CHAPTER 8
MINERAL NUTRITION
Minerals are essential for proper growth and development of plants. Soil is
the reservoir of all the nutrients. These nutrients are absorbed by the root system and is
then translocated to different parts through active transport. The study of the uptake of
mineral nutrients and their use by plants is called mineral nutrition. Most plants
require a relatively small number of nutrient elements in order to complete their growth
and life cycle. However more than 60 elements are reported in different angiosperms
but only 17 elements are considered as essential.
Methods to study the Mineral requirement
The Mineral requirement can be easily identified by a method called
Hydroponics. It is the culturing of plants in liquid nutrient medium without soil. So it is
called soil less culture. This method was developed by Julius von Sachs in 1860.
By adding removing a particular mineral element in to the medium or from the medium,
we can study its effect. When we remove a particular element from the medium, the
plant will show its deficiency symptoms. This symptoms can be rectified by supplying
the same element. Such elements that are essential for the proper growth and
development of plants are known as ESSENTIAL ELEMENTS.
***CRITERIA FOR ESSENTIALITY****
An element is said to be an essential, it should satisfy the following criteria...
1. The element should be necessary for proper growth and development, otherwise the
plant cannot complete its life cycle.
2.The requirement of one element should be specific. (The deficiency of one element
should not be rectified by the supply of another element)
3.The element should have direct involvement in the metabolism.
CLASSIFICATION OF ESSENTIAL ELEMENTS
Essential elements can be classified in to the following types.
a) Based on quantitative requirement.
b) Based on their function
1.Based on quantitative requirement: They are of two types
(i) Macro elements/macronutrients
(ii) Micro elements / Micronutrients
Macronutrients
They are present in plant tissues of more than 10 m mole /kg of dry matter. It
include C, H, O, N, P, K, Ca, Mg, and S.
Micronutrients or trace elements
They are needed for the plants in very small amount (less than 10 m mole /kg of
dry matter). It include Fe, Mn, Ca, Mo,Zn, B, Cl and Ni.
2.Based on the functions
(a) Essential elements that are components of biomolecules Eg:- C, H, O, and N.
(b) Essential elements that are components of energy related chemical compounds.
Eg:- 'P' in ATP and 'Mg' in chlorophyll
(c) Essential elements that can activate or deactivate enzymes.
Eg:- (a) Mg2+ is essential for the enzyme Rubisco and PEPCo.
(b) Zn2+ is essential for the enzyme Alcohol dehydrogenase.
(c) Mo is essential for the enzyme Nitrogenase.
(d) Essential elements that can create an osmotic potential.
Eg: K+ ions. Potassium plays an important role in the opening and closing
of stomata.
ROLE OF MACRO AND MICRONUTRIENTS
Essential elements perform several functions. They participate in various
metabolic processes in the plant cells such as permeability of cell membrane,
maintenance of osmotic concentration of cell sap, electron-transport systems, buffering
action, enzymatic activity and act as major constituents of macromolecules and co-
enzymes. Various forms and functions of essential nutrient elements are given below.
1.Nitrogen- required by plants in greatest amount. It is absorbed by plants as NO2 ,NO3
and NH 4+.It is one of the major constituent of proteins, nucleic acids and
vitamins.
2. Sulphur- Plants obtain sulphur in form of sulphate ( SO 4 2− ). Present in amino acids
(cysteine, methionine) and is main constituent of coenzymes and vitamins.
3. Phosphorus- Absorbed by plants from soil in the form of phosphate ions. It is the
constituent of cell membrane. All nucleic acids and nucleotides require
phosphorus.
4. Potassium – absorbed as potassium ions (K+). Help to maintain cation-anion balance
in cells. It is involved in protein synthesis, opening and closing of stomata.
5. Calcium – absorbed by plants from soil in form of calcium ions (Ca2+).During cell
division it is used in the synthesis of cell wall, particularly as calcium
pectate in the middle lamella. It is also needed during the formation of
mitotic spindle. It activates certain enzymes.
6. Magnesium- absorbed by plants in form of Mg2+ ions. It activates the
enzymes for respiration, photosynthesis, and involved in synthesis of
DNA and RNA. It is constituent of chlorophyll.
7. Iron- Obtained in the form of ferric iron (Fe3+). It is important constituent of protein
involved in electron transport system. It activates catalase enzyme, and
is essential for the formation of chlorophyll.
8. Manganese - Absorbed in form of Mn2+ ions. Main function is splitting of water to
liberate oxygen during photosynthesis.
9. Zinc-Obtained as Zn 2+ ions. Activate enzymes like carboxylase. Needed in
formation of Auxin.
10. Copper – Absorbed as cupric ions(Cu 2+). Involved in various metabolic
activities and redox reactions.
11. Boron-Absorbed as BO3 3− or B4O72− ions. Required for uptake of calcium,
cell elongation and pollen germination. Required in cell differentiation
and carbohydrate translocation.
12. Chlorine – It is absorbed in form of Cl− ions. Along with Na+ & K+,it helps
in determining the solute concentration and the anion-cation balance in
cells. It is essential for the water-splitting reaction in photosynthesis
that leads to oxygen evolution.
CRITICAL CONCENTRATION
It is the optimum amount of mineral elements required for the plants for their
proper growth and development. If the minerals are available above critical
concentration, it is toxic to the plants. This condition is called toxicity. But If It is
available below critical concentration, the plant will show its deficiency symptoms.
The common deficiency symptoms are
(a) Necrosis (Degradation of plant parts especially leaves)
(b) Chlorosis (Yellowing of leaves)
(c) Stunted growth (Retarded growth)
(d) Pre mature leaf and bud fall
(e) Inhibition of cell division.
