Chapter 18 Mineral Nutrition by Teaching Care online coaching classes

 

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
• The requirement of the element must be specific and cannot be replaced by another
• The requirement must be direct that is, not the result of any indirect effect 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.

• 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,
• Micronutrients (Microelements or minor elements or trace elements) : Which are required by plants in very small amounts, 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.

• 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 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.
• 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

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

• The essentiality of mineral
• The deficiency symptoms developed due to non-availability of particular
• Toxicity to plant when element is present in
• Possible interaction among different elements present in
• The role of essential element in the metabolism of

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

 Major role of nutrients.

Various elements perform the following major role in the plants :

• 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 They are described as protoplasmic elements.
• Maintenance of osmotic pressure : Various minerals present in the cell sap in organic or inorganic form maintain the osmotic pressure of the
• Maintenance of permeability of cytomembranes : The minerals, particularly Ca⁺⁺, K⁺ and Na⁺

maintain the permeability of cytomembranes.

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

 Specific role of macronutrients.

The role of different elements is described below :

• 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

• Source : The chief source of nitrogen for green plants is the 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

4

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.

• 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
• Deficiency symptoms : The symptoms of nitrogen deficiency are as follows :

• Impaired growth
• Yellowing of leaves due to loss of chlorophyll, e., chlorosis.
• Development of anthocyanin pigmentation in veins, sometimes in petioles and stems.
• Delayed or complete suppression of flowering and

Excessive supply of nitrogen produces following symptoms :

• Increased formation of dark green
• Poor development of root system.
• Delayed flowering and seed

(3)  Phosphorus

4

4

• 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 Soil solution contains phosphorus in

 

4

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– .

 

• 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
• It is present in plasma membrane, nucleic acid, nucleotides, many coenzymes and organic molecules as
• Phosphorus plays an indispensable role in energy metabolism 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

• Leaves become dark green or
• Sometimes development of anthocyanin pigmentation occurs in veins which may become necrotic (Necrosis is defined as localised death of cells).
• Premature fall of
• Decreased cambial activity resulting in poor development of vascular
• Root and shoot growth is
• Prolonged
• Sickle-leaf

(4)  Sulphur

 

• Source : Sulphur is present as sulphate

SO 2-

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

 

 

4

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

 

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

(iii) Deficiency symptoms

• Leaves remain small and turn pale green 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.
• Leaf tips and margins roll downwards and inwards g., tobacco, tea and tomato.
• Premature leaf
• Delayed flowering and
• Apical growth is retarded whereas premature development of lateral buds
• The tea yellow disease is caused in tea plants.
• Decrease in stroma lamellae and increase in grana stacking.
• Increase in starch and sucrose accumulation, and decrease in reducing

(5)  Potassium

• Source : Source of K⁺ to the plants is inorganic compounds like potassium sulphate, potassium nitrate, 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

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

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

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

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

(6)  Calcium

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

 

1. 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

 

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

 

 

 

• It acts as activator of several enzymes, g., ATPase, succinic dehydrogenase, adenylate kinase, etc.
• Along with Na⁺ and K⁺ it maintains the permeability of plasma
• It is involved in the organisation of spindle fibres during
• 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

• Ultimate death of meristems which are found in shoot, leaf and root tips.
• Chlorosis along the margins of young leaves, later on they become
• Distortion in leaf
• Roots poorly developed or may become
• Young leaves show malformation and leaf tips becomes
• Its deficiency checks flowering and causes the flowers to fall

(7)  Magnesium

• 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

(ii)  Functions

• It is an important constituent of chlorophyll.
• It is present in the middle lamella in the form of magnesium
• It plays an important role in the metabolism of carbohydrates, lipids and
• It acts as activator of several
• It is required for binding the larger and smaller subunits of ribosomes during protein
• 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

(iii) Deficiency symptoms

• Interveinal chlorosis followed by anthocyanin pigmentation, eventually necrotic spots appear on the 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.
• Stems become hard and woody, and turn yellowish
• Depression of internal phloem and extensive development of chlorenchyma.

 

 

 

 

 Specific role of micronutrients.

(1)  Iron

• 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- Therefore, sometimes it is also considered as a macronutrients.
• Functions : (a) Iron is a structural component of ferredoxin, flavoproteins, iron prophyrin proteins (Cytochromes, peroxidases, catalases, )

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

(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

• 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 Oxidising bacteria in soils render manganese unavailable to plants at pH ranging from 6.5 to 7.8.

(ii)  Functions

• It acts as activator of enzymes of respiration (malic dehydrogenase and oxalosuccinic decarboxylase) and nitrogen metabolism (nitrite reductase).
• It is essential for the synthesis of
• It is required in photosynthesis during photolysis of water.
• It decreases the solubility of iron by oxidation. Hence, abundance of manganese can lead to iron deficiency in
  • 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
• ‘Grey spot disease’ in oat appears due to the deficiency of manganese, which leads to total failure of

crop.

• ‘Marsh spot’s in seeds of (g) Deficiency symptoms develop in older leaves.
• Copper

 

 

 

• 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 It is found in natural deposits of chalcopyrite (CuFeS₂).

 

(ii)  Functions

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

(iii) Deficiency symptoms

• Both vegetative and reproductive growth are
• 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
• It also causes necrosis of the tip of the young leaves (g., Citrus). The disease is called ‘die back’.
• Carbon dioxide absorption is decreased in copper deficient
• Wilting of entire plant occurs under acute
• Grain formation is more severely restricted than vegetative

(4)  Molybdenum

• Source : Molybdenum occurs in the soil in three forms – dissolved, exchangeable and nonexchangeable 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

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

Deficiency symptoms

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

(5)  Zinc

• Source : Zinc occurs in the soil in the form of ferromagnesian minerals like magnetite, biotite and 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.

