Download Mineral Nutrition

1
Unit I (Botany)
Study material
B.Sc. III Year
Mineral nutrition
The process including the absorption and utilization of various mineral ions by plants for
their growth and development is called mineral nutrition.
80 – 90 % of tissue is composed of water. The part of the tissue left behind is called the
dry matter and typically it is about 10 -20 % of the original weight. The dry matter mainly
consists of organic compounds. Its 80 % consists of plant cell walls, primarily cellulose and
related carbohydrates. This can be eliminated in the form of gases on combustion at 6000 C. The
residue now left is ash which vary in different plant tissues from about 1- 0.15 % of the dry
weight. A careful analysis of the ash shows that it contains almost all of the chemical elements
present in the soil surrounding plant All these elements are not essential for the plant . Only 16
elements have been so far considered to be essential for the growth.
Criteria of essentiality of an element:- Arnon and Stout 1939 suggested certain criteria that an
element must fulfill in order to be classified as essential. These criteria are.
1. An element is essential if in its absence, the plant can not complete its life cycle.
2. An element is essential if it forms a part of any molecule or a constituent of plant that
in itself is essential for the plant e.g. Nitrogen in protein. Magnesium in chlorophyll. Iron in
cytochrome.
3. The element must act directly inside the plant or not enhance or suppress the
availability of some other element.
From a practical point of view an element is considered essential if plants show
deficiency symptoms when they are raised without that element in the medium, the symptom is
recovered only by injecting the same deficient element.
Sixteen elements have so far been found to fulfill the criteria of essentiality suggested by
Arnon and Stout in 1939.
Sachs and Knop have divided these sixteen elements into two categories based on the
quantity in which they are required by the plant.
1. Macroelements or Major elements or Macronutrients:- These elements are
required by the plants in their large quantities i.e. 1 – 10 mg./ gm of dry weight. The elements
are. C, H, O, N, S, P, Mg, K, Ca. Total = 09
2. Micro elements or Micronutrients or Trace elements:- These elements are required
by the plants in their lesser quantities i.e. 0.1 mg / gm of dry weight. These elements are Fe, Mn,
Bo, Cu, Zn, Mo, Cl. Total = 07
Scientists have added a few more elements to the list of sixteen though they have found
them essential for certain group of plants e.g. Vanadium(Va), Silicon(Si), and Iodine(I) are
essential for certain algae. Almunium(Al) is essential for some ferns. Selenium(Si) is essential
for weeds.
Sources of essential elements:- Depending upon the source of an element for the plant the
elements have been divided into.
1. Mineral elements:- These are the elements which the plants get from the soil e.g. S, P,
Mg, K, Ca, Fe, Mn, Bo, Cu, Zn, Mo, Cl, N.
2. Non mineral elements:- These are the elements which plant gets from the water and
air e.g. C, H, O. Carbon in the form of Co2 from air. Hydrogen in the form of water from the soil
and Oxygen from the air. Nitrogen is included in both the categories mineral and non mineral
element because its source is both atmosphere and soil.
2
Role and deficiency symptoms of Macro elements:
1.Carbon, Hydrogen, and Oxygen:- They are absorbed in the form of Co2 and Water. Co2 is
mostly obtained from air while water is got from the soil. The three elements enter the
composition of all types of organic compounds like carbohydrates, organic acids, fats, proteins,
amino acids, enzymes, hormones, etc. In short they build up the protoplasm. Deficiency of either
Co2 or water cause retardation of growth.
Apart from being a structural element, hydrogen in he form of H+ ions is highly
important. (i). the concentration of H+ ion determines the PH. PH value influences the majority
of reactions going on in the cells. (ii). In respiration oxidation of organic compounds involves the
transfer of hydrogen from them to certain acceptor substances (iii). There is an exchange of H+
ions with cations during salt absorption.
2. Nitrogen:- The available forms of the Nitrogen in the soil are No-3 and NH+4 ions .Although
nitrate ion is preferred and is translocated to leaves as such. Where it is first converted into
Ammonia and then to amino acids. Even plants can absorb Ammonium ions but are first
converted into amino acids in the roots and amino acids are translocated to leaves. NH+4
fertilizers when added to soil are converted to nitrates by the microbes then absorbed by the
plants. Dry plant material contains 2 – 4 % of nitrogen . In green plants protein Nitrogen is by far
the largest Nitrogen fraction and accounts for 80 – 85 % of the total Nitrogen.
Role:- Nitrogen is the mineral element that plants require in greatest amount. It is a
constituent of important organic compounds like. Amino acids, Proteins, Nucleic acids,
Chlorophyll, Hormones, Vitamins, ATP, NADP and NADPH coenzymes
Deficiency symptoms:
1. Poor growth of the plant.
2. The young leaves remain small and the older leaves fall off prematurely.
3. Chlorosis of leaves especially in the older leaves near the base of the plant. Young
leaves may not show this symptom because nitrogen can be mobilized from older leaves. Under
severe conditions they may turn yellow or Tan and fall off from the plant.
