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  Introduction.                                                                                                                                                           

Organism require many organic and inorganic substances to complete their life cycle. All such substances which they take from outside constitute their nutrition. On the basis of their nutritional requirements, organisms can be classified into autotrophs and heterotrophs. Autotrophs are those organisms which manufacture their organic food by themselves and require only inorganic substance from outside. Thus the nutrition of plants is only inorganic. All green plants (except for some saprophytes and parasites) and photosynthetic bacteria are autotrophs. The heterotrophs, on the other hand, require both organic and inorganic substances from outside. All non-green plants and animals, including human beings, are heterotrophs.

Autotrophic green plants obtain their nutrition from inorganic substances which are present in soil in the form of minerals, which are known as mineral elements or mineral nutrients and this nutrition is called mineral nutrition.

 Essential mineral elements.                                                                                                                               

A variety of mineral elements is present in the soil but all of them are not essential for plants growth. Besides, a particular element may be needed for the growth of one plant and may not be required at all by other plants. An essential element is defined as 'one without which the plant cannot complete its life cycle, or one that has a clear physiological role'. Therefore, in 1939 Arnon and Stout proposed the following characters for judging the criteria of essentiality of an element in the plant :

• The element must be essential for normal growth and reproduction, which cannot proceed without it.
• The requirement of the element must be specific and cannot be replaced by another element.
• The requirement must be direct that is, not the result of any indirect effect e.g. for relieving toxicity caused by some other substance.

Essential elements are divided into two broad categories, based on the quantity in which they are required by plants. Macro-elements and micro-elements. Their ionic forms are respectively called macronutrients and micronutrients. Cations may be absorbed on the surface of negatively charged clay particles. Anions (e.g., nitrate, phosphate, chloride, sulphate, borate) are held to soil particles to a lesser extent. Mineral salts dissolved in soil solution are constantly passing downwards along with percolating (gravitational) water. The phenomenon is called leaching. Leaching is more in case of anions.

1. Macronutrients (Macroelements or major elements) : Which are required by plants in larger amounts (Generally present in the plant tissues in concentrations of 1 to 10 mg per gram of dry matter). The macronutrients include carbon, hydrogen, oxygen, nitrogen, phosphorous, sulphur, potassium, calcium, magnesium.
2. Micronutrients (Microelements or minor elements or trace elements) : Which are required by plants in very small amounts, i.e., in traces (equal to or less than 0.1 mg per gram dry matter). These include iron, maganese, copper, molybdenum, zinc, boron and chlorine. Recent research has shown that some elements, such as cobalt, vanadium and nickel, may be essential for certain plants.

The usual concentration of essential elements in higher plants according to D.W. Rains (1976) based on the data of Stout are as follows :

 

 

Element	% of dry weight
Carbon	45
Oxygen	45
Hydrogen	6
Nitrogen	1.5
Potassium	1.0
Calcium	0.5
Magnesium	0.2
Phosphorus	0.2
Sulphur	0.1
Chlorine	0.01
Iron	0.01
Manganese	0.005
Boron	0.002
Zinc	0.002
Copper	0.0001
Molybdenum	0.0001

 

 Plant analysis.                                                                                                                                                       

1. Ash analysis : This is the simplest method. The plant tissue is subjected to a very high temperature (550-600°C) in an electric muffle furnace and is reduced to ash. The organic matter of the plant is completely oxidised. All carbon, hydrogen and oxygen molecules in the tissue are converted into carbon dioxide and water, both of which escape into the atmosphere as vapours. Besides some nitrogen is also lost as nitrogen gas and ammonia. The plant ash left behind forms a very small proportion of plants dry weight ranging from 2 to 10% only. Analysis of plant ash shows that about 92 mineral elements are present in different plants. Out of these, 30 elements are present in each and every plants and rest are in one or other plants. Out of these 30 elements, 16 elements are necessary for plants and are called essential elements. The ash is chemically analysed to determine these elements.
2. Solution culture (Hydroponics) : In this method plants are grown in nutrient solutions containing only desired elements. To determine the essentiality of an element for a particular plant, it is grown in a nutrient medium that lacks or is deficient in this element.

If the plant grows normally, it indicates that the element is not essential. However, if the plant shows deficiency symptoms then it indicates that the element is essential for that particular plant.

