Physiological Roles of Essential Elements

 1. Nitrogen (N)

Source

Soil is the chief source of nitrogen. Plants absorb N either in the form of nitrate or ammoniacal salts.

Some bacteria and heterocysts containing blue-green algae fix N of the atmosphere, which can be utilized by the

plants.

Physiological Roles

i. Present in the structure of the protein molecule

ii It is found in important molecules like purines, pyrimidines (which are essential

in protein synthesis), etc.

iii It is also found in the porphyrins found in chlorophyll and cytochrome enzymes

and hence it is essential for photosynthesis.

iv. It participates in the co-enzymes essential to the functions of many enzymes.

v. It is readily mobile within the plant tissues. When its deficiency occurs, It is

transferred from older to younger tissues where it can be reutilized in the growth

process. As a result, symptoms develop first on older leaves.

2. Phosphorus (P) Source

The plant absorbs P in the form of soluble phosphates such as H3PO4and HPO4. The absorption

ability differs from plant to plant,e.g. cabbage and alfalfa can absorb phosphate from rocks whereas

barley, corn, and oats cannot absorb so efficiently.

Physiological Roles

i. Phosphorus content is found to be 0.2 to0.8% of the total dry weight.

ii. It is found abundantly in the growing and storage organs such as fruits and seeds.

iii. It promotes healthy root development and fruit ripening through translocation of

carbohydrates.

iv. It is an essential element participating in the skeleton of the plasma membrane, nucleic

acids and organic molecules such as ATP (Adenosine Tri Phosphate) and other

phosphorylated compounds.

v. It is also found in plants as a constituent of nucleic acids, phospholipids, the

co-enzymes like NAD etc.

vii. Phospholipids along with protein may be important in the cell membranes.

viii. It is readily mobile within the plant.

3. Potassium (K)

Source

Potassium is widely distributed in soil minerals. Forms such as potash felspar, mica, and

glauconite are slowly converted into soluble forms by weathering processes. It is strongly fixed in

soils, largely as an exchangeable base. The K is found in less available forms. Small amounts are

normally present in the soil in an exchangeable form.

Physiological Role

i. It is concerned with the formation of carbohydrate and protein synthesis, photosynthesis,

Transpiration regulation, enzyme action, synthesis of nucleic acids and chlorophyll,

oxidative and photophosphorylation, translocation of solutes, etc.

ii. It acts as an activator of many enzymes involved in carbohydrate metabolism and

protein synthesis.

iii. It is actively involved in the opening and closing of stomata.

iv. It takes an important role in the drought tolerance of crops through water relations.

v. It offers resistance to pest and disease effects in crops.

vi. It is present in soluble forms and mostly contained in the cell sap and cytoplasm.

vii. It is readily mobile within the plant tissues.

viii. It is frequently present in all parts of the plant but in fairly large proportions

at growing tissues.

ix. Its function is partly replaced by rubidium.

4. Magnesium (Mg)

Source

Magnesium occurs as carbonates and is held in soils as an exchangeable base. It is easily

leached and for this reason, may become deficient in sandy soils during wet periods. Heavy application

of K fertilizers reduces its absorption.

Physiological Roles

i. It is a constituent of chlorophyll, and therefore, essential for its synthesis.

ii. It acts as a phosphorus carrier in the plant.

iii. It is essential for the synthesis of fats and the metabolism of carbohydrates and phosphorus.

iv. It is required in binding two subunits of ribosomes during protein synthesis.

v. It acts as an activator for many enzymes in phosphate transfer reactions in carbohydrate

metabolism and nucleic acid synthesis.

vi. It is involved in the formation of seeds of high oil contents containing a compound

called lecithin

vii. It is readily mobile within the plant tissues.

viii. Its function is partly replaced by manganese.

5. Sulphur (S)

Source

It is available to plants in the form of soluble sulfates of soil.

Physiological Role

i. It is an important constituent of some amino acids (cysteine, cysteine, and methionine),

vitamins (biotin, thiamine), coenzyme A, and volatile oils.

ii. It participates in protein synthesis

iii. Sulfyhydryl groups are necessary for the activity of many enzymes

iv. Disulphide linkages help to stabilize the protein structure.

v. It adversely affects chlorophyll synthesis.

vi. Sulphur affects the nodule formation in roots of leguminous plants.

vii. Characteristic odor of Cruciferous plants (onion, garlic, etc.) is due to the sulfur as

a constituent of volatile oils.

viii. It is known for its uneven distribution in various organs of the plant

(e.g)in mature corn, the content observed was -40% in leaves. 23% in the stem,

26% in gain and 11% in roots)

ix. It is immobile in the plant tissues. When its deficiency occurs, it is not transferred to the

younger leaves but accumulated in the older leaves only. As a result, deficiency

symptoms develop first on younger leaves.

6. Calcium (Ca)

source

Calcium occurs in soil with a variety of minerals. The soil derived from stone or chalk rocks

contains a larger percentage of carbonates of lime (calcium carbonate), white sandy soils show Ca

deficiency which is met by adding lime or limestone. The presence of CO2 dissolved in the soil

water promotes solubility of carbonate of lime in soil ensuring quick Ca absorption.

Physiological Roles

i. It is the important constituent of middle lamella in the cell wall

ii. It is essential in the formation of cell membranes.

iii. It helps to stabilize the structure of chromosomes.

iv. It is also an activator of many enzymes (ATPase, kinases, succinate dehydrogenase)

v. It provides a base for the neutralization of organic acids.

vi. It is concerned with the growing root apices.

vii. It is essential for fact metabolism, carbohydrate metabolism, and binding of nucleic

acids with proteins.

viii. It is also essential in the counteraction of metal toxicity

ix. It is immobile in the plant tissues. Therefore, when needed, the element is not transferred to the younger leaves but accumulated in the older leaves themselves. As a result,

deficiency symptoms develop first on younger leaves.