4. Under severe conditions necrosis of the leaves occurs.
5. In cereals there is poor tillering, reduction in number of ears and number of grains per
ear.
6. Flower formation is either suppressed or a few flowers are formed fruits and seeds
formed are small and less viable. Potato produces fewer tubers.
7. When nitrogen deficiency develops slowly plants may be markedly slender and often
woody stems. This woodiness may be due to build up of excess carbohydrates that can not be
used in the synthesis of amino acids and other nitrogen compounds and can be used in the
synthesis of anthocyanins which lead to purple colouration of leaves petioles and stems e.g. in
tomato and certain varieties of corn.
8. Branching restricted.
3. Phosphorus:- The major phosphorus containing ions in soil solution are monovalent (H2PO4), bivalent (HPo42-), Trivalent(Po43-), Monovalent is present in acidic soils. Bivalent is present in
neutral soils. Trivalent phosphate is present in alkaline soils. Monovalent is easily available to
roots while bivalent and trivalent are present in bound form. Roots are capable of absorbing
phosphate from solution low in phosphate content. The phosphate content of roots and xylem sap
is about 100 – 1000 fold higher than that of the soil solution.
Role:- Phosphorus is the constituent of nucleoprotein, ATP, NADP, phospholipids
.Another important phosphorus containing compound is phytin found in seeds. It is regarded as
3
the phosphate reserve. During seed germination it is mobilized and converted into other
phosphate forms that are needed in the metabolism of young plants.
Deficiency symptoms:
1. Stunted growth.
2. In cereals there is poor tillering, reduction in number of ears and number of grains per
ear.
3. Older leaves become darkish green in colour, contain small necrotic spots (dead tissue)
4. Stems of annual plants become reddish in colour due to formation of anthocyanin
pigments.
5. Vascular tissues are poorly developed.
6. Death of older leaves.
7. Delay in maturation of plant
8. Production of slender (bur not woody) stems.
9. Some species produce excess anthocyanins giving the leaves a slight purple
colouration but not associated with chlorosis.
10. Lateral buds show prolonged dormancy but active buds are not affected because they
can function as sinks at the expense of the remaining plant
11. Plants are unable to absorb and accumulate salts.
4. Potassium:- It is absorbed as k+ ions at high rates by the plants.
Role:1. It is necessary for meristematic growth.
2. It is involved in maintaining the water status of the plants by maintaining cell turgor.
3. It plays important role in opening and closing of stomata.
4. It is involved in the translocation of photosynthetates.
5. It is the activator of enzymes like,Diastase, Catalase, Invertase.
Deficiency symptoms:
1. Reduced growth rate.
2. First chlorosis and then necrosis on the margins and tips of older leaves.
3. Decrease in turgor.
4. Leaf wilting and abscission.
5. Roots become susceptible to rotting fungi in corn and the plant easily bends to ground
(Lodging).
6. Stem becomes slender and weak with abnormally short internodes.
7. Apical buds may die with the result loss of apical dominance is found.
8. In absence of K+ other ions present exert toxic effects.
5. Sulphur:- It is present in the soil in inorganic and organic form. The inorganic forms of
sulphur in soil consists mainly of So42-. The organic sulphur of the soil is made available to
plants by microbial activity and So42- is produced. Plants mainly absorb sulphur in the form of
So42. Plants can utilize So2 also as a sulphur source.
Role:
1. It is the constituent of most important compounds like amino acids (Cysteine and
Methionine) Vitamins (Lipoic acid , Thiamin , Biotin , Coenzyme A) electron carrier Ferredoxin.
2. Sulphur forms disulphide bridges in proteins.
3. It forms an alkaloid sinigrin (Diallyl disulphide) which gives pungent odour to Onion
and Garlic.
4
Deficiency Symptoms:
1. Rate of plant growth is reduced. Shoot growth is more effected than root growth
2. Chlorotic symptoms occur first in the younger then most recently formed leaves rather
than in old leaves because sulphur is not easily mobilized to the younger leaves in most species..
3. Premature leaf fall.
4. Reduction in nodule formation in legumes.
5. Anthocyanin accumulation.
6. Calcium:- It is absorbed from the soil in the form of Ca 2+ ions.
Role:
1. It is required for cell division because it is used in mitotic spindle during cell division.
2. It is essential for the stabilization of newly synthesized membranes.
3. It is involved in the normal functioning of cell membranes.
4. It is used in the synthesis of new cell walls particularly the middle lamella that
separates newly divided cells.
5. It is activator of enzymes like ATPase, Kinase, Phospholipase, α-amylase and
succinate dehydrogenase.
6. It functions as secondary messenger for hormonal and environmental signals for this it
combines with calmodulin protein present in cytosol and plays role in many metabolic processes.
7. Development of thick cuticle in potomogeton occurs when calcium is supplied in
abundance.