The growing of plants with their roots in dilute solutions of mineral salts instead of soil led to increased understanding of plant nutrition. This cultivation of plants by placing the roots in nutrient solution is called hydroponics. Probably the first recorded use of soilless culture was by Woodward in 1699. In early nineteenth century, plants were grown with their roots immersed in water solutions with inorganic salts alone, without the addition of soil or organic matter. By 1860, the culture solution technique was modernized by Sachs and he showed the essentiality of nitrogen for plant growth. Another significant worker for studying the essentiality of elements was Knop (1865). The method of growing plants in aqueous nutrient solutions as employed by Sachs and Knop is used experimentally and commercially today and known as hydroponic culture. The nutrient solution composition proposed by Knop (1865) and Arnon and Hoagland's (1940) are commonly used. Arnon and

 

 

Hoagland's nutrient medium has the advantage, that it contains micro-nutrients also. Iron was added in the form of ferrous sulphate which often precipitated out. Now a days a chelating agent Na²-EDTA (Disodium salt of ethylene diamine tetra acetic acid. EDTA is a buffer which is used in tissue cultures) is added.

Hydroponics or soilless culture helps in knowing

1. The essentiality of mineral element.
2. The deficiency symptoms developed due to non-availability of particular nutrient.
3. Toxicity to plant when element is present in excess.
4. Possible interaction among different elements present in plant.
5. The role of essential element in the metabolism of plant.

3. Solid medium culture : In this method either sand or crushed quartz is used as a rooting medium and nutrient solution is added to it. The nutrient medium is provided by one of the following methods :
   1. Drip culture : It is done by dripping over the surface.
   2. Slop culture : It is done by having the medium over the surface.
   3. Sub-irrigation : Here the solution is forced up from the bottom of the container.

 Major role of nutrients.                                                                                                                                        

Various elements perform the following major role in the plants :

1. Construction of the plant body : The elements particularly C, H and O construct the plant body by entering into the constitution of cell wall and protoplasm. They are, therefore, referred to as frame work elements. Besides, these (C, H and O) N, P and S also enter in the constitution of protoplasm. They are described as protoplasmic elements.
2. Maintenance of osmotic pressure : Various minerals present in the cell sap in organic or inorganic form maintain the osmotic pressure of the cell.
3. Maintenance of permeability of cytomembranes : The minerals, particularly Ca⁺⁺, K⁺ and Na⁺

maintain the permeability of cytomembranes.

4. Influence the pH of the cell sap : Different cations and anions influence on the pH of the cell sap.
5. Catalysis of biochemical reaction : Several elements particularly Fe, Ca, Mg, Mn, Zn, Cu, Cl act as metallic catalyst in biochemical reactions.
6. Toxic effects : Minerals like Cu, As, etc. impart toxic effect on the protoplasm under specific conditions.
7. Balancing function : Some minerals or their salts act against the harmful effect of the other nutrients, thus balancing each other.

 Specific role of macronutrients.                                                                                                                        

The role of different elements is described below :

1. Carbon, hydrogen and oxygen : These three elements, though can not be categorised as mineral elements, are indispensible for plant growth. These are also called 'framework elements'. Carbon, hydrogen and oxygen together constitute about 94% of the total dry weight of the plant. Carbon is obtained from the carbon

 

 

 

dioxide present in the atmosphere. It is essential for carbohydrate and fat synthesis. Hydrogen and oxygen would be obtained from water which is absorbed by the plants from the soil. Some amount of oxygen is also absorbed from the atmosphere.

(2)Nitrogen

1. Source : The chief source of nitrogen for green plants is the soil. It is absorbed mainly in the form of nitrate

 

3

ions (NO^- ) . The major sources of nitrate for the plants are sodium nitrate, potassium nitrate, ammonium nitrate

 

 

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and calcium nitrate. Under suitable conditions, ammonium ions (NH ⁺ ) may substitute for nitrate ions, being easily absorbed by plants. Ordinary green plants cannot utilize elemental nitrogen which constitutes about 79% of the air. It is also trapped by nitrogen fixing bacteria which live symbiotically in root nodules of the plants.