Micronutrients

7. Iron (Fe)

Source

It is fairly present in the form of oxides giving red or brown color to the soil. In

well-irrigated soils, Fe is present predominantly as ferric form and in waterlogged soils, ferrous

compounds are formed. The availability of Fe to plants increases with acidity and is depressed by

phosphates. It is absorbed in the ferric state; but, the ferrous form is only metabolically active for the

plants.

Physiological Role

i. It is an important constituent of iron-porphyrin proteins like cytochromes, peroxidases,

catalase, etc.

ii. It is essential for the synthesis of chlorophyll

iii. It acts as a catalyst and electron carrier during respiration

iv. It also acts as an activator of nitrate reductase and aconitase enzymes.

v. It is a very important constituent of ferredoxin, which plays an important role in biological

nitrogen fixation and primary photochemical reaction in photosynthesis.

vi. It is immobile in the plant tissues. Its mobility is affected by several factors like the presence

of magnesium, potassium deficiency, high phosphorus, and high light intensity.

8. Manganese (Mn)

Source

Like iron, the oxide forms of Mn are common in soil but the more highly oxidized forms

(manganous dioxide) are of very low availability to plants. Its solubility increases with increased

acidity and in strongly acid soils, it is frequently present in toxic concentrations. This might be

one of the chief reasons for crop failure due to soil acidity.

The absence of organic matter and poor drainage conditions of soil causes the unavailability of Mn in

the soil. Sometimes, Oxidising bacteria in the soils may also cause Mn unavailable over the pH

range of 6.5 to 7.8.

Physiological Roles

i. It acts as an activator of some respiratory enzymes like oxidases, Peroxidases, dehydrogenases,

kinases, decarboxylase, etc.

ii. It is essential in the formation of chlorophyll

iii. It decreases the solubility of iron by oxidation; in certain cases, an abundance of Mn leads to Fe

deficiency.

iv. It is necessary for the evolution of O2 during photosynthesis.

v. It is immobile in the plant tissues. When its deficiency occurs, it is not transferred to the

younger leaves but accumulated in the older leaves only. As a result, deficiency symptoms

develop first on younger leaves.

9. Copper (Cu)

Source

Copper is found in smaller quantities in soils due to the additions of growing plants and its

added residue. Organic matter, soil organism, and pH are the important factors affecting the availability

of copper. Soils neighboring the copper deposits are normally toxic to plants.

Physicological Roles

i. It acts as a catalyst and regulator

ii. It is a constituent of several oxidizing enzymes like ascorbic oxidase, lactase, tyrosinase,

phenoloxidase, plastocyanin, etc.

iii. It is essential for photosynthesis, respiration and to maintain carbon/ nitrogen balance.

iv. Its higher concentration is toxic to plants.

v. It is immobile in the plant tissues.

10. Zinc (Zn)

Source

Like copper, it is also found in soils in very small quantities and largely it results from the

concentration and addition from growing plants and added residue. Its uptake is reduced by a large

or prolonged supply of phosphate fertilizers. It is generally found to be toxic in the neighborhood of

zinc deposits.

Physiological Role

i. It is a component of enzymes like carbonic anhydrase, alcohol dehydrogenase, glutamic

dehydrogenase, lactic dehydrogenase, alkaline phosphatase, and carboxypeptidase.

ii. It is essential for the evolution and utilization of CO2, carbohydrate, and phosphorus

metabolism.

iii. It is also essential for the biosynthesis of the growth hormone, Indole-3-acetic acid

(IAA)and also for the synthesis of RNA.

iv. It is readily mobile within the plant tissues.

v. It is closely involved in chlorophyll formation.

11. Molybdenum (Mo)

Source

It is found widely distributed in small amounts in soils and plants and relatively higher

concentration occurs in mineral oils and coal ashes.

Physiological Roles

i. It is associated with the prosthetic group of enzymes, nitrate reductase, and thus involved in

nitrate metabolism.

ii. It acts as an activator of some dehydrogenases and phosphatases and as cofactors in the synthesis

of ascorbic acid.

iii. It is necessary for the formation of nodules in legumes for the fixation of atmospheric

nitrogen.

12. Boron (B)

Source

Boron occurs in rocks and marine sediments. It is absorbed in the form of borate ions and

it has some sort of antagonistic effect with other cations like calcium, potassium, and others.

Physiological Roles

i. It is necessary for the translocation of sugars within the plant system

ii. It is involved in the reproduction and germination of pollens (tube)

iii. It is concerned with water reactions in cells and regulates the intake of water into the cell

iv. It keeps Ca insoluble form within the plant and may act as a regulator of K ratios (K/Ca etc.)

v. It is also concerned with nitrogen metabolism and with oxidation and reduction

equilibrium in cells.

vi. It is immobile in the plant tissues.

13. Chlorine and others

Chlorine occurs in soils as chlorides and moves freely in soil solution and form which it is ]

available to the plant.

Chlorine increases the water content of tobacco cells; it affects carbohydrate metabolism

and speeds up photosynthesis.

Cobalt is needed by the leguminous crop in the absence of nitrogen. Because it is required by

the symbiotic bacteria for fixation of atmospheric nitrogen.

Elements like aluminum (Al), silica (Si), and selenium (Se) possess stimulating effects of

certain non-essential elements by counteracting the toxicity of certain elements present in the soil.

Elements like, Al and Si are called ballast elements. Some plants are also accumulators. They

can accumulate larger quantities of some elements like Al and Si. Silica is highly essential for the

paddy crop and is actively involved in its metabolism.