 

 

 

 

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

• It is a constituent of enzymes like carbonic anhydrase, hexokinase, alcohol dehydroge-nase, lactic dehydrogenase and
• It is required for metabolism of phosphorus and
• Zinc also appears to play an important role in protein synthesis because in its absence there is substantial increase in soluble nitrogenous
  • Deficiency symptoms : (a) The first symptom appears in the form of interveinal chlorosis of the older leaves, starting at the tips and the
• Growth becomes stunted due to formation of smaller leaves and shortened Reduced stem growth is due to less synthesis of auxin.
• The leaves become distorted and sickle shaped and get clustered to form This effect is known as

‘little leaf disease’.

• In maize, zinc deficiency produces ‘white bud disease’ which leads to greatly reduced flowering and fruiting as well as poorly differentiated root
• Its deficiency causes khaira disease of rice and mottled leaf of apple, Citrus and

(6)  Boron

• Source : Boron is present in the soil in very small 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

7

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

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

(iii) Deficiency symptoms

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

 

 

 

• 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
• Fruits when affected are severely deformed and
• Its deficiency checks the cells division of cambium but continues cell elongation.

(7)  Chlorine

• Source : It is absorbed from the soil as chloride 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

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

• Passive absorption
• Active absorption
• Passive absorption : Absorption of ions without the use of metabolic energy is known as passive 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

 

 

 

 

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

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

• Mass flow hypothesis : According to Hylmo (1953, 1955), the ion absorption increases with increase in 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.
• 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 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.
• Facilitated diffusion hypothesis : According to this concept, the ions are transported across the membrane by a carrier 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.
• 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

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

• 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
• 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

 

 

Root    H+

 

 

K+

 

Clay micelle

 

A

 

Root     CO2

H₂O H⁺HCO₃^–

12_K+

 

 

 

Clay micelle

 

K⁺HCO₃^–

B

 

• Donnan equilibrium : This mechanism, given by 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^– = Anions outside

 

Positive ions inside Positive ions outside

= Negative ions outside Negative ions inside

 

Let us denote these indiffusible anions as R^– which are electrically balanced by an equal amount of cations say K⁺. If the anion Cl^– enters the cell due to diffusion gradient, it is accompanied by an equal amount of cations. The absorption of this cation may be against concentration gradient. The equilibrium so achieved is called donnan equilibrium.

Out

Fixed ions

 

5K⁺

4K⁺

10R^–

1Cl–          Absorption

 

5Cl^–

4Cl^–

10K⁺

1K⁺

against concentration gradient

 

Root

 

 

Fig : The concept of Donnan equilibrium

• Active absorption : Generally, the lipid-protein membrane of a cell is largely permeable to free The energy is considered to be involved in the transport of such free ions across the membrane. The absorption of ions, involving use of metabolic energy, is called active absorption. Energy used in these mechanisms comes from metabolic activities, especially respiration. Mineral absorption is mainly active process. Hoagland (1944) indicated active ion absorption and their (ions) accumulation against concentration gradient in green algae Nitella and Valonia.

Following evidences show the involvement of metabolic energy in the absorption of mineral salts :

• Higher rate of respiration increases the salt accumulation inside the

 

 

 

• Respiratory inhibitors check the process of salt
• By decreasing oxygen content in the medium, the salt absorption is also

Active transport in necessary for living cells because certain substances must be concentrated and others must be excluded. Active uptake of minerals by roots mainly depends on availability of oxygen. Depending upon the nature of the carrier and participation of metabolic energy, several theories are proposed to explain the mechanism of active absorption. Some of these are discussed below :

In side

Out side

• Carrier concept : This concept was proposed by Van den Honert (1937). The space in a cell or tissue where mineral ions enter by the usage of metabolic energy is called inner space. The boundries of outer and inner spaces are not well Perhaps the two are separated by the plasma membrane. According to this concept

 

there are separate carriers for cations and anions. A carrier forms an ion-carrier complex on the outer surface of the membrane. This complex breaks up and releases the ion into the inner space and this release is perhaps mediated by the

 

 

Ion

Plasma Membrane (Barrier)

 

ADP ATP

 

 

Respiration

 

enzyme phosphatase. The inactivated carrier is again activated

by the enzyme kinase and in this process an ATP is used up. ATP molecule combine with carrier molecules and allow

Ion carrier reaction

Activated carrier

Enzymatic activation of carrier

 

passage of substances against concentration gradient. The activated carrier again accepts new ions and the entire cycle is repeated.

 

Ion

Inactiviated

carrier

 

 

Transport

 

Carrier* + Ion (+/–)

¾¾¾® Ion – Carrier complex*

across the membrane

Ion-carrier

Enzyme

Ion

 

Ion – Carrier complex*

¾¾^Pho¾^sph¾^atas¾^e¾^(?) ®  Carrier + Ion

complex

mediated release of ion

 

Carrier + ATP

(Inactivated)

¾¾Kin¾a¾se ®

Carrier *

(Activated)

+ ADP

Fig : The ion-carrier hypothesis

 

• Cytochrome – pump hypothesis : This theory was proposed by Lundegardh (1950, 1954). According to this explanation only anions are absorbed actively, e., anion uptake requires energy and the absorption of cations does not require energy, (i.e., they are absorbed passively). At the outer surface of the membrane, the cytochrome undergoes oxidation and loses one electron and in exchange picks up an anion. This is then transported to the inner side of the membrane through to the cytochrome chain and on the inner surface of the membrane the anion is released and the cytochrome gets reduced by the action of dehydrogenase involved in

 

respiration.

The cations move passively along the electrical gradient created by the accumulation of anions at the inner surface of the membrane.