8. It plays role in binding proteins and nucleic acids in chromosomes.
9. It controls metabolism of carbohydrates.
10. Calcium counteracts toxities of other elements eg. Oxalo-aceticacid is converted into
calcium oxalate which is non toxic.
Deficiency symptoms:
1. Reduced meristematic activity because of necrosis of young meristematic regions.
2. Young leaves become deformed (rolling and curling).
3. The surface of apples is pitted with small brown necrotic spots called bitter pit disease.
In tomato this disease is called blossom end rot.
4. Premature drop of flower and fruit by breaking of their stalks
5. Chlorosis starts from margins to middle or in the leaf area between the veins.
7. Magnesium:- Magnesium is absorbed in the form of divalent Mg 2+.
Role:
1. In plant cells magnesium ions have a specific role in the activation of enzymes
involved in respiration, photosynthesis, (rubisco) and the synthesis of DNA and RNA.
2. It is the part of ring structure of chlorophyll molecule.
3. Association of two sub units of ribosomes occurs in presence of Mg.
4. It is essential for the formation of carotenoids.
5. It is involved in the synthesis of magnesium pectate of middle lamella.
Deficiency symptoms:
1. Chlorois between the leaf veins occurring first in the older leaves because of mobility
of this element. This pattern of chlorosis results because the chlorophyll in the vascular bundles
remain unaffected for longer periods than the chlorophyll in the cells between the bundle does. If
the deficiency is extensive the leaves may become yellow or white.
2. Premature leaf fall.
3. Chlorosis is followed by necrosis. Defoliation may also occur.
5
4. Phloem and pith becomes reduced or remain under developed.
5. There is reduced vegetative and reproductive growth.
6. In deficiency of Mg tomato fruits develop pale orange colour, reduced pulp and wooly
flesh.
Role and deficiency symptoms of Micronutrients.
1. Iron:- Plants obtain iron in the form of ferric ions Fe3+
Role:
1. Iron has an important role as a component of enzymes involved in the transfer of
electrons (redox reactions) such as cytochrome. In this role it is reversibly oxidized from Fe2+ to
Fe3+ during electron transfer.
2. It is essential for the development of chloroplasts and maintenance of chlorophyll
though no iron dependent enzyme is involved in chlorophyll synthesis.
3. It is essential for the formation of or activity of nucleic acids and synthesis of proteins.
Deficiency symptoms:
1. Chlorosis between the leaf veins occurring first in the younger leaves because iron can
not be readily mobilized from older leaves.
2. Under conditions of extreme deficiency the veins may also become chlorotic and the
whole leaf may become white.
3. In the leaves of cereals the deficiency is shown by alternate yellow and green stripes
along the length of leaf.
4. Reduced growth because iron deficiency disturbs various plant activities like
photosynthesis. Respiration, utilization of nitrate and protein synthesis.
2. Manganese:- It is absorbed in the form of Mn2+
Role:
1. Manganese activates several enzymes in plants. In particular decarboxylases,
dehydrogenases, involved in the tricarboxylic acid (Krebs) cycle are specially activated by
manganese.
2. The best defined function of manganese is in the splitting of water to liberate oxygen
during photosynthesis
3. It is concerned some what in the formation of chlorophyll and maintenance of lamellar
structure of chloroplast.
4.It is essential for nitrogen metabolism by being required for nitrite and hydroxylamine
reductase.
5. It affects absorption of Calcium and Potassium
6. It is involved in Auxin synthesis.
Deficiency symptoms:
1.The major symptom of manganese deficiency is interveinal chlorosis associated with
the development of small necrotic spots. This chlorosis may occur first in the younger or older
leaves depending upon the species and growth rate.
2. In severe deficiency the leaves show premature fall or do not develop at all.
3. Both roots and stem apices may die back and will show stunted growth.
4. Flowers are often sterile.
5. Its deficiency causes disorganization of lamellar system of chloroplasts.
3. Boron:- It is absorbed in the form of borate (Bo33-).
Role: 1. It plays role in cell elongation, nucleic acid synthesis, hormone responses and
membrane function.
6
2. It plays role in carbohydrate and auxin translocation
3. Pollen germination.
4. It is required for uptake and mobilization of calcium.
5. Maintenance of sugar and starch balance.
6. Pectin formation.
7. Production of root nodules in legumes.
Deficiency symptoms:
1. Death of root and shoot tip, with the result apical dominance is lost causing the plant to
become highly branched.
2. Black necrosis of the young leaves and terminal buds.
3. Fruits, fleshy roots and tubers may exhibit necrosis or abnormalities related to the
breakdown of internal tissues causing diseases like, Heart root in beats (disintegration of internal
tissues), Brown rot or water core of turnip, Internal cork of apple etc.
4. Absence of root nodules in leguminous plants.
5. Disturbance in pollen germination.
6. Stunted growth.
7. Reduced level of pectin.
8. Accumulation of fat.
9. Sugar accumulate, starch is also formed because of their non utilization in the
formation of amino acids.
