RBSE Class 12 Biology Notes Chapter 8 Mineral Nutrition in Plants

Rajasthan Board RBSE Class 12 Biology Notes Chapter 8 Mineral Nutrition in Plants

Introduction :
The remaining part is weighed and dry weight of this Sill part is obtained.

1. All living organisms need to derive some substances from their surrounding in order to grow’ develop and complete their normal life cycle. These substance are termed as nutrients.

2. The basic requirement of nutrients of all organisms are more or less the same.

3. On the basis of the mode of nutrition organisms can be categorized as

(i) Heterotrophic Organisms
(ii) Autotrophic organisms.

(i) Heterotrophic organisms:
Such organisms which depend of external sources for both organic as well as inorganic substances for their nutrition are called heterotrophic. Example: Fungi, bacteria, animals.

(ii) Autotrophic organisms:
Such organism which themselves synthesize all required organic substance and derive only inorganic substances from outside source are called autotrophic. Example Chlorophyllous plants and some bacteria.

Nutrition in green plants nonnally means mineral nutrition.
Dependence of plants on minerals was first worked out by De Saussure (1804) and later by Von Sachs (1860).

Methods for study of mineral nutrients in plants

Scientists have adopted different methods to learn about the mineral requirements of plants.
Some of these are as follows.

Analysis of plant ash.
Sand culture experiment.
Hydroponics or water culture experiment.

Analysis of plant ash

This method is used to determine presence of relative amount of various elements in the plant.
In this method when fresh plants are dried at 70-80° C in oven for one or two days, all water present in the plant is driven off.
The main constituents of the dry matter are the polysaccharides, lignin, proteins, fats, organic acids and some other elements.
This dry matter is burnt in a furnace at 600° C, as a result all organic compounds are oxidized and are driven off in the form of gases (CO₂, NH₃ and SO₂ etc.).
The remaining material is called plant ash, and contains only mineral elements.
The relative amount of various minerals can be determined by analysis of this ash.
However, the extent of their utilization and essentially cannot be determined by this method.

Sand culture experiment

1. In this method sterile soil is made by washing it with hydrochloric acid. This sand is devoid of minerals.

2. This sand is properly washed with distilled water and then plants are grown in this soil.

3. Some experimental plants are grown in sand to which solution containing all minerals is added. These are referred as control plants.

4. Other experimental plants are grown in sand to which solution deficient in one or more nutrient elements have been added. These are referred as deficient plants.

5. Deficiency symptoms are noted in plants grown in sand containing solution deficient in one or more than one specific elements by comparison between control plants and the deficient plants.

6. By this method, the effect of deficiency of one or more specific mineral elements on plants can be determined.

7. Several difficulties are encountered in growing plants in sand,

8. Hence, in place of sand, quartz, plastic chips, vermiculite or artificial soil etc. is used.

9. Vermiculite is a mineral found in soil in natural form and is used now a days in place of sand. This is heated in furnace at 2000°F and the product so obtained is used in culturing plants

10. The culture experiments carried out by culturing plants in vermiculite are called vermiculoponics.

Properties of vermiculite:-

This is light weight, chemically inert, sterile substance.
Water holding capacity is more than soil. It is a loose textured substance and does not resist growth and development of roots.
It does not undergo degradation and so it can be used to growS plants repeatedly.

Water culture experiments or hydroponics

Sachs (1860) first demonstrated that plants can be cultured in soil less nutrient solution.
The methods of growing plants in nutrient solutions in place of soil are referred as hydroponics.
“Hydroponics” is a Greek term which literally means “working with water”.
In this method normally plants are grown in nutrient solution proposed by Sachs (I860); by Knop (1865) or byHoagland (1938). ‘
In order to ensure supply of oxygen to roots air is circulated by a tube in the solution.

Essential elements.

Green plants synthesize carbohydrate by photosynthesis using CO₂ absorbed from atmosphere and water absorbed from soil
Plants require several inorganic mineral elements and these are derived from soil or atmosphere.
Out of 105 elements found on earth, about 60 elements have been found present in plants, but all these are not essential for plants.
Various studies have revealed that for nonnal growth of plant, seventeen (17) elements are necessary. These are called essential elements.
Many non-essential elements present beyond a certain limit may be toxic to plants.