The evidence in favour of Lundegardh’s hypothesis is that the respiration increased when a plant is transferred from water to salt solution. The

 

Anion A^–      A^–

 

 

e–

 

1

Plasma membrane Cytochrome pump

 

A–

 

 

e–

A–                                          A–

 

A^–     Anion

 

increased respiration was called salt respiration or

4 O2

1 H O

Dehydrogenase activity

 

anion respiration.

This theory was critised on the following grounds –

 

Cation C+

2   2

Passive transport

H+

C+ Cation

 

Fig : The cytochorome pump hypothesis

 

 

 

 

• It is applicable to absorption of anions
• It fails to explain selective absorption of
• It has been observed that even cations can stimulate
• ETS is poorly developed in anaerobically respiring
  • Protein-lecithin carrier concept : Bennet-Clark (1956) proposed that the carrier could be some

 

amphoteric molecule which can carry anions as well as cations. He suggested it to be a membrane-bound protein

which is conjugated with a phosphatide called as lecithin.       A^–

Plasma membrane

 

C+

Phospho- tidic acid

 

Lecithin functions as a carrier. According to this theory, the phosphate group in the phosphatide acts as the cation binding site and choline acts as the anion binding site. During transport, ions are picked up by lecithin to forms an ion- lecithin-complex. The ions are released on the inner surface of the membrane due to hydrolysis of lecithin by the enzyme lecithinase into phosphatidic acid and choline.

Lecitin

C+

 

Choline esterase

 

 

  Lecithinase  

 

 

 

 

ATP ATP

 

+

 

Choline               A^–

 

 

 

Inside

Choline acetylase

 

Lecithin is resynthesised from these components in the

Fig : The protein-lecithin carrier concept

 

presence of enzyme choline acetylase and choline esterase which requires ATP.

Goldacre, 1952 proposed a mechanism of ion transport where contractile proteins act as ion carrier. They bind ions in unfolded condition on the outer face of the membrane and then contract releasing the ion into the cell and again become unfolded. The energy for this folding and unfolding is provided by ATP.

In hydrophytic plants, water and salts are absorbed by outer layer of plants.

 Factors affecting mineral absorption.

The process of mineral absorption is influenced by the following factors :

• Temperature : The rate of absorption of salts and minerals is directly proportional to temperature. But it holds good only within a narrow

The absorption of mineral ions is inhibited when the temperature has reached its maximum limit, perhaps due to denaturing of enzymes.

• Light : The effects of light on mineral absorption are indirect and are mainly due to the effect of light on transpiration and Transpiration is responsible for mass flow and photosynthesis provides energy and oxygen. When there is sufficient light, more photosynthesis occurs. As a result more food energy becomes available and salt uptake increases.
• Oxygen : A deficiency of O₂ always causes a corresponding decrease in the rate of mineral It is probably due to unavailability of ATP. The increased oxygen tension helps in increased uptake of salts.
• pH : It affects the rate of mineral absorption by regulating the availability of ions in the medium. At normal physiological pH monovalent ions are absorbed more rapidly whereas alkaline pH favours the absorption of bivalent and trivalent
• Interaction with other minerals : The absorption of one type of ions is affected by other type. The absorption of K⁺ is affected by Ca⁺⁺, Mg⁺⁺ and other polyvalent It is probably due to competition for binding

 

 

 

sites on the carrier. However, the uptake of K⁺ and Br^– becomes possible in presence of Ca⁺⁺ ions. There is mutual competition in the absorption of K, Rb and Cs ions.

• Growth : A proper growth causes increase in surface area, number of cells and in the number of binding sites for the mineral As a result, mineral absorption is enhanced.

 Mineral translocation.

P.R. Stout and D.R. Hoagland (1939) proved that mineral salts are translocated through xylem. After absorption of minerals by root, ions are able to reach xylem by two pathways.

• Apoplast
• Symplast

In apoplast pathway, inflow of water takes place from the cell to cell through spaces between cell wall polysaccharides. Ions thus are able to move from cell wall of epidermis to cell walls of various cells in cortex, cytoplasm of endodermis, cell wall of pericycle and finally into xylem. In symplast pathway, ions move through cytoplasm of epidermis and finally move through cytoplasm of cortex, endodermis, pericycle through plasmodesmata and finally into xylem.

Minerals in xylem are carried along with water to other parts of the plant along transpiration stream. Minerals reaching leaves take part in assimilation of organic compounds and then transported to other parts of the plant through phloem.

 Nitrogen nutrition in plants.

Nitrogen is an essential constituent of protoplasm. Nitrogen is the component of amino acids, proteins, enzymes, nucleotides and nucleic acids.

Nitrogen is picked up as inorganic compound and is changed into organic form by plants and some prokaryotes. Though atmosphere contains 79% of nitrogen in gaseous state, yet animals cannot use it directly. Nitrogen is a highly inert gas and it is energetically difficult for most of the living organisms, including the higher plants, to obtain it directly for their use. It must be fixed (i.e., combined with other elements such as C, H and O) to form nitrates, nitrites, ammonium salts, etc. before it is absorbed and utilized by the plants. Higher plants generally

utilize the oxidized forms such as nitrate (NO ^– ) and nitrite (NO ^– ) or the reduced form (NH ⁺ ) of nitrogen which is

 

made available by a variety of nitrogen fixers. Nitrogen can be fixed by three methods :

 

Process of Nitrogen fixation

On the basis of agency through which the nitrogen is fixed the process is divided into two types :

• Atmospheric nitrogen fixation : By photochemical and electrochemical reactions, oxygen combines with nitrogen to form oxides of nitrogen. Now they get dissolved in water and combine with other salts to produce

 

 

 

 

• Biological nitrogen fixation : Some blue-green algae (Anabaena, Nostoc), symbiotic bacteria (Rhizobium) and free living bacteria (Azotobacter) pick up atmospheric nitrogen, reduce it to ammonia, combines with organic acid to form amino
• Industrial nitrogen fixation : Nitrogen and hydrogen combines to form ammonia industrially, under pressure and

 

 

• Physical nitrogen fixation : Out of total nitrogen fixed by natural agencies approximately 10% of this occurs due to physical processes such as lightening (e., electric discharge), thunder storms and atmospheric pollution.