10. There is a shift from common path way of respiration to pentose phosphate path way.
4. Copper:- It is absorbed as Cupric ions(Cu2+).
Role:
1. It is the constituent of enzymes involved in redox reactions being reversibly oxidized
from Cu+ to Cu2+ like Plastocyanin which is involved in electron transfer during the light reaction
of photosynthesis.Ascorbicacid oxidase and cytochrome oxidase helping in transferring
electrons to oxygen.
2. It enters in composition of enzyme rubisco which takes part in carbon dioxide
assimilation. Tyrosinase which is required in the formation of chlorophyll. Superoxide
dimutase takes part in detoxification of superoxide (O2-) radicals.
3. It is connected with maintenance of carbohydrate/nitrogen balance of plant.
Deficiency symptoms:
1. Production of dark green leaves which may contain necrotic spots. The necrotic spots
appear first at the tips of young leaves and then extend towards the leaf base along the margins.
2. The leaves may also be twisted or malformed
3. Under extreme copper deficiency leaves may abscise prematurely.
4. Its deficiency causes two diseases (a).Exanthema:- In this disease tree bark shows
deep slits from which gum exudes also called die back disease. (b). Reclamation disease:- Tips
of leaves undergo chlorosis, hence it is also called leaf tip disease.
5. Stunted growth.
6. Fruits formed are fewer. They may show necrosis and skin splitting
7. Blackening of potato is also caused by shortage of copper.
5. Zinc:- Zinc is absorbed as Zinc ions (Zn2+).
Role:
1. It is a component of many enzymes like Carbonic anhydrase, alcohol dehydrogenase,
lactic dehydrogenase, glutamic dehydrogenase, carboxypeptidase and alkaline phosphatase.
7
2. It is required for some processes in carbohydrate metabolism, RNA synthesis, Protein
synthesis, and synthesis of auxin or its precursor.
3. It is required for activity of copper containing superoxide dimutase.
4. It is involved in chlorophyll synthesis in some plants.
Deficiency symptoms:
1. Reduction in internodal growth as a result plants display a rosette habit of growth in
which the leaves form a circular cluster radiating at or close to ground (called rosette
disease).The leaves may also be small distorted with leaf margins having a puckered appearance
(little leaf disease). This symptom may result from loss of the capacity to produce sufficient
amount of auxin.
2. In some species older leaves may become intervenously chlorotic and then develop
white necrotic spots called white bud. This may be an expression of zinc requirement for
chlorophyll biosynthesis.
3. Protein synthesis and protein levels are markedly lowered and amino acids
accumulation occurs because of reduction in RNA synthesis.
4. Seed and fruit formation is reduced
5. Accumulation of fatty material but reduction in carbohydrate content.
6. Molybdenum:- It is available in the soil solution as monovalent and divalent molybedates
(HMoO4,- MoO42-) exists as Mo4+ to Mo6+
Role:
1. It is a component of several enzymes including nitrogenase and nitrate reductase.
Nitrogenase converts nitrogen gas to ammonia in nitrogen fixing organisms. Nitrate reductase
catalyses the reaction of nitrate to nitrite.
2. It plays role in the synthesis of ascorbic acid.
3. It plays role in the hydrolysis of organic phosphates.
Deficiency symptoms:
1. Nitrogen deficiency in plants.
2. General chlorosis between veins.
3. Necrosis of older leaves.
4. Whip tail disease in crucifers, in which leaves twist and subsequently die. Leaf now
consists of petiole midrib and some laminar tissues.
5. Flower formation is prevented or fall prematurely.
7. Chlorine:- It is absorbed as Chloride anion (Cl –)
Role:
1. It is required for the water splitting reaction of photosynthesis through which oxygen is
produced.
2. It is required for cell division both in leaves and roots.
3. With Na + and K+ it helps in determining solute concentration and anion cation balance
in cells.
4. It is required for normal production of fruits.
Deficiency symptoms:
1.Reduced growth and wilting of leaf tips
2.Chlorosis and necrosis of the leaves, turn bronze coloured.
3.Stunted roots thickened at the tips
4. Reduced fruiting
8
Absorption and uptake of minerals:- Mineral nutrients are found in soil in usually three forms.
(i). Dissolved:- These mineral ions are dissolved in water in the soil, constituting soil
solution. The soil solution serves as the medium for ion movement to the root surface.
(ii). Adsorbed:- The adsorbed mineral ions are held by soil colloids both inorganic and
inorganic . The soil colloids are mostly negatively charged and hold cations over their surface.
These are not easily lost when the soil is leached by water and they provide a nutrient reserve
available to plant roots.
Mineral anions, nitrate (No-3) and Cl- remain in dissolved form in the soil solution .
Phosphate is bound to soil particles as it replaces their hydroxyl groups. Sulphate (So42-) in
presence of calcium (Ca2+ forms gypsum (CaSo4). It is slightly soluble but it releases sufficient
sulphate to support plant growth.