Criterion for determining essentiality of 17 essential elements.

(i) These elements are necessarily required for normal growth, development and reproduction of plants.

(ii) The function of the essential element should be specific and its deficiency can be fulfilled only by this element and not by any other element.

(iii) The element must be directly involved in the metabolism of plant.

(iv) A specific symptom must appear in plant due to deficiency of a specific element and the symptom must disappear by supply of that element. On the basis of relative amount present in the plants, the seventeen essential elements have been further divided into two categories:

(A) Macronutrients: Elements which are found in the plant in amount from 1.0 to 10.0 mg per gm of dry weight of the plant are called macronutrients. These are nine in number – C, H,P,O,N,P,K,S, Mg, and Ca.
Among these N, P and K are considered as primary macronutrients and also called as critical elements whereas Ca, Mg and S are considered as secondary macronutrients.

(B) Micronutrients: – Elements which are found in plants in amount less than 1,0 mg per gm of dr}7 weight are called minor or micronutrients. Although required in very small amount, they play significant role in plant metabolism. These are also called as trace elements. These are nine in number – B, Cu, Cl, Ain, Mo, Ni, Zn, and Fe.

Besides the above 17 elements, some other elements have also been found beneficial in higher plants. These are called beneficial or functional elements. Ex. Na, Si, Co and So.
Essential elements are also classified on the basis of their function or role in plant nutrition. These categories are

(a) Structural elements: the essential elements which are constituents of bio chemicals are called structural elements. Example e, h, o, n.

(b) Energy related elements: Such essential elements are useful in energy related biochemical reactions in plant. Example: Phosphorous in ATP and Alg in chlorophyll.

(c) Enzyme activators: These essential elements act as activator or inhibitors of different enzymes. Example: Ain, Zn, Mo, Mg, Etc.

Note : Some essential elements regulate osmotic activity of plant cells. Example K, Cl, etc. Potassium plays significant role in stomatal movement (opening and closing of stomata).

The 17 essential elements are generally referred as mineral nutrients, but source of four of these elements is atmosphere and water and not the soil. Some scientists consider these elements as non-mineral elements. Example Carbon, Hydrogen, Oxygen and Nitrogen.

Although nitrogen is a maj or component of atmosphere but higher plants cannot derive it directly from atmosphere.

As nitrogen is obtained by plants in the fonn of nitrate or nitrite through soil, it is categorized among mineral elements.

Hence out of 17 essential elements only C, H, and O are non-mineral elements and remaining 14 elements (N, P,K, S, Mg, Ca, Fe. B, Mn, Cu, Zn, Mo, Cl, Ni) are mineral elements.

Role of macro and micronutrients in plant nutrition

1. Essential elements participate in different physiological activities of plants.

2. These elements play important role in. regulating permeability of the plasma membrane, controlling osmotic pressure, electron transport system, balancing various biochemical reactions with the help of enzymes, storage of reserve food material in storage organs, butler actions, etc.

3. A brief account of availability, uses and effect of deficiency and excess of essential elements on tire plants is given below.

Carbon, Hydrogen and Oxygen

These are non-mineral essential elements which are absorbed by plants from atmosphere and soil in the form of CO₂, O₂, and H₂O.
These are constituents of all organic compounds such as carbohydrates, fats, proteins, etc.
About 90-95% of the dry weight of most higher plants consists of C, H and Oxygen.
‘Normally plants do not suffer from deficiency of these elements.

Nitrogen

The amount of nitrogen in air is about 78% by volume.
Plants absorb nitrogen from soil in the form of NO₂, NO₃ and as an exception as NH₄.
It is main component of amino acids, proteins, nucleic acids, vitamins and phyto hormones.
Nitrogen is specially required in meristematic cells, buds and other metabolically active cells.
Chemical fertilizer urea is main source of nitrogen.

Deficiency Symptoms :

Deficiency of nitrogen results into chlorosis, first in old leaves followed by young leaves.
Due to degradation of chlorophyll leaves begin to appear pinkish due to anthocyanin effect.
Plants show stunting due to reduced rate of respiration and protein synthesis.

