Biology: Mineral Nutrition

Mineral Nutrition

Hydroponics:

It was given by Julius Von Sachs. Hydroponic is growing of plants in a defined nutrient solution, in the absence of soil.  It helps us to study the effect of adding, removing or varying the concentration of any particular mineral element.  Essential elements can be identified by this method, and their deficiency symptoms can be noted.

Criteria for the essentiality of an element are:

• Absolutely necessary for the completion of the life cycle of a plant; necessary for its growth and reproduction. 
• Its requirement is specific, and not replaceable by any other element.
• Directly involved in the metabolism of the plant

Categories of Essential Elements

• Essential elements are 17.
• Basically categorized according to:

Their requirements:

• Macronutrients – Present in large amounts in tissues (C, H, O, N, P, S, K, Mg, Ca).
• Micronutrients – Present in small amounts in tissues (Fe, Mn, Cu, Mo, Zn, B, Cl, Ni)

Functions performed in a plant:

• Components of bio-molecules (C, H, O, N)
• Components of energy-related chemical compounds (Mg – chlorophyll ; P – ATP)
• Activation / Inhibition of enzymes – Mo (enzyme nitrogenase)
• Elements that activates osmotic potential of cell – K (opening and closing of stomata)

Deficiency Symptoms of Essential Elements

·         If essential elements are below their Critical concentration (amount of nutrients required for normal growth and development of plants), plants show certain morphological and observable characters. Those characters are called as Deficiency symptoms.

Deficiency symptoms:

·         Chlorosis (Loss of Chlorophyll) - leads to yellowing of leaves - N, K, Mg, S, Fe, Mn, Zn, Mo.

·         Necrosis (Death of Tissue) - Ca, Cu, K, Mg

·         Delayed flowering - N, S, Mo

·         Inhibition of Cell Division - N, K, S, Mo

Toxicity of Micronutrients

• Any mineral ion concentration that reduces the dry weight of tissues by 10% is considered toxic.
• Toxicity of one element may lead to deficiency of other elements since the former may inhibit the uptake of latter.
• For example; Mn competes with Fe, Mg for uptake and also inhibits Ca translocation to shoot apex. Therefore, Mn toxicity symptoms are actually same as deficiency symptoms of Fe, Mg, and Ca.

Nitrogen Metabolism

Nitrogen Cycle:

• Nitrogen fixation: The process of conversion of nitrogen (N2) into ammonia (NH3)
• Ammonification: The process of decomposition of organic nitrogen of plants and animals into ammonia.
• Nitrification: The ammonia so formed may volatilise and re-enter the atmosphere, or some of the ammonia may be converted into nitrate by soil bacteria.
• 
  
  These are the steps involved in nitrification.
  The nitrate so formed can be easily absorbed by the plants, and transported to leaves. In leaves, nitrate is reduced to ammonia to form the amine group of amino acids.
• Denitrification: Process of reduction of the nitrate present in soil to nitrogen. Carried out by bacteria like Pseudomonas and Thiobacillus.

Biological Nitrogen Fixation

• Reduction of nitrogen to ammonia by living organisms is called Biological Nitrogen Fixation.
• Certain prokaryotes (bacteria) are able to fix nitrogen because the enzyme nitrogenase is present exclusively in them.
  N ≡ N NH3
• Nitrogen-fixing microbes can be classified as follows:
• Free living : Aerobic (Azotobacter), Anaerobic (Rhodospirillum), Cyanobacteria (Nostoc, Anabaena).
• Symbiotic – with leguminous plants (Rhizobium), with non-leguminous plants (Frankia).

·         It needs three biological components :

o    A reducing agent to transfer hydrogen atom to dinitrogen (N ≡ N)

o    ATP to provide energy

o    Enzyme system , Nitrogenase, Mo- Fe protein and leghaemoglobin.

·         Leg haemoglobin: It is a pink colour pigment similar to haemogolobin of vertebrates and functions as an oxygen scavanger and protects nitrogenase from oxygen.

N2 + 8e− + 8H+ + 16ATP → 2NH3 + H2 + 16ADP + 16Pi

Nodule Formation

·         Root hair comes in contact with Rhizobium. It becomes curved and deformed due to the chemical secretion.

·         Plant forms an infection thread, grows inside and delivers bacteria to the cortical  tissue.

·         Bacteria produce cytokinin and auxin which is produced by the plant to stimulate cell division and enlarge to form nodules.

·         Nodules form contact with vascular tissues and get food.

·         Formation of root nodules and nitrogen fixation occur under the control of nod genes of legumes and nod, nif and fix genes of bacteria.

Synthesis of amino acids



·         Ammonia formed by nitrogen fixation is used for the synthesis of amino acids.

·         There are 2 processes by which amino  acids are synthesized

o    Reductive amination

§  NH4+ reacts with  - ketoglutaric  acid and forms glutamic acid.

§  It is catalysed by glutamate dehydrogenase enzyme.

o    Transamination 

§  Amino group of one amino acid is transferred to keto group of a keto – acid.

§  Glutamic acid is the main amino acid which transfers its amino group (NH2) to form 7 other amino acids by the enzyme transaminase.

o    Amides

§  By the replacement of OH- of the amino acid by NH2 radical.

§  Asparagine  and glutamine   are amines formed from aspartic acid and glutamic acid In the presence of enzyme asparagines synthetase  and glutamine synthetase.