(iii). Combined form:-These minerals have complex combinations with inorganic or
organic substances. . Minerals are released when complexes break down due to various forces
(physical, Chemical or organic. They then become part of the soil solution or yet adsorbed over
colloidal particles.
The mineral substances available to plants from the soil occur in the form of either
cations or anions. K, Mg, Ca, Fe, Mn, Cu, Mo, and Cl form anions. Nitrogen occur both as
cations(NH4+) and anions (No3- or No2- ). The most common form is anion nitrate.
Some of the adsorbed ions can easily be displaced from the surface of the colloid when
the concentration of the same decreases in the soil solution. This maintains ionic equilibrium .
Others are released by ionic exchange (cation and anion exchange).
Areas of root, absorbing mineral nutrients:- Plants absorb their mineral salts from the soil
through the roots. The most active areas of the root for mineral absorption are the zone of
elongation and root hair.
Mechanism of mineral absorption:Absorption of minerals occurs by two mechanisms. Passive and Active.
1. Passive absorption:-The absorption of minerals by the cell along their concentration
or chemical potential gradient i.e. from higher concentration / chemical potential to low
concentration / chemical potential is called passive absorption. It does not involve the
expenditure of energy.
2. Active absorption:- The absorption of minerals by the cell against concentration or
chemical potential gradient i.e. from low concentration or low chemical potential to high
concentration or high chemical potential is termed active absorption. It involves the expenditure
of energy.
Outer and apparent free space theory:- It is found frequently that when a plant cell or tissue is
transferred from a medium of low salt concentration to the medium of relatively higher
concentration, there is an initial rapid uptake of ions in the outer or free space which is followed
by a slow and steady uptake in the inner space. During rapid initial uptake of ions the metabolic
energy is not involved. If the above plant tissue is returned to the lower salt solution or to the
pure water, some of the ions taken up will diffuse out in the external medium. In other wards a
part of the cell or tissue is open to free diffusion of ions. Since free diffusion implies that ions
can move freely in or out of the tissue, the part of the tissue opened to free diffusion will reach
equilibrium with the external medium and the ion concentration of this part will be same as
found in external medium. The part of the plant cell or tissue which allows diffusion is called an
outer space.
9
This outer or free space is the primary cell wall of the cell consisting of mainly cellulose
microfibrills embedded in an amorphous matrix of two polysaccharides hemicelluloses and
pectin substances. Which are partly made from Polygalacturonic acids having weak carboxylic
acid groups and give negative charge on which hydrogen ions are loosely held when positively
charged ions such as K, Mg, and Ca pass through the cell wall, they displace hydrogen ions.
Cellulose microfibrills which are loosely arranged have pores. The intercellular spaces, the
negatively charged region (Donnan free space) in the amorphous matrix and the pores in the
cellulose microfibrills is the free space or outer space. Entry of ions into outer space seems to be
passive where as entry into and exit from the inner space is active process. Movement of ions is
called flux, inflow is called influx outward is called efflux.
Passive absorption of Minerals:- The various methods of passive absorption can be as follows.
1. Diffusion:- It is the movement of substances from their high concentration to their
lower concentration. Diffusion of solutes to the cell interior can normally take place provided
their concentration is higher in the external medium. Nutrient uptake by the roots lowers the
concentration of
nutrients at the root surface generating concentration gradient in the soil solution surrounding
root.
Diffusion is of two types. Simple diffusion and facilitated diffusion
1. Simple diffusion:- In this process of diffusion membrane do not provide any
special pathway . It is believed to occur through aquaporins through which water soluble gas O2
and Co2 diffuse and lipid matrix through which lipid soluble O2, Co2 and ammonia can pass.
2. Facilitated diffusion:- It occurs through channels and carriers:(i).Channels:- These are transmembrane proteins that function as selective
pores through which molecules and ions can diffuse across the membranes. As long as channel
pore is open solutes that can penetrate the pore diffuse through it extremely rapidly about 108
ions per second. These channels are gated that open and close the pore in response to external
signals including voltage changes, hormone binding etc.
(ii).Carriers:- These do not have pores. The substance to be transported
initially bounds to a specific site on the carrier protein. This requirement for binding allows
carriers to be highly selective for a particular substrate to be transported Binding causes a
conformational change in the protein which exposes the substance to the solution on the other
side of the membrane. The rate of transport by carriers is slower than the channel proteins
because transport of individual molecules or ions occurs. Rate of transport is 102 to 104
molecules per second. Carrier mediated passive transport is some times called facilitated
diffusion because the transport of the ion occur down its concentration gradient or
electrochemical gradient
Channel Protein
10
2. Mass flow:- A number of workers have observed increased salt uptake in rapidly
transpiring plants . Transpiration causes a mass flow of water which can also drive the salt along
with it.
3. Ion exchange:- The anions and cations of the plant cells are exchanged for the anions
and cations of the equivalent charge from the external medium in which tissue is immersed. It
has been experimentally confirmed in excised barley roots that when the roots were placed in the
solution of Potassium bromide having radioactive K+. It was then excised and placed in distilled
water. Radio active K+ did not leak out; however, when the excised root was kept in solution of
KBr with non radioactive K+, there was an exchange of K+ ions. An appreciable number of
radioactive K+ ions come out of the root. Even then the concentration of total K+ remained
constant in the external solution.
