Frankia), Parasponia (with Rhizobium)
Some plants form leaf nodules as well: Dioscorea, Psychotria
Even gymnosperms can have nitrogen fixing nodules: Podocarpus (root nodules), Pavetta zinumermanniana, Chomelia (leaf nodules)

Non-Nodulation

Nitrogen fixation can occur in symbiotic relationships where nodules are not formed
Examples:
- Lichens (cyanobacteria)
- Anthoceros (Nostoc)
- Azolla (Anabaena azollae)
- Cycas (Nostoc and anabaene)
- Gunnera macrophylla (Nostoc)
- Digitaria, Maize and Sorghum (Spirillum notatum)
- Paspalum notatum (Azotobacter paspali)

Formation of Root Nodules in Legumes

Rhizobium bacteria infect the roots of legumes to form nodules.
Free-living bacteria cannot fix nitrogen until they are within the nodule.
Root secretion of growth factors promotes bacteria multiplication.
The interaction between lectins (carbohydrate containing plant proteins) on legume roots and specific carbohydrate receptors on rhizobial cells allows for recognition and attachment.
Bacterial entry occurs through softened root hairs, eventually forming a tubular infection thread that carries the bacteria deeper into the root.
Within the root hairs, the bacteria multiply, induce cortical cell division, and form nodules on the root surface.

Biochemistry of Nitrogen Fixation

Numerous requirements for nitrogen fixation include:
- Nitrogenase enzyme
- Protective mechanism against Oxygen
- Ferrodoxin
- Hydrogen releasing system or electron donor (Pyruvic acid or glucose/sucrose)
- Constant supply of ATP
- Coenzymes and cofactors like TPP, CoA, inorganic phosphate and Mg+2
- Cobalt and Molybdenum
- A carbon compound

Nitrogenase Enzyme

Crucial role in nitrogen fixation
Active in anaerobic conditions
Consists of two protein subunits:
- Fe-protein (or nitrogen reductase): Smaller subunit with 4Fe & 4S atoms
- Mo Fe-protein (or nitrogenase): Larger subunit with 1-2 Mo, 12-32 Fe, and 24 S atoms

Nitrogen Fixation Process

The overall equation for nitrogen fixation: N2 + 8H+ + 8e- + 16 ATP = 2NH3 + H2 + 16ADP + 16 Pi
- The reduction of N2 to NH3 requires 6 protons and 6 electrons.
- 12 mols of ATP are required.
- One pair of electrons needs 4 ATP.
Simplified equation: N2 + 8H+ + 8e- = 2NH3 + H2
Hydrogenase facilitates the conversion of H2 into protons and electrons.

Pathway of Nitrogen Fixation in Root Nodules

Glucose-6-phosphate acts as an electron donor, starting the process.
Glucose-6-phosphate is converted to phosphogluconic acid, which releases protons and reduces ferrodoxin.
Reduced ferrodoxin is an electron carrier that delivers electrons to the Fe-protein.
ATP and Mg+2 help the reduced Fe-protein activate and reduce the Mo Fe-protein.
The reduced Mo Fe-protein then donates electrons to N2, converting it to NH3.
The enzyme is released once N2 is fully reduced to NH3.

Ammonification

The process of converting organic nitrogen compounds into ammonia

Nitrification

Occurs in two main stages:
- Stage 1: Nitrosomonas bacteria oxidize ammonia to nitrite.
- Stage 2: Nitrobacter bacteria oxidize nitrite to nitrate.

Nitrogen Assimilation

Nitrates and ammonium are the essential nitrogen sources for plants.
Nitrate assimilation involves nitrate uptake, reduction to nitrite, and further reduction to ammonium.
Nitrate Reduction:
- Nitrate is taken up by plant roots.
- Nitrate reductase enzyme reduces nitrate to nitrite in the cytosol.
- Nitrite enters the plastids (chloroplasts in shoots) and is reduced to ammonium by nitrite reductase.
Ammonium Fixation:
- The GS/GOGAT pathway combines ammonium with glutamate to form glutamine and then glutamate.
- These amino acids serve as starting points for the production of other amino acids for protein synthesis.

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Description

Explore the critical functions of nitrogen in plants, including its contributions to growth, reproduction, and essential compounds like chlorophyll. Understand the various sources of nitrogen and the processes of nitrogen fixation, including both biological and non-biological methods.