Due to lightening and thundering of clouds, N₂ and O₂ of the air react to form nitric oxide (NO). The nitric oxide is further oxidised with the help of O₂ to form nitrogen peroxide (NO₂).

NO₂ combines with H₂O to form nitrous acid (HNO₂) and nitric acid (HNO₃). The acid falls along with rain

 

water. Now it acts with alkaline radicals to form water soluble

 

NO ^–

(nitrates) and

NO^– (nitrites).

 

 

Ca or K Nitrates

 

The nitrates are soluble in water and are directly absorbed by the plants.

• Biological nitrogen fixation : The conversion of atmospheric nitrogen into inorganic or organic usable forms through the agency of living organisms is called biological nitrogen The process is carried by two main types of microorganisms, those which are “free living” or asymbiotic and those which live in close symbiotic association of with other plants.
  • Asymbiotic biological nitrogen fixation : This is done by many aerobic and anaerobic bacteria, cyanobacteria (blue green algae) and some fungi g. :

(a)  Free living bacteria

Aerobic            –       Azotobacter

Anerobic          – Clostridium Photosynthetic – Chlorobium Chemosynthetic – Thiobacillis

• Cyanobacteria (blue-green algae) g., Anabaena, Nostoc,

 

Tolypothrix cylindrospermum, Calotherix and Aulosira etc.

• Free living fungi g., Yeast cells and Pullularia.
  • Symbiotic biological nitrogen fixation : Symbiotic bacteria are found in the root nodules of the members of family

 

 

Secondary root

 

Root nodules

Primary root

 

 

 

 

Leguminosae. The best known nitrogen fixing symbiotic bacterium is Rhizobium leguminosarum (Bacillus radicicola).

Members of the family Leguminosae such as beans, gram, groundnut and soyabean etc. on their secondary, tertiary and sometimes primary roots bear small nodule like swellings. Rhizobium penetrates to the cortex of root through infection thread. Simultaneously cortical cells or root are stimulated to divide more vigorously to form nodules on the root. Neither bacterium nor plant alone can fix nitrogen in such cases. Nitrogen fixation is actually the outcome of symbiotic relationship between the two. When a section of root nodules is observed the presence of a pigment, leghaemoglobin is seen to impart pinkish colour to it. This pigment is closely related to haemoglobin and helpful in creating optimal condition for nitrogen fixation. Like haemoglobin, leghaemo-globin is an oxygen scavenger. Fixation of nitrogen is done with the help of enzyme nitrogenase, which functions under anaerobic conditions. Leghaemo-globin combines with oxygen and protects nitrogenase.

Symbiotic bacteria have also been found to occur in root nodules of Casuarina, Cycas, Alnus, etc. Leaf nodules develop in some members of family Rubiaceae, the bacteria being Mycobacterium. Some cyanobacteria also have symbiotic association with plants e.g., lichens; Anthoceros (a liverwort) and Azolla (a water fern).

Mechanism of biological nitrogen fixation : It is believed that nitrogen is bound to the enzyme surface and is not releasd until it is completely reduced to ammonia. Nitrogen bound to the enzyme surface is reduced in step-wise reaction before N–N bond is ruptured. Several schemes incorporating such idea have been proposed and Burris (1966) accepts that the total reduction of nitrogen occurs on an enzyme complex (Nitrogenase) without release of intermediates less reduced than ammonia.

The enzyme complex nitrogenase consists of two sub-units

• A non-heme iron protein commonly called Fe protein (or dinitrogen reductase, component I)
• An iron molybdenum protein called MoFe protein (or dinitrogenase, component II)

According to Burris (1966) hypothesis for nitrogen fixation suggesting the function of ATP and ferredoxin at each step in the reduction of nitrogen. The pretty function of ATP donor is furnished by pyruvate which also acts as electron donor for N₂ reduction as well.

Pyruvate on one hand acts as ATP donor while on other hand it supplies hydrogen ions and electrons for nitrogen reduction via NADH₂ and ferredoxin. The nitrogenase enzyme require 16 ATP molecules, 8 hydrogen ions and 8 electrons to reduce one molecule of nitrogen to 2NH₃ molecules.

 

Explaining the mechanism of nitrogenase activity, its now believed that electrons are transferred from the reducing agent (Ferredoxin, Flavoprotein or Dithionite) to complex of Mg-ATP and Fe-protein (component II). From here electrons flow to Mo-Fe protein (component I) and then to substrate (nitrogen) which is finally reduced (to NH₃).

 

 

Flavodoxin or Ferredoxin or Dithimite

–                                            e–

Fe-protein

e           (reduced)

2(Mg²⁺ ADP

 

Mo-Fe protein (reduced)

 

 

Flavodoxin or Ferredoxin              Fe protein (oxidised) 2(Mg²⁺ ADP)

Mo-Fe protein (oxidised)

 

In most diazotrophs (N₂– fixing organisms) ferredoxin and flavodoxin are probably the natural electron carriers for the reduction of Fe-protein. The reduced Fe-protein binds to Mg-ATP (Mg²⁺ ATP), creating a complex with Mo- Fe protein. Dissociation of two proteins occur between electron transfer events. The oxidised Fe-protein dissociates and becomes reduced again which recombines randomly with another nitrogenase untill all the electrons needed for reduction of substrate (e.g. 8 for N₂) are accumulated. Apart from H⁺, substrates such as N @ N or HC @ CH are believed to be bound to the same site in Mo-Fe protein (component I).