 

2. Functions : Nitrogen is an essential constituent of proteins, nucleic acids, vitamins and many other organic molecules as chlorophyll. Nitrogen is also present in various hormones, coenzymes and ATP etc. It plays an important role in protein synthesis, respiration, growth and in almost all metabolic reactions.
3. Deficiency symptoms : The symptoms of nitrogen deficiency are as follows :

1. Impaired growth
2. Yellowing of leaves due to loss of chlorophyll, i.e., chlorosis.
3. Development of anthocyanin pigmentation in veins, sometimes in petioles and stems.
4. Delayed or complete suppression of flowering and fruiting.

Excessive supply of nitrogen produces following symptoms :

1. Increased formation of dark green leaves.
2. Poor development of root system.
3. Delayed flowering and seed formation.

(3)Phosphorus

1.  

4

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Source : Phosphorus is present in the soil in two general forms, organic and inorganic. Plants do not absorb organic phosphorus, either from the solid or solution phase of soil. However, organic compounds are decomposed and phosphorus is made available to plants in inorganic form. Soil solution contains phosphorus in

 

 

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inorganic forms as the phosphate ions

 

H 2 PO^-

and

HPO^2- . When pH is low phosphate ions are present in the

 

4

form of H ₂ PO^- . When pH is high, phosphate ions are represented in

 

(ii)Functions

HPO^2- .

1. Phosphorous is present abundantly in the growing and storage organs such as fruits and seeds. It promotes healthy root growth and fruit ripening by helping translocation of carbohydrates.
2. It is present in plasma membrane, nucleic acid, nucleotides, many coenzymes and organic molecules as ATP.
3. Phosphorus plays an indispensable role in energy metabolism i.e., hydrolysis of pyrophosphate and various organic phosphate bonds being used to drive chemical reactions. Thus it is required for all phosphorylation reactions.

 

 

 

 

(iii)Deficiency symptoms

1. Leaves become dark green or purplish.
2. Sometimes development of anthocyanin pigmentation occurs in veins which may become necrotic (Necrosis is defined as localised death of cells).
3. Premature fall of leaves.
4. Decreased cambial activity resulting in poor development of vascular bundles.
5. Root and shoot growth is checked.
6. Prolonged dormancy.
7. Sickle-leaf disease.

(4)Sulphur

1. Source : Sulphur is present as sulphate

SO 2-

in mineral fraction of soil. It is also found in FeS and FeS₂

 

 

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forms, which are not available to plants. In industrialized areas, atmospheric sulphur dioxide trioxide (SO3 ; in low concentration) may be important sources of sulphur nutrition.

 

(ii)Functions

(SO2 )

and sulphur

1. Sulphur is a constituent of amino-acids like cystine, cysteine and methionine; vitamins like biotin and thiamine, and coenzyme A.
2. It increases the nodule formation in the roots of leguminous plants. It favours soluble organic nitrogen and there is decrease in the quantity of soluble nitrogen with its increase.
3. The characteristic smell of mustard, onion and garlic is due to the presence of sulphur in their volatile oils.
4. Sulphur in plants is required in stem and root tips and young leaves. It is remobilised during senescence.

(iii)Deficiency symptoms

1. Leaves remain small and turn pale green i.e., symptoms of chlorosis. Chlorosis affects young leaves more because of immobile property of the sulphur. The young leaves develop orange, red or purple pigment.
2. Leaf tips and margins roll downwards and inwards e.g., tobacco, tea and tomato.
3. Premature leaf fall.
4. Delayed flowering and fruiting.
5. Apical growth is retarded whereas premature development of lateral buds starts.
6. The tea yellow disease is caused in tea plants.
7. Decrease in stroma lamellae and increase in grana stacking.
8. Increase in starch and sucrose accumulation, and decrease in reducing sugars.

(5)Potassium

1. Source : Source of K⁺ to the plants is inorganic compounds like potassium sulphate, potassium nitrate, etc. Potassium is usually present in sufficient amount in clay soils, where it is firmly bound (largely as an exchangeable

 

 

 

base). It is prevalent cation in plants and may be involved in the maintenance of ionic balance in cells. It contains approximately 0.3 to 6.0 percent of whole plant. In seeds, it is found in less amount.

(ii)Functions

1. It differs from all other macronutrients in not being a constituent of any metabolically important compound.
2. It is the only monovalent cation essential for the plants.
3. It acts as an activator of several enzymes including DNA polymerase.
4. It is essential for the translocation of photosynthates, opening and closing of stomata, phosphorylation, synthesis of nucleic acid and chlorophyll.

