Mineral Nutrition

MINERAL NUTRITION

POINTS TO REMEMBER :

• Autotrophs : An organism that synthesize its required nutrients from simple and inorganic substances.
• Heterotrophs : An organism that cannot synthesize its own nutrients and depend on others.

Essential Mineral elements :

• More than sixty elements found in different plants.
  
  
  
• Some plant accumulates selenium, some other gold.

Criteria for Essentiality :

• Element absolutely necessary for normal growth and reproduction.
• In the absence of the element the plant can not complete their life cycle.
• Role of the element can not be replaced by any other elements.
• The element must be directly involved in the metabolism of plant.

Macronutrients : are generally present in the plants tissues in large amount (in excess of 10 mmole
Kg^-1 of dry matter).
Micronutrients : or trace elements are needed in very small amounts (less than 10 mmole Kg^-1 of dry matter)
Four group of essential elements :

• As components of biomolecules and forms structural elements of cells (e.g. carbon, hydrogen, oxygen and nitrogen)
• As components of energy-related chemical compounds in plants. (magnesium in chlorophyll and phosphorous in ATP)
• Element that activate or inhibit enzymes  (Mg²⁺, Zn²⁺)
• Alter the osmotic potential of a cell. (K⁺)

Role of macro and micro-nutrients :
Nitrogen :

• Absorbed in the form of NO₂^- or NH₄⁺
• Required by meristematic tissue and metabolically active tissue.
• Constituent of proteins, nucleic acids, vitamins and hormones.

Phosphorus :

• Absorbed in the form of H₂PO₄- or HPO₄^2-.
• Constituents of cell membrane certain proteins, all nucleic acids and required in phosphorylation reaction.

Potassium :

• Absorbed as potassium ion (K⁺)
• Required in meristematic tissues.
• Maintain cation and anion balance in cell.
• Opening and closing of stomata.
• Activation of enzyme.
• Maintenance of turgidity of cells.

Calcium :

• Absorbed in the form of calcium ions (Ca²⁺).
• Required by meristematic and differentiating tissues.
• Used in synthesis of cell wall particularly as calcium pectate in middle lamella.
• Required during formation of mitotic spindle.
• Involved in normal functioning of cell membrane.
• Activate certain enzyme.
• Important role in regulating metabolic activity.

 Magnesium :

• Absorbed in the form of Mg²⁺.
• Activates enzymes of respiration, photosynthesis.
• Involved in the synthesis of DNA and RNA.
• Constituent of the ring structure of chlorophyll.
• Maintain ribosome structure.

Sulphur :

• Absorbed in the form of sulphate SO₄^2-.
• Present in two amino acids cystine and methionine
• Main constituent of several coenzyme, vitamins and ferredoxin.

Iron :

• Obtained in the form of ferric ions (Fe³⁺).
• Required in larger amount in comparison to other elements.
• Constituent of proteins involved in the transfer of electron like ferredoxin and cytochromes.
• Activates catalase enzyme.
• Essential for formation of chlorophyll.

Manganese :

• Absorbed in the form of manganous ions (Mn²⁺).
• Activates many enzymes of photosynthesis, respiration and nitrogen metabolism.
• Photolysis of water and evolution of oxygen during light reaction.

Zinc :

• Obtained in the form of Zn²⁺.
• Activates enzymes like carboxylase.
• Required in synthesis of auxin.

Cupper :

• Absorbed in the form of cupric ions (Cu²⁺).
• Essential for overall metabolism.
• Associated with enzyme involved in redox reactions.

Boron :

• Absorbed in the form of BO₃^3- or B₄O₇^2-.
• Required in uptake and utilization of Ca²⁺.
• Pollen germination.
• Cell elongation.
• Cell differentiation.
• Carbohydrate translocation.

Molybdenum :

• Obtained in the form of molybdate ions (MoO₂^2-).
• Component of enzyme like nitrogenase and nitrate reductase.
• Required in nitrogen metabolism.

Chlorine :

• Absorbed in the form of chloride anion (Cl^-).
• Along with Na⁺ and K⁺ it determines the solute concentration.
• Maintain anion cation balance of the cell.
• Essential for photolysis of water during light reaction of photosynthesis.