Pyruvate                                               Acetylphosphate + CO₂ 2H⁺+2e

 

Reduced ferredoxin

Acetate + ATP

 

NH₃ + Enzyme

Fig : Scheme suggesting the role of ATP and ferredoxin at each step in the reduction of nitrogen. Enzymes is nitrogenase (Burris, 1966)

 

The ammonia formed in biological nitrogen fixation is not liberated. It is highly toxic and is immediately converted into amino acids.

Ammonia + a-ketoglutarate + NADH  ¾¾^Deh¾^ydr¾^ogen¾^a¾^se ®  Glutamate + NAD⁺ + H²O.

The amino acids are transported through phloem to other parts of the plant.

Ammonification and nitrification

Thus symbiotic nitrogen fixing organisms give a part of their fixed nitrogen to the host in return for carbohydrate food and shelter. But the free living nonsymbiotic nitrogen fixing organisms do not enrich the soil immediately. It is only after their death that the fixed nitrogen enters the cyclic pool by the two steps namely the ammonification and nitrification.

• Ammonification : The nitrogenous organic compounds in the dead bodies of plants and animals are converted into ammonia or ammonium ions in the This is carried out by ammonifying bacteria. Ammonia is toxic to the plants but ammonium ions can be safely absorbed by the higher plants.

2

• Nitrification : Once ammonia has been produced it is converted into nitrates by nitrifying activities and process is called nitrification. Soil bacteria such as Nitrosomonas and Nitrosococcus convert ammonia into nitrite (NO ^– )

 

Nitrites are then oxidised to nitrates by Nitrobacter.

 

The nitrifying bacteria are chemoautotrophs and are benefited by utilising energy released in oxidation, which is used in chemosynthesis. At soil temperatures 30°C – 35°C in alkaline soils and with sufficient moisture and aeration, the activity of ammonifying and nitrifying bacteria is found to be maximum.

Some bacteria such as Thiobacillus denitrificans, Pseudomonas aoruginosa and Micrococcus denitrificans also occur in the soil which convert the nitrate and ammonia into atmospheric free elemental nitrogen. Such bacteria are called denitrifying bacteria and the process is called denitrification. These bacteria act very well in soil where there is more water and less oxygen and there are high level of the carbohydrate.

Nitrate assimilation in plans

Nitrate is the most important source of nitrogen for the plants but it cannot be used as such. It is first reduced to ammonia and then incorporated into organic compounds.

The process of nitrate reduction to ammonia occurs in the following steps :

Nitrate ® Nitrite ® Hyponitrite ® Hydroxylamine ® Ammonia

• Reduction of nitrate to nitrites : First the nitrate is reduced to nitrite by an enzyme called nitrate reductase. The reductase enzyme is a flavoprotein and contains FAD (Flavin adenine dinucleotide) as prosthetic group which receives hydrogen from reduced NADP or Molybdenum in enzyme serves as electron carrier.

 

Fig : Steps for nitrate reduction

 

• Reduction of nitrites : The nitrite ions are reduced to ammonia by an enzyme called nitrite reductase. This change occurs in leaves in the presence of light more rapidly and in dark with lesser This is due to the reducing power of reaction from photochemical splitting of water.

2HNO2 + 2H 2 O ® 2NH 3  + 3O2

Nitrite reductase does not need molybdenum but may require the presence of iron and copper. NADH and NADPH act as hydrogen donors.

Application of fertilizers

Application of fertilizers : Most of the soil usually contain sufficient amounts of essential mineral elements for the better crop production. Some of them are, however, deficient in certain elements. These elements are required to be supplemented externally by adding the appropriate fertilizers. Moreover, constant agricultural cultivation in field may also cause depletion of certain elements which must be replenished in order to improve the

fertility of soil. The important elements need to be replenished in crop fields are nitrogen, phosphorus and potassium. These are grouped as nitrogenous fertilizers, phosphate fertilizers and potash fertilizers. These are abbreviated as NPK. Common sources of NPK are ammonium chloride, ammonium sulphate, ammonium nitrate, bone meal, calcium magnesium phosphate and nitrate of soda.

The common fertilizers that supplements NPK is nitrophosphate with potash in varying proportions. The percentage of nitrogen, phosphorous and water soluble potassium are labelled on the bags as 17-18-9 or 15-15-15 and so on. The amount of fertilizer needed varies according to change in season, soil, nature of crop and other climatic conditions.

Important Tips

• Woodward (1699) reported that plants grow better in muddy water as compared to fresh rain
• De Saussure (1804) first of all demonstrated that plants obtain minerals from soil through root
• Liebig for the first time discovered the presence of elements in plant
• Liebig’s law of minimum states that the productivity of soil depends upon the proportionate amount of that essential element which is deficient in that
• Tracer elements : These are radioactive isotopes of elements, which are used to detect various metabolic pathways in plants, g., C¹⁴, N¹⁵, P³², S³⁵, etc.).
• If dried plant parts are heated in silica crucible at 600°C, all organic substances vaporize and the remaining plant ash contains only inorganic substances or mineral elements.
• Aeroponics : Growing plants in stands provided with fine mist of solution having all the required inorganic
• Hydroponics developed by
• Sodium (Na) regulates the transport of amino acids to the
• Aluminium (Al) is accumulated in
• Veledium (V) is required by alga
• Selenium (Se) is required by Atriplex and
• Iodine is required by marine alga
• The elements taken in the form of gas by prokaryotes only is
• Critical elements are the elements in which soil is generally deficient g. N, P and K. These are given in form of fertilizers.
• In addition to 16 essential elements, some plants require some more essential micronutrient elements such as
• Silica : Found in grasses and
• Sodium : Found in
• Cobalt : Found in ferns (e.g. Lycopodium), taking part in
• Nickel : Enzyme urease used it to hydrolyse urea by living
• In Rhizobium cobalt play an important role in nitrogen fixation and is an essential constituents of vitamin B₁₂. It is used in ‘cancer therapy’.
• Cytozyme is a water soluble commercial preparation which contains essential mineral element for use as foliar
• Khaira disease of rice and white bud of maize is due to zinc
• Die back of Citrus and reclamation disease of cereals and legumes and exanthema in fruit trees are due to deficiency of
• Whiptail disease of cauliflower is caused by Mo
• The symptoms produced by the deficiency of mineral substances are called ‘hunger sign‘.
• Mineral salt absorption is independent of water absorption.
• Maximum mineral salt absorption occurs by zone of No mineral salt absorption occurs by hair zone. Mineral salt absorption occurs directly by cells of epiblema and not by root hair.