It takes part in the formation of cell membrane and it is also responsible for maintenance of turgidity of cells. It is considered that whole of potassium in plant is found in soluble form and most of it is contained in cell sap and cytoplasm.

(iii)Deficiency symptoms

1. Mottled chlorosis followed by the development of necrotic areas at the tips and margins of the leaves.
2. K⁺ deficiency inhibits proteins synthesis and photosynthesis. At the same time, it increases the rate of respiration.
3. The internodes become shorter and root system is adversely affected.
4. The colour of leaves may turn bluish green.
5. Widespread blackening or scorching of leaves may occur as a result of increased tyrosinase activity.
6. Rosette or bushy habit of growth may be seen in plants.
7. Reduction of stem growth, weakening of stem.
8. Lowered resistance to pathogens.

Destruction of pith cells of tomato and increased differentiation of phloem elements.

(6)Calcium

1. Source : The element is abundant in most soils and plants under natural conditions are seldom deficient in

it. It is absorbed by the plants in the form of

Ca 2+

from calcium carbonate etc. It occurs abundantly in a non-

exchangeable form such as anorthite onto the surface of clay micelle.

(ii)Functions

(CaAl ₂ . Si₂O₈ ). Much of the exchangeable calcium of the soil is absorbed

1. It is necessary for formation of middle lamella of plants where it occurs as calcium pectate.
2. It is necessary for the growth of apical meristem and root hair formation.

 

 

3. It acts as activator of several enzymes, e.g., ATPase, succinic dehydrogenase, adenylate kinase, etc.
4. Along with Na⁺ and K⁺ it maintains the permeability of plasma membrane.
5. It is involved in the organisation of spindle fibres during mitosis.
6. It antagonises the toxic effects of Na⁺ and Mg⁺⁺.

It is essential for fat metabolism, carbohydrate metabolism, nitrate assimilation and binding of nucleic acids with proteins.

(iii)Deficiency symptoms

1. Ultimate death of meristems which are found in shoot, leaf and root tips.
2. Chlorosis along the margins of young leaves, later on they become necrotic.
3. Distortion in leaf shape.
4. Roots poorly developed or may become gelatinous.
5. Young leaves show malformation and leaf tips becomes hooked.
6. Its deficiency checks flowering and causes the flowers to fall early.

(7)Magnesium

1. Source : Magnesium occurs in the soil in the form of magnesite (MgCO₃), dolomite (MgCO₃, CaCO₃), magnesium sulphate (MgSO₄) and as silicates. It is absorbed from the soil in the form of (Exchangeable cation) ions (Mg⁺⁺). It is easily leached and thus become deficient in sandy soils during rainy season.

(ii)Functions

1. It is an important constituent of chlorophyll.
2. It is present in the middle lamella in the form of magnesium pectate.
3. It plays an important role in the metabolism of carbohydrates, lipids and phosphorus.
4. It acts as activator of several enzymes.
5. It is required for binding the larger and smaller subunits of ribosomes during protein synthesis.
6. It is readily mobile and when its deficiency occurs, it is apparently transferred from older to younger tissues, where it can be neutralised in growth processes.

(iii)Deficiency symptoms

1. Interveinal chlorosis followed by anthocyanin pigmentation, eventually necrotic spots appear on the leaves. As magnesium is easily transported within the plant body, the deficiency symptoms first appear in the mature leaves followed by the younger leaves at a later stage.
2. Stems become hard and woody, and turn yellowish green.
3. Depression of internal phloem and extensive development of chlorenchyma.

 

 

 

 Specific role of micronutrients.                                                                                                                         

(1)Iron

1. Source : It is present in the form of oxides in the soil. It is absorbed by the plants in ferric as well as ferrous state but metabolically it is active in ferrous state. Its requirement is intermediate between macro and micro- nutrients. Therefore, sometimes it is also considered as a macronutrients.
2. Functions : (a) Iron is a structural component of ferredoxin, flavoproteins, iron prophyrin proteins (Cytochromes, peroxidases, catalases, etc.)

2. It plays important roles in energy conversion reactions of photosynthesis (phosphorylation) and respiration.
3. It acts as activator of nitrate reductase and aconitase.
4. Although iron is not a component of the chlorophyll molecules, it is essential for the synthesis of chlorophyll.