The deficiency of easily mobilisable elements tend to appear in the
mature plant parts. Eg :- Nitrogen, potassium and magnesium.
Similarly the deficiency of not easily mobilisable elements appear in
the younger parts. Eg: - Calcium.
The deficiency of one element can produce multiple symptoms and different
elements can produce the same symptom. So in this case it is compared with standard
table to identify the mineral which cause the deficiency symptom.
Toxicity of Micronutrients
If the minerals are available above critical concentration, it can cause toxicity.
(A mineral element concentration in tissues that can reduce dry weight of tissues by 10%
is considered as toxic. Eg:- Manganese toxicity
The excess manganese cause toxicity that appear in the form of brown spots.
* Manganese competes for Iron and Magnesium for uptake.
* It competes with 'Mg' to bind with enzymes.
* It inhibits calcium translocation. So Manganese toxicity induce deficiency of Iron,
Magnesium and calcium.
MECHANISM OF ABSORPTION OF ELEMENTS
The process of absorption of minerals takes place in two stages.
(1) Initial rapid uptake of minerals to the free space" or "outer space" of the cell. It is the
apoplast pathway, which is a passive process and takes place through
"porins".
(2) The second phase is the movement of ions slowly in to the "inner space" of the cell.
It is symplast pathway and is an active process. The movement of ions are
called "FLUX". The inward movement of ions in to the cell is called
"INFLUX" and outward movement is called "EFFLUX"
NITROGEN METABOLISM IN PLANTS
Nitrogen is an essential element which is important for the synthesis of
amino acids, proteins, enzymes,hormones,vitamins and nucleic acids. Atmosphere
contains more than 78% nitrogen. But it is not available directly to plants or animals
because it is triple bonded and less reactive.
Nitrogen is available to the plants by the following ways..
(1) Physical Nitrogen fixation =Through lightening.
(2) Industrial N2 fixation =In the form of fertilizers
(3) Biological N2 fixation = With the help of living organisms (microbes)
BIOLOGICAL N2 FIXATION
SYMBIOTIC NITROGEN FIXATION
Symbiosis is a type of interaction between two organism in which both of
them are equally benefited. Symbiotic fixation takes place between Rhizobium and
leguminous plants.
Symbiotic N 2 fixation takes place in two steps.
(i) Nodule formation.
(ii)Nitrogen fixation within the nodule using Nitrogenase enzyme.
I. NODULE FORMATION
Rhizobium is a Gram -ve, rod shaped, motile bacterium. Initially it is free
living and aerobic. During symbiosis all the bacteria in the soil are attracted towards the
root system of plants and group around the root hair. It is achieved by some chemicals
like flavonoids and homoserine produced by the root system (Rhicadhesin proteins of
bacterial cell wall helps in attachment with root hair).
The bacteria cannot enter directly in to the root hair until they get the
signal from the plant. Curling of root hair tip is the signal for the entry. [Rhizobium
secrete nod D proteins that recognizes flavonoid of root system. This protein get
activated end return back to bacterial cell and induce transcription of nod genes (A,B
and C). This nod gene A, B, C produce enzymes for the synthesis of nod factors. Nod
factors are N. acetyl glucosamine. This nod factors induce curling of root hair). When
they get the signal they will enter in to the root system by breaking the cell wall of root
hair tip and enter in the form of linear chain called INFECTION THREAD (in folding
of root hair plasma membrane).
This infection thread finally reach the cortical cells of root and create
uncontrolled cell division, so that the corticalregion bulges outward forming a balloon
like structure called NODULE.
II. NITROGEN FIXATION WITHIN THE NODULE
Nitrogen fixation takes place with the help of an enzyme called
"Nitrogenase".This enzyme requires the following things for its activity.
(1) Anaerobic condition
(2) Mo-Fe complex.
Anaerobic condition is produced by a special type of pigment produced
within the root system called "LEG HAEMOGLOBIN” (It is a pink coloured pigment).
It will absorb all the O2 molecules. Hence it is known as “Oxygen Scavenger.”
ACTIVITY OF NITROGENASE ENZYME
After N 2 fixation, two molecules are ammonia is produced. It is an energy
consuming process which consume 16 ATP molecules.
Nitrogenase
N2 + 8e - + 8H + + 16 ATP 2NH 3 + H 2 + 16ADP + 16Pi
Leg haemoglobin
FATE OF AMMONIA
The ammonia produced is highly toxic and cannot be stored in that form. So it
is converted in to ammonium (NH4+) through protonation. This ammonium is later
used up for the synthesis of aminoacids. It takes place in two steps namely
(a) Reductive amination
(b)Transamination
Reductive amination
In this step, ammonium combines with alpha-ketoglutaric acid to form an amino
acid called Glutamic acid. It is catalysed by the enzyme glutamate dehydrogenase.
Glutamate dehydrogenase
NH 4+ + alpha ketoglutaric acid+NADPH Glutamic acid+ H 2 O + NADP
Transamination
In this step, amino group (NH 2 ) from glutamic acid is transferred
to a keto acid and new amino and is produced. It is catalysed by the enzyme
transaminase. transminase
Glutamic acid + keto acid New amino acid
Similarly amides are also produced in plants. Eg:- Asparagine and Glutamine.
They are formed from the amino acids aspartic acid and glutamic acid by adding an
amino group to the hydroxyl part. So these compounds are rich in nitrogen than amino
acids. They can be directly used up by plants. In some plants like soybean, exports
nitrogen in the form of ureids from nodules along with transpiration stream.
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