Deficiency symptoms of essential elements :

• Critical concentration: the concentration of the essential element below which plant growth is retarded.
• The element is said to be deficient when present below the critical concentration.
• For the elements that are actively mobilized within the plant that show the deficiency symptoms in the older tissues. (nitrogen, potassium and magnesium)
• The deficiency symptoms tend to appear first in the young tissues whenever the elements are relatively immobile and are not transported out of the mature organs.(sulphur and calcium)
• Deficiency symptom includes chlorosis, necrosis, and stunted growth, premature fall of leaves and buds, and inhibition of cell division.
• Chlorosis: is the loss of chlorophyll.
• Necrosis: death of cells and tissues.

Toxicity of Micronutrients :

• Micronutrient required in low amount.
• Moderate decrease causes the deficiency symptoms.
• Moderate increase causes toxicity.
• Any mineral ion concentration in tissues that reduces the dry weight of the tissues by 10 percent is considered toxic.
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Nitrogen cycle :

• Nitrogen fixation: conversion of molecular nitrogen into ammonia.
• Biological nitrogen fixation: Conversion of atmospheric into organic compounds by living organisms.
• Ammonification: decomposition of organic nitrogen of dead plants and animals into ammonia is called Ammonification. (Nitromonasbacteria)
• Nitrification. Ammonia oxidized into nitrite by Nitrosomonasand Nitrococcus bacteria. The nitrite further oxidized to nitrate with the help of Nitrobacter.These steps are called nitrification.
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• Assimilation:
  • Nitrates absorbed by plant from soil and transported to the leaves.
  • In the leaves nitrates reduced to form ammonia that finally forms the amine group of amino acids.
• Denitrification: Nitrate in the soil is also reduced to molecular nitrogen. This process is carried by bacteria like Pseudomonas and Thiobacillus.

Biological nitrogen fixation :

• Reduction of nitrogen to ammonia by living organisms is called biological nitrogen fixation.
• The enzyme nitrogenase which catalyses the process are present in prokaryotes, called nitrogen fixer.
• Nitrogen fixing microbes could be free-living or symbiotic.
• Free-living nitrogen fixing aerobic microbes are Azotobacter and Beijernickia.
• Free-living nitrogen fixing anaerobic microbes are Rhodospirilium.
• A number of cyanobacteria like Anabaena and Nostocare free-living nitrogen fixer.

Symbiotic nitrogen fixation :

• Best example of symbiotic nitrogen fixation is observed in legume-Rhizobium bacteria.
• Rhizobium form root nodules in leguminous plants.
• Frankia also produces nitrogen-fixing nodules on the roots of non-leguminous plants (e.g. Alnus).
• Both Rhizobium and Frankia are free living in soil, but as symbiont, can fix atmospheric nitrogen.
• The root nodules contain pink coloured pigment contains a protein called leg-haemoglobin.

Nodule formation :

• Nodule formation involves a sequence of multiple interactions between Rhizobium and roots of the host plant.
• Rhizobia multiply and colonize the surroundings of roots and get attached to the epidermal and root hair cells.
• An infection thread is produced carrying the bacteria into the cortex of root.
• Bacteria released from the thread into the cells which differentiated into special nitrogen fixing cells.
• Nodule develops vascular connection for exchange of nutrients.
• The nodule contains an enzyme called nitrogenase.
• Nitrogenase is a Mo-Fe protein and catalyses the conversion of atmospheric nitrogen to ammonia.
• Nitrogenase is highly sensitive to molecular oxygen; it requires anaerobic condition.
• Nodule contains a special protein called leg-haemoglobin.
• Leg-haemoglobin acts as oxygen scavenger and provides anaerobic condition to the bacteria inside the nodules; protect the enzyme nitrogenase from oxidation.
• Ammonia synthesis by nitrogenase is energetically expensive process; 8 ATP required synthesizing each molecule of NH₃.
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Fate of ammonia :

• At physiological pH, the ammonia is protonated to form NH₄+.
• Most of plant assimilated nitrate and ammonium ions.
• Reductive amination: the ammonia reacts with α-ketoglutaric acid and forms Glutamic acid.
• Transamination: it involves the transfer of amino group from one amino acid to the keto group of a keto acid.
• Glutamic acid is the main amino acid from which by the process of transamination other amino acids are synthesized.
• Two important amides – asparagines and glutamine found in the protein of plant.
• They are formed from two amino acids namely aspartic acid and Glutamic acid respectively.