(iii)Deficiency symptoms

(a) Chlorosis particularly in younger leaves, the mature leaves remain unaffected. (b) It inhibits chloroplast formation due to inhibition of protein synthesis. (c) Stalks remain short and slender. (d) Extensive interveinal white chlorosis in leaves. (e) It may develop necrosis aerobic respiration severely affected. (f) In extreme deficiency scorching of leaf margins and tips may occur.

(2)Manganese

1. Source : Like iron, the oxide forms of manganese are common in soil. However, manganese dioxide (highly oxidised form) is not easily available to plants. It is absorbed from the soil in bivalent form (Mn⁺⁺). Increased acidity leads to increase in solubility of manganese. In strong acidic soils, manganese may be present in toxic concentrations. Oxidising bacteria in soils render manganese unavailable to plants at pH ranging from 6.5 to 7.8.

(ii)Functions

1. It acts as activator of enzymes of respiration (malic dehydrogenase and oxalosuccinic decarboxylase) and nitrogen metabolism (nitrite reductase).
2. It is essential for the synthesis of chlorophyll.
3. It is required in photosynthesis during photolysis of water.
4. It decreases the solubility of iron by oxidation. Hence, abundance of manganese can lead to iron deficiency in plants.
   3. Deficiency symptoms : (a) Chlorosis (interveinal) and necrosis of leaves. (b) Chloroplasts lose chlorophyll, turn yellow green, vacuolated and finally perish. (c) Root system is poorly developed. (d) Formation of grains is badly affected.
5. 'Grey spot disease' in oat appears due to the deficiency of manganese, which leads to total failure of

crop.

6. 'Marsh spot's in seeds of pea. (g) Deficiency symptoms develop in older leaves.

3. Copper

 

 

1. Source : Copper occurs in almost every type of soil in the form of complex organic compounds. A very small amount of copper is found dissolved in the soil solution. The bivalent copper cation Cu²⁺ is available in plants in exchangeable forms. It is found in natural deposits of chalcopyrite (CuFeS₂).

 

(ii)Functions

1. It activates many enzymes and is a component of phenolases, ascorbic acid oxidase, tyrosinase, cytochrome oxidase.
2. Copper is a constituent of plastocyanin, hence plays a role in photo-phosphorylation.
3. It also maintains carbohydrate nitrogen balance.

(iii)Deficiency symptoms

1. Both vegetative and reproductive growth are reduced.
2. The most common symptoms of copper deficiency include a disease of fruit trees called 'exanthema' in which trees start yielding gums on bark and 'reclamation of crop plants', found in cereals and legumes.
3. It also causes necrosis of the tip of the young leaves (e.g., Citrus). The disease is called 'die back'.
4. Carbon dioxide absorption is decreased in copper deficient trees.
5. Wilting of entire plant occurs under acute shortage.
6. Grain formation is more severely restricted than vegetative growth.

(4)Molybdenum

1. Source : Molybdenum occurs in the soil in three forms – dissolved, exchangeable and nonexchangeable forms. It is available to the plants mostly as molybdate ions. It is required in extremely small quantities by plants. It is found relatively in higher concentration in mineral oil and coal ashes.

(ii)Functions

1. Its most important function is in nitrogen fixation because it is an activator of nitrate reductase.
2. It is required for the synthesis of ascorbic acid.
3. It acts as activator of some dehydrogenases and phosphatases.

Deficiency symptoms

1. Mottled chlorosis is caused in the older leaves as in nitrogen deficiency, but unlike nitrogen-deficient plants, the cotyledons stay healthy and green.
2. It is also known to inhibit flowering, if they develop, they fall before fruit setting.
3. It leads to drop in concentration of ascorbic acid.
4. Its deficiency causes 'whiptail disease' in cauliflower and cabbage. The leaves first show an interveinal mottling and the leaf margins may become gray and flaccid and finally brown.

(5)Zinc

1. Source : Zinc occurs in the soil in the form of ferromagnesian minerals like magnetite, biotite and hornblende. When weathering of these minerals takes place, zinc is liberated in bivalent Zn²⁺ form. Increase in soil pH decreases the availability of zinc.