 

• Mineral salts are absorbed mostly in form of ions e. anions and cations.
• Path of transport of mineral salts is
• Cytochromes act as anion
• Phytotron is the place or laboratory where plants can be maintained and studied under wide range of controlled conditions.
• Nif gene : Nitrogen fixing gene is nif A cluster of 18 genes (nif gene) encode the protein required for nitrogen fixation in Klebsiella.

 

 Special modes of nutrition.

Nutrition is an important characteristic of living organisms. Plants need energy for its various life activities. Energy is provided by the oxidation of different foods. The method of taking in and synthesis of various types of foods by different plants and animals is called nutrition.

Generally plants are autotrophic in their mode of nutrition, but there are some examples which are heterotrophic in their mode of nutrition. These plants are unable to manufacture their own food due to lack of chlorophyll or some other reasons, e.g., some bacteria, fungi, some bryophytes, pteridophytes and few angiospermic plants also, but special mention is of angiospermic plants. There are 4 special modes of nutrition.

• Parasites
• Saprophytes
• Symbiotic plants
• Insectivorous plants

• Parasites : These plants obtain either their organic food prepared by other organisms or depend upon other plants only for water and minerals with the help of which they can synthesize their own food. The living organism from which the parasite obtains its organic food or water and minerals is called host. Any part of the body of parasite is modified into a special organ called haustorium which enters into the cells of host and absorbs food or water and minerals from the host.

A plant parasite may live on the root or stem of the host plant partially or totally. The total parasites remain permanently attached to the host whereas the association of partial parasites is only short lived. Accordingly, parasites can be classified into two categories :

• Total
• Semiparasites or partial
• Total parasites : These plants never possess chlorophyll, hence they always obtain their food from the They may be attached to branches, stem (stem parasites) or roots (root parasites) of the host plants.
• Total stem parasite : Cuscuta is a rootless, yellow coloured, slender stem with small scale leaves, which twines around the host. The parasite develops haustoria (Small adventitious sucking roots) which enter the host plant forming contact with xylem and phloem of the It absorbs prepared food, water and minerals from the host plant.

 

• Total root parasite : Total root parasites are common in the families like Orobanchaceae, Rafflesiaceae, Balanophoraceae, Orobanche, Rafflesia and Balanophora are some of the common root parasites.

Orobanche is commonly known as broom rape. It has scale leaves and pinkish or bluish flowers. The tip of the root of parasite makes haustorial contact with the root of host and absorbs food from the host. Orobanche is

Flower

 

Host Parasite

 

 

 

Host

 

A                         B

22                           Fig : Cuscuta (dodder), a total parasite A : Parasite coiled around host plant

B : Relationship between vascular tissues of host and parasite

 

 

 

 

usually parasitic upon brinjal, tobacco. In Rafflesia (stinking corpse lily) another root parasite, vegetative parts of the plant are highly reduced and represented by cellular filaments resembling fungal mycelium. These filaments get embedded in the soft tissue of the host while the flowers emerge out in the forms of buds.

Balanophora occurs as a total stem parasite in the roots of forest trees.

• Semiparasite or partial parasite : Such parasitic plants have chlorophyll and, therefore, synthesize their organic food themselves. But they fulfill their mineral and water requirements from their host plants. Like total parasites, they grow on the stem and roots of the host plants and can be grouped into

 

following two categories :

• Partial stem parasites : The well known example of partial stem parasite is Viscum album (mistletoe) which parasitizes a number of shrubs and The mature plant of Viscum is dichotomously branched with green leaves born in pairs attached on each node of stem. The shoots are attached to the host by means of haustoria. The primary haustoria reaches upto cortex of the host

 

 

 

 

Host

Parasite plant

 

 

 

Haustoria

 

which runs logitudinally. It sends secondary haustoria which make connection with the xylem of the host and absorb water and minerals Loranthus is another partial stem parasite.

Fig : Viscum plant attached to the host stem (part of host stem is cut open to show the haustorium

 

• Partial root parasites : The common example of partial (semi-parasite) root parasite is Santalum album

(Sandal wood tree) which is an evergreen partial root parasite which grows in South India. It grows on the roots of Dalbergia sisso, Eucalyptus. Like other partial parasites, it also has green leaves and absorbs only minerals and water from the host plants.

Similarly, Striga on roots of sugarcane and Thesium on the roots of grasses are other partial root parasites.

• Saprophytes : These plants live upon dead organic matter and are responsible for conversion of complex organic substances into simple inorganic substances (minerals), g., some bacteria, some fungi (Yeast, Mucors, Penicillium, Agaricus), few algae (Polytoma), few bryophytes (Buxbaumia, Hypnum and Splanchnum), few pteriophytes (like Botrychium) and some angiosperms

(Monotropa and Neottia) also.