The ion exchange mechanism has been explained by the following two theories.
(a). Contact exchange theory:- It was proposed by Jenny and overstreet in 1939.
According to this theory the ions are transferred from soil particles to the root without passing
into free solution. It is well known fact that the ions are adsorbed electrostatically to the surface
of the root cells or clay particles. Due to thermal agitation each of them oscillates within the
small volume of the space. It is termed oscillation volume. When the oscillation volume of the
two ions with same charge overlap, one ion is exchanged with the other.
Suppose H+ is adsorbed on the root cell surface K+ is adsorbed on the clay micelle and
both oscillate in such a way that oscillation volume of H+ overlap with that of the K+. It will
result in the transfer of H+ to clay micelle and K+ to the root surface.
(b). Carbonic acid exchange theory:-The theory explains that the Co2 released during
respiration of the root cells combine with the water to form carbonic acid (H2Co3) in the soil
solution. Carbonic acid dissociates into H+ and HCo3- ions. A cation eg. K+ adsorbed on the clay
micelle may be exchanged with H+ of the soil solution. The cation K+ may diffuse to the root
surface in exchange for H+ ion. Bicarbonate ions may act simultaneously and release anions or
may accompany released cation into root.
Mode of ion absorption through contact exchange
Mode of absorption of ions through carbonic acid exchange
4. Donnan equilibrium:- According to this theory there are certain pre-existing fixed or non
difusable ions inside the cells which can not diffuse out side through membrane. However the
cell membrane is permeable to both anions and cations present in the external medium. If the cell
is immersed in an external salt solution and on the inner side of the membrane there are fixed
ions (-). The movement of equal number of anions and cations take place until product of anion
and cation in the internal solution becomes equal to the product of anions and cations in the
external solution.
Suppose a cell has potassium salt of a large indifusable organic anion (A-). It is placed in
a solution of KBr. K+ is present already in the cell, therefore the external K+ can not directly
diffuse inwardly, however Br- will enter the cell due to its diffusion gradient. Since the internal
(A-) does not diffuse out to balance the ionic equilibrium of the external solution the externally
11
situated K+ ions also passes inwardly to maintain electrical or Donnan equilibrium. The result
will be increased K+ and decreased anion Br- concentration in the interior as compared to
external solution
Passive absorption and accumulation of minerals due to Donnan equllibrium
Active absorption of minerals:- Hober 1945 reported that fresh water alga Nitella accumulated
K+ ions in the cell about 1,000 times more than the concentration of. K+ in the surrounding
medium. The absorption of ions and their retention within the cells at higher concentration
require an expenditure of energy by the plants. There is a direct relationship between metabolic
energy and salt uptake. It has been demonstrated experimentally by the number of workers.
1. Steward (1932) and Hopkins91956) observed a close relationship between salt
accumulation and respiration.
2. Robertson and Turner observed that respiratory inhibitors inhibit salt absorption.
3. Lundergardh observed that salt uptake or absorption increases with the increase in rate
of respiration.
Mechanism of active salt uptake can be explained by the carrier concept.
Carrier concept:-. The substance to be transported initially bounds to a specific site on
the carrier protein. This requirement for binding allows carriers to be highly selective for a
particular substrate to be transported Binding causes a conformational change in the protein
which exposes the substance to the solution on the other side of the membrane. This carrier
mediated transport is active also involving the movement of molecules or ions against
concentration gradient with the involvement of metabolic energy.
Active Transport is of two types. Primary and secondary.
1. Primary active transport:- It is directly coupled to a source of energy such as ATP
hydrolysis. The membrane proteins that carry out primary active transport are called pumps. In
the plasma membrane of plants H+ is the principal ion that is electrogenically pumped across the
membrane. The plasmamembrane H+ ATPase generates the gradient of electrochemical potential
of H+ across the plasma membrane. In plant membranes the most prominent pump is H+ and
Ca2+. Therefore another mechanism is needed to drive the active uptake of most mineral
nutrients. The other important way that solutes can be actively transported across the membrane
against their gradient of electrochemical potential is by coupling of the uphill transport of one
solute to the down hill transport of another . This type of carrier mediated co transport is termed
secondary active transport.
2. Secondary active transport:- Protons are extruded from the cytosol by electrogenic
H+ ATPase operating in the plasma membrane. Consequently a membrane potential and a PH
12
gradient are created at the expense of ATP hydrolysis. This gradient of electrochemical potential
for H+ or the proton motive force represents stored free energy in the form of H+ gradient.
The proton motive force generated by electrogenic H+ transport to drive the transport of
many other substances against their gradient of electrochemical potentials
There are two types of secondary transport Symport (Protein = Symporter) and Antiport
(Protein = Antiporter)..