Bivalent form of zinc (Zn⁺⁺) is exchangeable and is readily available in the soil. Plants require this mineral only in traces and its higher concentrations are highly toxic.

 

 

 

2. Functions : (a) It is required for the synthesis of tryptophan which is a precursor of indole acetic acid-an auxin.

2. It is a constituent of enzymes like carbonic anhydrase, hexokinase, alcohol dehydroge-nase, lactic dehydrogenase and carboxypeptidase.
3. It is required for metabolism of phosphorus and carbohydrates.
4. Zinc also appears to play an important role in protein synthesis because in its absence there is substantial increase in soluble nitrogenous compounds.
   3. Deficiency symptoms : (a) The first symptom appears in the form of interveinal chlorosis of the older leaves, starting at the tips and the margins.

2. Growth becomes stunted due to formation of smaller leaves and shortened internodes. Reduced stem growth is due to less synthesis of auxin.
3. The leaves become distorted and sickle shaped and get clustered to form rosettes. This effect is known as

'little leaf disease'.

4. In maize, zinc deficiency produces 'white bud disease' which leads to greatly reduced flowering and fruiting as well as poorly differentiated root growth.
5. Its deficiency causes khaira disease of rice and mottled leaf of apple, Citrus and walnut.

(6)Boron

1. Source : Boron is present in the soil in very small amounts. It appears in exchangeable soluble and

nonexchangeable forms in the soil

BO3-

or B4

O² . It occurs in highly complex forms such as borosilicates, boric

3

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acids and calcium and manganese borates. It is absorbed from the soil as boric acid (H ₃ BO₃ ) and tetraborate anions. Its calcium and magnesium salts are soluble. Its availability to plant decreases with increase in pH.

 

(ii)Functions

1. It facilitates the translocation of sugars.
2. It is involved in the formation of pectin.
3. It is also required for flowering, fruiting, photosynthesis and nitrogen metabolism.
4. Boron is required for uptake and utilisation of Ca²⁺, pollen germination, seed germination and cell differentiation.
5. It regulates cellular differentiation and development.

(iii)Deficiency symptoms

1. The first major symptom of boron deficiency is the death of shoot tip because boron is needed for DNA synthesis.
2. Generally flowers are not formed and the root growth is stunted.
3. The leaves develop a thick coppery texture, they curve and become brittle.

 

 

4. Some of the physiological diseases caused due to boron deficiency are internal cork of apple, top rot of tobacco, cracked stem of celery, browning of cauliflower water core of turnip, hard fruit of Citrus and heart rot of sugar beets and marigold. These diseases can be cured by application of small doses of sodium tetraborate in the soil.
5. Fruits when affected are severely deformed and useless.
6. Its deficiency checks the cells division of cambium but continues cell elongation.

(7)Chlorine

1. Source : It is absorbed from the soil as chloride ions. It is required in very small amounts and almost all types of soils contain enough chlorine for the plants. Hence, it is rarely supplied as fertilizer.

(ii)Functions

1. It is required for photolysis of water during photosynthesis in photosystem-II.
2. In tobacco, it increases water volume inside the cell and also regulates carbohydrate metabolism.
3. With Na⁺ and K⁺, chlorine helps in determining solute concentration and anion cation balance in the cells.
4. It is essential for oxygen evolution in photosynthesis.
   3. Deficiency symptoms : (a) The deficiency symptoms of chlorine consist of wilted leaves which later become chlorotic and finally attain a bronze colour.

2. Roots become stunted or thickened and club shaped and fruiting is reduced.
3. Photosynthesis is also inhibited.

 Mechanism of absorption of mineral elements.                                                                                           

Plants absorb the minerals from the soil and translocate them to other parts of the body. Soil serves as a main source of mineral salts in which clay crystals with a central nucleus is called micelle. The micelles are negatively charged. To maintain the balance, they hold positively charged ions on their surface. When this balance is disturbed by salt absorption, the equilibrium is again restored by transferring some of the absorbed ions into the solution. The movement of ions is called as flux. The movement of ions into the cell is called influx and outward migration of ions is known as efflux. Various theories have been proposed to explain the mechanism of mineral salt absorption and can be placed under the following two categories.

1. Passive absorption
2. Active absorption

1. Passive absorption : Absorption of ions without the use of metabolic energy is known as passive absorption. This type of absorption is carried out by purely physical forces.