Phosphorus

Phosphorus is absorbed from soil in the form of soluble inorganic phosphate ions either as 11,PC) ( or HPC).
It is transported in plant in inorganic form but it is found in the plant in the form of organic compounds. Normally it’s amount in plants is 0.2 to 0.8% of dry weight of the plant and is next to nitrogen among the mineral nutrients derived from soil.
It is main constituent of DNA, RNA, Phospholipids, NAD. NADP, ATP, ADP, etc.
It plays significant role in oxidation reduction reactions, respiration, photosynthesis and fatty acid synthesis.
It is also required in all phosphorylation reactions.
It is founds in high amount in meristematic regions.

Deficiency Symptoms

Plants show stunted growth and leaves become dark green in color.
Leaves and stem show anthocyanin pigmentation.
Acute deficiency results in to formation of necrotic area s in leaves and fruits.
Leaves become deformed and may fall early.
Activity of cambium is suppressed.

Calcium

Calcium is found in the form of calcite and dolomite ore in soil in sufficient amount.
It is absorbed from soil in the form of calcium ions (Ca⁺⁺).
It is an important component of middle lamella where it is found as calcium pectate.
It is also needed during formation of mitotic spindle. Hence is required by meristematic and differentiating cells.
It provides elasticity to the cell wall.
Calcium plays important role in binding of nucleic acids with proteins and formation of chromosomes.
It also plays important role in transportation of carbohydrates and amino acids.

Deficiency Symptoms

Cell walls lose their elasticity and become rigid.
Cellular differentiation is adversely affected.
Young leaves become distorted and the leaf tip bends downwards. This is called hooked leaf symptom.
Chlorosis occurs along the margins of young leaves.
Premature drop of flowers takes place.

Potassium (K)

It is found in soil in soluble and exchangeable form.
It is absorbed in the form of potassium ion (K⁺).
Potassium is not a component of any biochemical compound and is not a structural element.
It plays important role in four biochemical reactions viz cation neutralization, transport across membrane, enzyme activation and osmotic potential regulation.
The best function of potassium is in regulating opening of stomata.
It also plays important role in photosynthesis and normal growth of seeds and fruits.

Deficiency Symptoms

The first symptom of deficiency of potassium is marginal chlorosis.
The leaves become mottle due to chlorosis.
Shortening of intemodes results in to stunted growth and bushy appearance of plants.
Necrotic spots appear on the margins of more mature leaves.
Disease resistance of plants decreases.
Rate of protein synthesis decreases.

Magnesium

Magnesium is found in soil as magnesium carbonate (Mg CO3) and is absorbed as magnesium ion (Mg⁺⁺).
It is an important component of chlorophyll pigment.
This element activates the enzymes of respiration, protein synthesis and photosynthesis.
It helps in joining the two subunits of ribosomes.
It is also involved in synthesis of DNA and RNA.
It is found in abundant amount in oil seeds as it plays important role in formation of oil seeds.

Deficiency symptoms

Magnesium deficiency causes extensive interveinal chlorosis.
Leaves show reddish and yellow and orange spots due to predominance of anthocyanin pigments.
There is a decline in photosynthesis.
In acute cases leaves become nearly white.

Sulphur (S)

This is absorbed from soil in the form of sulphate ion (S04⁺⁺). Besides this sulphur dioxide (SO₂) present as environmental pollutant may also be absorbed by plants.
It is helpful in synthesis of amino acids such as cysteine, methionine, etc. needed in protein synthesis.
It is also found in vitamin B and coenzyme-A
It is an important component of protoplasm.
In the roots of plants of pea family growing in sulphur rich soil, the nodules are more well developed.
The characteristic pungent odour of cruciferous plants, & onion and garlic is due to sulphur rich volatile oils.

Deficiency symptoms

Leaves show chlorosis.
Shortage of sulphur containing amino acids occurs.
Plants become stunted and fail to form fruits.
Plants become stiff and woody due to formation of more of thick walled tissue such as sclerenchyma and xylem.

Iron (Fe)

Plants absorb iron from soil in the form of ferric ions (Fe⁺⁺⁺).
It plays important role in cell division, respiration and different steps of electron transport system.
It is an important constituent of cytochrome and ferredoxin and acts as activator of aconitase, catalase, peroxidase and some Krebs cycle enzymes.
It is helpful in chlorophyll synthesis but is not a part of chlorophyll molecules.
It is found as fixed protein (Phytopheritin) in leaves and in the chromatin network of nucleus.
In metabolic reactions it participates as Fe⁺⁺ (Ferrous).