Symport:- In this transport two substances are moving in the same direction through the
membrane.
Antiport:- In this transport down hill movement of protons drives the active (up hill)
transport of a solute in opposite direction.
Translocation of solutes:
There are two paths through which water and dissolved ions might move into the xylem cells of
the root.
1. Apoplast:- In the apoplast pathway mineral ions moves exclusively through the cell
walls without crossing any membrane. The apoplast is the continuous system of the cell walls.
2. Symplast:- In the symplast pathway mineral ons move from one cell to next via the
plasmodesmata. The symplast consists of the entire net work of the cell cytoplasm
interconnected by the plasmodesmata.
At the endodermis ion movement through the apoplast pathway is obstructed by the
casparian strips. The casparian strip is a band of radial cell walls in the endodermis that is
impregnated with the wax like hydrophobic substance suberin. Suberin acts as a barrier to water
and solute movement. The casparian strips breaks the continuity of the apoplast pathway and
forces water and solutes to cross the endodermis through symplast pathway.
Most experts have assumed that
both the symplastic and the
apoplastic pathways contribute
for the transport of most ions.
Experiments with radioactive
isotopes of Cl36 on the water
plant vallisneria show that
these ions take only symplastic
pathway probably through
plasmodesmata. Studies using
radio active isotopes of Ca45 in
Apoplast and symplast pathway of solute translocation
Barley seedlings show that it is transported along apoplast pathway. Mg2+ is also
transported through apoplast pathway.
13
The transport of the mineral elements from the root to the shoot is driven by the gradient
of water potential developed due to transpiration. An increase in transpiration rate enhances both
the uptake and the translocation of mineral elements in the xylem. The lateral transport of ions
from stem xylem to leaves probably take place via xylem transfer cells whose walls facing xylem
are corrugated for providing large surface area for absorption and the cells contain many
mitochondria that are located close to the corrugated wall in order to supply ATP for the active
transport that take place across these walls.
Biological nitrogen fixation
The process by which molecular nitrogen (N2) is reduced to Ammonium is called nitrogen
fixation. It is carried out by prokaryotic organisms, so called biological nitrogen fixation.
Principal nitrogen fixers include.
1. Free living soil Bacteria e.g. Azotobacter, Clostridium, Rhodospirillum,
Beijerinkia, Chromatium. Methanococcus, Derxia, Klebsiella.
2. Free living Cyanobacteria e.g. Anabaena, Nostoc , Aulosira , Cylindrospermum
Totypothrix , Stogonema Calothrix.
3. Symbiotic cyanobacteria e.g. Nostoc with Gunnera a herb, Anabaena with Azola a
water fern, Acetobacter with Sugarcane.
4. Bacteria which live in symbiotic association with members of leguminous pants.
E.g. Rhizobium, Azorhizobium, Bradyrhizobium, Photorhizobium, Sinirhizobium. 90 % ie.
20,000 of the members of family Leguminoceae have root nodules. And 15 % of these plants
have been examined for nitrogen fixation.
5. An actinomycetes Frankia living in association with number of non leguminous
trees and shrubs living in nitrogen deficient soils including members of genera Alnus, Myrica,
Shepherdia , Coraria , Hipophae , Elaegnus , Causurina Datisca etc.
Nitrogen fixation Occurs under anaerobic conditions:
Oxygen irreversibly inactivates the nitrogenase enzyme involved in nitrogen fixation.
Thus each nitrogen fixing organism either functions under natural anaerobic conditions or can
create an internal anaerobic environment in the presence of oxygen.
In Cyanobacetria anaerobic conditions are created in specialized cells called
heterocysts. Heterocysts are thick walled cells that differentiate when filamentous Cyanobacteria
are deprived off NH4+. These cells lack oxygen producing system of chloroplasts which is
photosystem II. Non heterocyst Cyanobacetria fix atmospheric nitrogen in the soil of flooded rice
fields, and die when the fields dry, releasing nitrogen in the soil.
Free living bacteria that are capable of fixing nitrogen are aerobic, facultative or
anaerobic.
1. Aerobic:-These are thought to maintain reduced oxygen conditions through their
high levels of respiration. E.g. Azolla. Other bacteria evolve oxygen photosynthetically during
the day and fix nitrogen during night
2. Facultative generally fix nitrogen under anaerobic conditions
3. Anaerobic: For anaerobic nitrogen fixing bacteria oxygen does not pose any
problem. These can be photosynthetic (Rhodospirillum) or non photosynthetic (Clostridium).
Symbiotic nitrogen fixation in legumes:- It is completed in following steps.
1. Recognition :- The symbiosis between legume and Rhizobia is not obligatory.
Legume seedling germinates without any association with Rhizobia and they may remain
unassociated throughout their life cycle. Rhizobia also occur as free living organism in the soil as
saprophyte. Under nitrogen limited condiotiond however the symbionts seek out one another
14
through an elaborate exchange of signals. This signaling the subsequent infection process and the
development of nitrogen fixing nodules involve specific genes in both host and symbionts.