In most of the cases, the movement of mineral ions into root occurs by diffusion. Diffusion of molecules is their net movement down a free energy or chemical potential gradient. The rate of diffusion varies with the chemical potential gradient or the difference in activity (essentially equivalent to concentration) across the diffusion distance.

Briggs and Robertson (1957) demonstrated the passive absorption of ions by root system. They showed :

• Mineral salt absorption is not affected by temperature and metabolic inhibitors.

 

 

 

• Rapid uptake of ions occurs when plant tissues are transferred from a medium of low concentration to high concentration.

Some of the important theories explaining the mechanism of passive absorption of minerals are given below :

1. Mass flow hypothesis : According to Hylmo (1953, 1955), the ion absorption increases with increase in transpiration. The ions have been considered to move in a mass flow with water from the soil solution through the root and eventually to the shoot. The theory was supported by Kramer (1956), Russel and Barber (1960), etc. Later, Lopushinsky (1960) using radioactive P³² and Ca⁴⁵, has supported this experiment.
2. Simple diffusion hypothesis : According to this hypothesis, if the concentration of solutes inside the plant is lower than the soil, the mineral ions are thought to migrate into the root by simple diffusion. As a result, a state of equilibrium is reached. The part of plant cell or tissue that permits free diffusion is sometimes called outer space. The apparent volume that accomodates these ions has been referred to by some workers as apparent free space. In the model of plasma membrane proposed by Danielli and Davson 1935, there are pores of 7Å diameter through which ions can diffuse into the cytoplasm. However, these pores are thought to be unstable in the fluid mosaic model. The accumulation of ions in the cell against concentration gradient can not be explained by this concept.
3. Facilitated diffusion hypothesis : According to this concept, the ions are transported across the membrane by a carrier protein. When the ions enter the cell through protein channels and not through the lipid layer the phenomenon is called facilitated diffusion. The ions combine with the carrier before they move to and fro across the membrane by thermal diffusion. In bacteria this action is performed by certain antibiotics, which are small polypeptide units. These antibiotics are called ionophores. They transport cations into the cell. In this phenomenon there is no participation of metabolic energy.
4. Ion exchange hypothesis : According to this view the ions adsorbed to the cell surface are exchanged from the external medium. A cation is exchanged for a cation and anion for anion. If a particular ion is absorbed by the plant, in exchange it offers H⁺ or OH^– ions which are made available by the dissociation of water molecule.

There are two theories to explain the mechanism of ion exchange.

1. Contact exchange theory : According to this theory, ions are not completely static, they are always oscillating around their absorption surface and when the oscillation volume of the ions on the roots and on the colloidal particles overlap each other, ion exchange occurs. An equilibrium is maintained between the dissolved fractions as any depletion in the soil solution is covered by movement of ions.
2. Carbonic acid exchange theory : In this case, CO₂ released by roots during respiration reacts with water to produce carbonic acid which dissociates into hydrogen ions and bicarbonate ions. Hydrogen ion exchanges itself with the cations adsorbed on the colloidal particles and the bicarbonate ions release the adsorbed anions to supply both anions and cations nearby.

 

Root    H+

 

 

K+

 

 

Clay micelle

 

A

 

 

 

Root     CO2

H₂O H⁺HCO₃^–

12_K+

 

 

 

 

Clay micelle

K⁺HCO₃^–

B

 

 

 

 

 

 

 

 

 

 

 

5. Donnan equilibrium : This mechanism, given by F.G. Donnan (1927), takes into account the effect of non-diffusible ions, which may be present on one side of the membrane. Unlike diffusible ions, the membrane is not permeable to non-diffusible ions. Such ions are termed as fixed ions. They may be anions or cations. In a system, in which there are no fixed ions, there are equal number of anions and cations on both sides of the membrane at equilibrium. But in Donnan equilibrium, in order to balance the charge of the fixed ions (say anions), more ions of the other charge (say cations) would be required.

Mathematically, the Donnan equilibrium may be represented by following equation :

[C⁺ ][A^- ] = [C⁺ ][A^- ]

i              i                     o            o

Here : C⁺ = Cations inside; C⁺ = Cations outside

i                                                                   o

 

 

i

o

A^- = Anions inside; A^-