Deficiency symptoms

Young leaves show extensive chlorosis and may become white or yellow white.
Derailment of reactions of photosynthesis, respiration and protein synthesis occurs.
Cell division activity is inhibited.
Plant growth becomes slow.

Manganese

It is found in soil normally in the form of manganese dioxide (MnO₂) and plants absorb it in the form of Mn ion.
Manganese acts as activator of several enzymes related with respiration, photosynthesis, and nitrogen metabolism reactions.
It plays important role in synthesis of chlorophyll and development of chloroplasts.
The best defined function of Mn is photolysis of water and evolution of O₂.

Deficiency symptoms

Young as well as old leaves show chlorosis.
Limited development of roots .
It’s deficiency causes specific plant diseases.

Example :
1. Marsh spot of pea
2. Grey speck of oats

Boron (B)

Boron is absorbed from soil in the form of borate ions \(\left(\mathrm{BO}_{3}^{3-} ; \mathrm{B}_{4} \mathrm{O}_{7}^{2-}\right)\)
It reacts with calcium present in soil to form calcium borate which cannot be absorbed by roots.
Because of this, availability of boron is low in calcium rich soils.
Boron plays important role in translocation of carbohydrates, cell division, nitrogen metabolism, pollen germination and functioning of plasma membrane.

Deficiency symptoms

The leaves become thick and dark green in color due to deficiency of boron.
Storage tissue and fleshy parts become disorganized.
Number of flowers is reduced. Flowers become sterile due to effect on pollen germination.
Death of growing points results in dwarfing.

Zinc (Zn)

This element is absorbed from soil in the form of zinc ions (Zn⁺⁺).
Zinc is required for synthesis of growth hormone indole acetic acid.(IAA).
It also controls absorption of phosphorus.
Zinc is component of several types of enzymes especially carboxylases.

Deficiency symptoms

Deficiency of zinc causes litteling of leaves.
Leaves become distorted and small in size.
Discolorised necrotic spots appear on leaves.
Phloem tissue shows malformation (poorly formed).
Little leaf disease is main symptom of zinc deficiency.

Copper (Cu)

It is absorbed from soil by roots in the form of divalent cupric ions (Cu⁺⁺) or monovalent cuporous ions (Cu⁺).
Copper acts as electron carrier in oxidation reduction reactions.
It is a component of plastocyanin and cytochrome oxidase which act as electron carrier in photosynthesis.

Deficiency symptoms

Wilting and curling of leaves occurs.
The leaf tip shows chlorosis and becomes discolored.
In Citrus deficiency of copper causes die back disease of leaf.

Molybdenum (Mo)

This element is absorbed in the form of molybdenum oxide (Mo O₂).
This is required for nitrogen fixation in leguminous plants.
Molybdenum ions are part of enzyme nitrogenase and nitrate reductase which are involved in nitrogen fixation.

Deficiency Symptoms

Mottling of leaves is charateristic symptom of molybdenum deficiency.
Chlorosis of leaves and poor flower setting is observed.
In cauliflower, the leaves become distorted and subsequently die. This is called whip tail disease and is caused due to molybdenum deficiency.

Chlorine (Cl)

Chlorine is absorbed from soil in the form of chloride ions (Cl^–)
This element is not component of any biochemical substance.
It plays important role in carbohydrate metabolism and in balancing anion-cation concentration in cell.
It is helpful in photolysis of water leading to evolution of oxygen.

Deficiency symptoms:

Leaves show variegated chlorosis followed by necrosis.
Fruit formation is reduced and size of fruit is also reduced.

Nickel (Ni)

Dalton (1988.) included nickel as essential element.
It is found in soil in sufficient amount and is absorbed in the form of nickel ions (Ni++).
Nickel is neither a component of any biochemical compound nor its function and deficiency are properly understood.
It is an essential part of enzyme urease. It is probably helpful in transportation of nitrogenous compounds.