 

Monotropa, commonly known as Indian pipe, lacks chlorophyll and is colourless or ivory white. It is found in Khasi hills and in the dense forests of Shimla. Monotropa, though usually referred to as a saprophyte, actually gets its nourishment

A                        B

Fig : Saprophytic plants

• : Neottia (Birds nest plant)
• : Monotropa (Indian pipe)

 

from fungal mycelium which surround its roots. Such association between roots of higher plants and fungi is known as mycorrhiza. Neottia (Bird’s nest orchid) grows in the humus rich soil of

 

the forests. It has very few reduced leaves and thick pale yellow stem. The

roots lack root hairs and the nutrients are absorbed by mycorrhiza.

• Symbiotic plants : Sometimes two different species of organisms spend much or all of their lives in close physical association, deriving mutual Such an association is known as symbiosis and each organism is known as symbiont. Symbiotic association is so close that symbionts appear to be different parts of the same plant.

 

Upper cortex

Algal cells intermingled with fungus hyphae

 

 

Fruiting body of fungus

 

 

Fig : A lichen thallus in T.S.

 

 

 

 

Symbiotic association may be between two higher plants or between a higher plant and a lower plant. Some common examples of symbiosis are described below.

• Lichens : Lichens is a special group of plants, when an algae and fungi live together and are mutually benefitted (algae provides food and fungi provides water minerals and protection of algae).

The fungus component of the lichens, called mycobiont, is generally a member of Ascomycetae or occasionally a Basidiomycetae. The algal component of the lichen is known as phycobiont and is generally a member of Chlorophyceae (e.g., Trebouxia) or Cyanophyceae (e.g., Nostoc, Gloeocapsa).

• Mycorrhiza : It is a mutually beneficial association between a fungus and the root of higher plant. The fungus absorbs water, salts (from organic matter) and

protects the plant from soil borne pathogens. In return, it gets shelter and nourishment from the plant. In such association the fungal mycelium forms a mantle over the root surface and some of the hyphae penetrate between cortical cells and metabolites are transferred in both directions (i.e., from fungus to the root cells and vice- versa).

Usually the roots in the upper part of the soil, where organic matter is abundant, are mycorrhizal, and the roots penetrating deep in the soil are not associated with fungi. Generally, mycorrhizal roots have few or no root hairs.

 

Water and minerals are absorbed by the fungus and

Rootlet

Root hairs

Root cell

 

passed on to the host. The fungus digests starch grains

Fig : Micorrhizal roots

 

stored in the cortical cells of the host and uses the digested products in its own metabolism.

In some plants the mycorrhizal association is essential for normal growth and development. For example, seedlings of orchids fail to survive if the soil is free from fungus. Pine seedlings grow poorly unless mycorrhizal fungi are introduced in to the soil.

 

• Root nodules of leguminosae : Members of the sub- family Papilionaceae of the Leguminosae (g., pea, beans, trifolium) harbour species of Rhizobium, a nitrogen fixing bacteria. The bacteria form nodules in the roots. They fix elemental nitrogen of the atmosphere and make it available to the plant in forms that can be utilized. In turn they derive food and shelter from the leguminous plant.
• Myrmecophily : It is the symbiotic relationship between

Nodules

 

Root hair infected by bacteria         B

 

ants and some higher plants. The ants obtain food and shelter from the plant. They protect the plant (e.g., Mango) from other animals. In Acacia sphaerocephala the stipules are hollowed to function as ant shelter. Leaflet tips (Belt’s corpuscles) and rachis (extrafloral nectaries) possess feeding materials. A higher plant which is

Nodule Bacteria

Root

A                                          C

Fig : Symbiotic plants : A : A leguminous plant with root nodules, B : A root hair infected with bacteria, C : T.S. of a root nodule showing many bacteria

 

 

benefitted by association with ants is called myrmecophyte. The term myrmecophily is also used for pollination by ants.

 

• Insectivorous plants : These plants are autotrophic in their mode of nutrition but they grow in marshy or muddy soils, which are generally deficient in nitrogen and in order to fulfil their nitrogen requirement, these plants catch small insects. The organs and specially leaves of these plants are modified variously to catch the These plants have glands secreting proteolytic enzymes which breakdown complex proteins into simple nitrogenous substances, which inturn are absorbed by these plants. Some

 

Endodermis

 

 

Palisade like tissue

Secretory cells

 

Drop of secreted liquid

 

of these plants are as follows :

• Drosera (Sundew) : It is a herbaceous plant having spathulate or lunate leaves. The leaves are covered by glandular hair with a

Vascular tissue

 

Fig : Insectivorous plant : Drosera (Sundew) Fig : One grandular tentacle

 

swollen tip. The glands secretes a sticky purple juice which shines like a dew drop in bright light sunshine, hence the name sundew. These long special hair are generally referred to as ‘tentacles‘. When an insect alights on the leaf, the tentacles curve due to thigmonasty. The insect is killed and its proteins are digested by pepsin hydrochloride. Similar tentacles are also found in Drosophyllum.

• Utricularia (Bladderwort) : It is submerged floating aquatic herb which lack Some of the species of Utricularia also occur in moist soil. The leaves are dissected into fine segments and appear like roots. Some of the leaf segments are modified into pear-shaped sacs called bladders or utricles.

 

Leaf segment

Bristles

 

 

 

Bristle

 

 

 

 

Single bladder

B

 

 

Door valve

Outer wall

 

Inner wall Hairs

 

D

 

The bladders are triangular or semicircular structures having a single opening guarded by a valve. There are numerous bristles near the mouth and digestive glands inside.

Fig : Insectivorous plant : Utricularia (Bladderwort)  A

– Complete plant, B – One bladder, C  –  Part  of  leaf with several bladders, D – Internal structure of bladder

 

The bladders show special trap mechanism. The valve of the bladder opens on the inner side. When small aquatic animalcules enter the bladder along with water current, they get trapped inside. Their proteins are digested enzymatically. When a bladder is full of undigested matter, it degenerasis.