The first stage in the formation of symbiotic relationship between the nitrogen fixing
bacteria and their host is migration of the bacteria towards the roots of the host plant. This
migration is chemo tactic response mediated through chemical attractants especially (iso)
flavinoids and betaines secreted by the roots. These attractants activate the rhizobial nod D
proteins which then induce transcription of other nod genes.
The nod genes activated by nod D code for nodulation proteins, most of which are
involved in the biosynthesis of nod factors. Nod factors are lipoprotein oligosaccharide signal
molecules all of which have a chitin 1
4 linked N- acetyl glucose amine backbone varying in
length from 3- 6 sugar units and fatty acid accyl.
Host specific nod genes that vary among rhizobial species are involved in the
modification of the fatty accyl chain or the addition of groups important in determining host
specificity.
A particular legume host responds to a specific nod factor. The legume receptors for
nod factors appear to be special lectins (sugar binding proteins) produced in root hairs. Nod
factors activate the lectins. This lectin facilitates attachment of rhizobium to the cell walls of root
hair.
2. Infection : During the infection process rhizobia that are attached to the root hairs
release nod factors that induce a pronounced curling of root hair cells. The curling of the thread
facilitates infection by providing a close environment to the bacterium, where it can enter the
root hair cell by causing its localized enzymatic dissolution. Following the entry of Bacterium
the root hair wall degrades and plasma membrane invaginates and grows into a tube like
structure called infection thread which goes deeper into the cortex near xylem.
3. Nodule formation:- Due to infection of the infection thread containing rhizobia the
cortical cells start dividing forming a distinct area within cortex called as nodule primordia from
which nodule develops. The nodule cells become tetraploid. This nodule develops vascular
system which facilitates the exchange of fixed nitrogen and for nutrition contributed by plants.
When the infection thread with rhizobia reaches to nodule its tip fuses wit the plasma
membrane of host cell releasing bacterial cells that are packed in the membrane derived from
host cell called peribacteroid membrane. Branching of the infection thread inside the nodule
enables the bacteria to infect many cells.
As first bacteria continue to divide soon it stops its division and enlarges to
differentiate into nitrogen fixing endosymbiotic organelles called bacteroids.
Nodules contain an oxygen binding heme protein called leghemoglobin. It is present
in the cytoplasm of infected nodule cells at high concentrations and gives the nodule pink colour.
The host plant produces the globin portion of the leghemoglobin in response to infection by the
bacteria, The bacterial symbiont produces heme protein. Leghemoglobin has a high affinity for
oxygen about ten times higher than the B chain of human hemoglobin. Leghemoglobin stores
enough oxygen so keeps oxygen away from nitrogenase.
Mechanism of nitrogen fixation:●The enzyme catalyzing the conversion of molecular N2 to Ammonia NH3 is called
nitrogenase. It is produced by a group of bacterial genes called nif genes.
●The reduction of N2 to NH3requires electrons and protons. The source of protons are
NADH, NADPH produced during oxidation of carbohydrates Tranlocated from leaves to host in
15
the form of sucrose. The electron donors to nitrogenase enzyme are reduced ferredoxin or
reduced flavodoxin. The bacteria have provision for the same.
●Mo and Fe are integral constituents of nitrogenase and therefore they must be available
to plant in order to ensure the formation of nitrogenase.
● Reduction of molecular nitrogen to ammonia is high energy requiring process
consuming 16 -24 ATP molecules per nitrogen reduced
●Nitrogenase enzyme is made of two subunits. 1. One of these contains non heme iron
protein called Fe- protein or component II or dinitrogenase reductase. 2. Other contains Fe and
Mo and is therefore called Fe- Mo protein or component I or dinitrogenase. Both the components
are required for conversion of N2 to NH3.
In the overall nitrogen reduction reaction ferredoxin serves as an electron donor to the
Fe protein which inturn hydrolyzes ATP and reduces the MoFe protein . The MoFe protein can
reduce the N2 into NH3. The overall reaction for nitrogen fixation is.
N2 + 8 electrons + 8H + 16 ATP
2NH3 + 16ADP + 16Pi + H2.
It is estimated that two to three molecules of ATP are consumed for the transfer of one
electron in the nitrogenase reaction.
The symbiotic nitrogen fixing prokaryots release ammonia that to avoid toxity, must be
rapidly converted into organic forms in the root before being transported to the shoot via xylem.
It is exported in the form of amides eg.asparagine or glutamine by temperate region legumes eg.
Pea (Pisum), Clover (Trifolium) Broad bean (Vicia) and Lentil (Lens). It is also exported in the
form of Ureids (allantoin, allantoic acid, citrulline). by legumes of tropical origin eg. Soyabean
(Glycin), Kidney bean (Phaseolus), Pea nut (Arachis).
A
C
B
D
Different stages of recognition, infection and development of nodule in biological nitrogen fixation.
Nodule structure
The reaction catalyzed by nitrogenase