Deficiency symptoms

Plants show chlorosis and formation of necrotic tissue in leaves.
Plants growing in soil normally do not suffer deficiency of nickel because it is required in very minute quantity.

Note: It is important to note that deficiency symptoms of mobile elements first appear in old leaves whereas the deficiency symptoms of immobile elements first appear in young leaves.

Deficiency symptoms due to deficiency of Ca and B appear in terminal buds.
Deficiency symptoms first appear in old leaves due to deficiency of mobile elements such asN,P,K,Mg,Zn,Mo.
Deficiency symptoms first appear in young leaves due to deficiency of immobile elements such as Cu,S,Fe,Mn.

Mechanism of Absorption of Mineral Salts

Minerals are absorbed from soil by the roots in the form of ions.
Normally the minerals are absorbed by active absorption and energy is used in this process. Absorption of minerals may take place by two methods.

(i) Passive absorption
(ii) Active absorption.

Passive absorption

According to this theory mineral elements enter the cell from the soil solution along their electrochemical potential gradient i.e. from their higher concentration to the place of their lower concentration without any expenditure of energy of the cell. Three theories have been put forwarded in support of passive absorption of minerals.

1. Mass Flows Hypothesis:

According to this theory, under the influence of transpiration pull, mineral ions are also absorbed along with the stream of water. Mass flow, also called as bulk flow is the unidirectional movement of molecules or ions through the root along with stream of water due to the suction force created by transpiration pull. An increase in rate of transpiration results in to increase in rate of passive absorption.

2. Ion Exchange theory:-

Exchange of cations and anions in between the external solution (soil solution) and the surface of root is called ion exchange. This hypothesis assumes that exchange of cations and anions takes place by ions of similar charge i.e. K+of the external solution can exchange with H+on the surface of membrane and similarly the anion can also exchange with free hydroxyl ion.

3. Donnan equilibrium theory:-

This theory was proposed by Donnan (1927).
According to this theory there are some fixed ions in the cell which cannot pass out of the cell membrane. These are called stable ions, whereas both anions and cations can enter the membrane from outside.
Normally there is an equilibrium of ions betw een external and the internal solution.
This theory assumes that a cell has a concentration of fixed ions to which its membrane is impermeable. Equal number of anions and cations from the external solution will diffuse across the membrane till it reaches to the state of equilibrium.
This theory explains the accumulation of anions against a concentration gradient without participation of metabolic energy.

Note:-It is assumed that at least a part of the total salt uptake may result from passive absorption and it may account for salt accumulation with in the plant tissue. The rate of mineral salt absorption is too rapid and cannot be explained by passive absorption which requires metabolic energy.

Active absorption

According to this theory mineral elements move against the concentration gradient or the electro chemical potential and metabolic energy of cell is used in the process. This process requires ATP molecules. This absorption is called active absorption. Following theories have been put forwarded in support of this theory.

1. Carrier concept

This theory was proposed by VanJDen Honert (1937).
According to this concept the part of the cell or tissue in which ions are absorbed by the use of metabolic energy is called inner space and the area outside this is called outer space.
The boundary or barrier (cell membrane or plasma membrane and tonoplast) between inner and outer space is impermeable to movement of free ions.
According to Honert (1937), protein molecule present at the surface of membrane acts as carrier and combines on outer space with a specific ion in the soil solution, transports it across the membrane and releases it in the inner space.

The process can be explained with the help of following steps.
a. Carrier + ATP (Activation of Carrier)
b. Activated carrier +Ion —Carrier ion complex.
c. Carrier ion complex—Carrier +Ion

2. Ion Pump or Cytochrome Pump Theory

This theory was proposed by Lundegardh and Burstroem (1933).
According to this theory there is direct relationship between the rate of respiration and absorption of anions.
Anions are transported from outer surface of membranes to the inner surface by cytochromes.
To balance the electro chemical potential, the cations move from outer surface to inner surface of membrane.
This theory assumes that anions are taken up by expenditure of energy whereas movement of cations is passive.

3. Electrochemical gradient hypothesis.

This hypothesis has been proposed by Peter Mitchel (1968).
According to this hypothesis, anions are transported across the membrane due to electrochemical potential developed on the outer and inner face of the membrane.
In this process ATPase enzyme plays important role.