• Nepenthes (Pitcher plant) : They are commonly found in tropical areas like Assam and Meghalaya (e. N. Khasiana). In this plant the leaf base is winged, petiole is tendriller and the lamina is modified into pitcher. The pitcher has a distinct collar at the mouth and the apex is modified into the lid. The undersurface of the lid has alluring glands whereas the inner surface of pitcher is lined by numerous

Winged petiole

Pitcher

Fig : Insectivorous plant : Nepenthes

 

 

Tendrillar petiole

 

(Pitcher plant) A pitcher plant with pitcher

 

 

 

 

digestive glands and several downward directed hair. The lid attracts insects which slide down into the pitcher. The downward directed hair check their escape. The insect is killed and

its   proteins  are   digested  by   pepsin   hydrochloride.   Other insectivorous plants having leaf pitchers are Sarracenia, Cephalotus,

 

Heliamphora, etc.

• Dionaea (Venus fly trap) : It is a small herbaceous plant found mainly in America. The plant has a rosette of radiating leaves. The petiole is winged and The lamina is bilobed and the midrib acts like a hinge between the two lobes of the lamina. Each lobe has 15-20 trigger hairs or bristles. These hairs are very sensitive to nitrogenous substances. When an insect alights on the leaf and touches the sensitive hairs, the two lobes of lamina fold

Leaf

Lamina

 

 

Fig : Insectivorous plant : Dionea (Venus fly trap)

 

along the midrib. Thus the insect is trapped in between the lobes. Pepsin hydrochloride secreted by the digestive glands, present in the upper part of the lobes digests the insect. The simple digested substances are absorbed by the plant. Soon after the digested matter has been translocated to other parts

of the plant, the lobes of the lamina reopen.

• Sarracenia (Pitcher plant; Devil’s boot) : This pitcher plant is found in the temperate regions. It has a very reduced stem which bears a rosette of leaves. The leaves are modified into pitchers. It can easily be distinguished from Nepenthes on the basis of its trumpet-shaped sessile Contrary to Nepenthes, the pitchers of Sarracenia lack digestive enzymes and here the insects are decomposed by bacteria.
• Pinguicula (Butterwort) : It is a herbaceous plant having a basal rosette of ovate leaves. The leaf margins are slightly curved in upward The dorsal (upper) surface of leaf has two types of

 

glands stalked and sessile. The stalked glands secrete mucilage while the sessile glands secrete digestive enzymes.

As soon as the insect sits on the leaf surface, it sticks to the mucilage

A                                 B

Fig : Sarracenia (Pitcher plant)

 

secreted by stalked glands. Meanwhile the margins of the leaf roll inward due to stimulation received by the insect.

Thus   the  insect  gets  enclosed within   the   leaf.   The   protein

 

contents of the insect are digested by the enzymes secreted by the sessile glands. The leaf reopens when the stimulation is over.

• Aldrovanda (Water flea trap) : It is also a rootless, submerged aquatic plant (bog plant) recalling the habit of Utricularia. The leaves are bilobed with long petioles. There are five bristle like outgrowths associated with the lamina. The leaf surface is covered by visid stalked

 

 

Open leaf

 

Closed leaf

Open

leaf

 

 

Glands          Trigger hair

 

 

 

 

Closed leaf

 

 

 

A                                B

 

Fig : Entire plant of Pinguicula                            Fig : Aldrovanda vesiculosa

A : A floating twig B : An open leaf

 

 

 

 

The two halves of the lamina rise upward on stimulation by an insect, the midrib acting as hinge. The proteins of the insect are digested enzymatically.

Modes of nutrition in plants

 

Autotrophic

These are photosynthesizing plants and thus make their food by themselves.

Heterotrophic

These are non-photosynthesizing plants which obtain ready made food from other plants.

 

 

 

 

Photosynthetic

Use sunlight as a source of energy to manufacture food.

Examples :

• Most green
• Green sulphur
• Purple sulphur bacteria.
• Purple non-sulphur

 

Chemosynthetic

Use energy released in chemical reactions to manufacture food Examples :

• Nitrifying
• Iron
• Hydrogen

Parasitic

It obtain food only from living plants

 

Total                                           Partial

 

• Sulphur
• Methane bacteria.

Stem Example :

• Cuscuta.

Root Examples :

• Orobanche.
• Rafflesia.
• Balanophora.

Stem Examples :

• Loranthus

Root Example :

• Santalum.

 

 

 

 

Saprophytic

Obtain food from dead and decaying organic matter.

Examples :

• Monotropa
• Neottia
• Agaricus.

Symbiotic

It is a close association between two different kinds of plants and both the plants obtain their food by different methods.

Examples :

• Lichens
• Mycorrhiza
• Root

 

Insectivorous

These are green photosynthesizing plants but fulfill their nitrogen requirement from insects.

Examples :

• Drosera
• Utricularia
• Nepenthes
• Dionaea
• Pinguicula
• Sarracenia
• Aldrovanda

 

Important Tips

• Term ‘symbiosis’ was given by De
• Rafflessia (largest flower in the world) was discovered by Sir Stamford Raffles from Flower measures about a meter in diameter, about 11 kg in weight, smell is like rotten fish, pollination by elephants and found on roots of Vitis and Cissus.
• Sapria himalayensis (largest flower in India), measures 15 cm – 30 cm in
• Insectivorous plants are example of predation (i.e. first killing and then eating).
• Cephalotus (Fly Catcher). A deep rooted carnivorous herb with a rosette of pitchers for trapping small
• Cuscuta/Amarbel/Akashbel/Dodder : A dicot with no cotyledon (some workers consider it to have a single cotyledon). It is a total stem parasite but initially grows on
• Dischidia : The pitcher is without lid and is used only for storing rain water with some
• Epiphytes are plants which live on other plants for space (shelter/support) only. They are therefore, called space parasites
• Bird of paradise flower is Sterilitzia reginae.