Physiological Roles of Essential Elements

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 heterocyst containing blue green algae fix N of atmosphere, which can be utilized by the plants.
    Physiological Roles
    1. Present in the structure of the protein molecule
    2. It is found in important molecules like purines, pyrimidines (which are essential in protein synthesis)etc.
    3. It is also found in the porpyrines found in chlorophyll and cytochrome enzymes and hence it is essential for photosynthesis.
    4. It is participated in the co-enzymes essential for the function of many enzymes.
    5. 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 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
    1. Phosphorus content is found to be 0.2 to 0.8% of the total dry weight.
    2. It is found abundantly in the growing and storage organs such as fruits and seeds.
    3. It promotes healthy root development and fruit ripening through translocation of carbohydrates.
    4. It is an essential element participating in the skeleton of plasma membrane, nucleic acids and organic molecules such as ATP (Adenosine Tri Phosphate) and other phosphorylated compounds.
    5. It is also found in plants as a constituent of nucleic acids, phospholipids, and the coenzymes like NAD etc.
    6. Phospholipids along with protein may be important in cell membrane.
    7. 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
    1. It is concerned with the formation of carbohydrate and protein synthesis, photosynthesis, transpiration regulation, enzyme action, synthesis of nucleic acids and chlorophyll, oxidative and photo phosphorylation, translocation of solutes etc.
    2. It acts as an activator of many enzymes involved in carbohydrate metabolism and protein synthesis.
    3. It is actively involved in the opening and closing of stomata.
    4. It takes an important role in drought tolerance of crops through water relations.
    5. It offers resistance to pest and disease which affects the crops.
    6. It is present in the soluble forms and mostly contain in the cell sap and cytoplasm.
    7. It is readily mobile within the plant tissues.

    4. Magnesium (Mg)
    Source
    Magnesium occurs as carbonates and 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
    1. It is a constituent of chlorophyll, and therefore, essential for its synthesis.
    2. It acts as a phosphorus carrier in the plant.
    3. It is essential for the synthesis of fats and metabolism of carbohydrate and phosphorus.
    4. It is required in binding two subunits of ribosome during protein synthesis.
    5. It acts as an activator for many enzymes in phosphate transfer reactions in carbohydrate metabolism and nucleic acid synthesis.
    6. It is involved in the formation of seeds of high oil contents containing a compound called lecithin
    7. It is readily mobile within the plant tissues.
    5. Sulphur (S)
    Source : It is available to plants in the form of soluble suphates of soil.
    Physiological Role
    1. It is an important constituent of some amionacids (cystine, cysteine and methionine), vitamins (biotine, thiamine), coenzyme A and volatile oils.
    2. It participates in protein synthesis
    3. Sulfohydryl groups are necessary for the activity of many enzymes
    4. Disulphide linkages help to stabilize the protein structure.
    5. It adversely affects chlorophyll synthesis.
    6. Sulphur affects the nodule formation in roots of leguminous plants.
    7. Characteristic odour of Cruciferous plants (onion, garlic etc.) is due to the Sulphur as constituent of volatile oils.
    8. 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 variety of minerals.
    • The soil derived from stone or chalk rocks contains larger percentage of carbonates of lime (calcium carbonate), while sandy soils show Ca deficiency which is met by adding lime or lime stone.
    • The presence of CO2 dissolved in the soil water promotes solubility of carbonate of lime in soil ensuring the quick Ca absorption.
    Physiological Roles
    1. It is the important constituent of middle lamella in the cell wall
    2. It is essential in the formation of cell membranes.
    3. It helps to stabilize the structure of chromosomes.
    4. It is also an activator of many enzymes (ATPase, kinases, succinate dehyrogenase)
    5. It provides a base for the neutralization of organic acids.
    6. It is concerned with the growing root apices.
    7. It is essential for fact metabolism, carbohydrate metabolism and binding of nucleic acids with proteins.
    8. It is also essential in the counteraction of metal toxicity
    9. It is immobile in the plant tissues.


    Micronutrients
    1. Iron (Fe)
    Source: 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 decreased by phosphates. It is absorbed in ferric state; but, ferrous form is only metabolically active form for the plants.
    Physiological Role
    1. It is an important constituent of iron-porphyrin proteins like, cytochromes, peroxidases, catalases etc.
    2. It is essential for the synthesis of chlorophyll.
    3. It acts as a catalyst and electron carrier during respiration.
    4. It also acts as an activator of nitrate reductase and aconitase enzymes.
    5. It is a very important constituent of ferredoxin, which plays an important role in biological nitrogen fixation and primary photochemical reaction in photosynthesis.
    6. It is immobile in the plant tissues. Its mobility is affected by several factors like presence of magnesium, potassium deficiency, high phosphorus and high light intensity.
    2. 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.
    • Absence of organic matter and poor drainage condition of soil cause 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
    1. It acts as an activator of some respiratory enzymes like oxidases, Peroxidases, dehyrogenases, kinases, decarboxylases etc.
    2. It is essential in the formation of chlorophyll
    3. It decreases the solubility of iron by oxidation; in certain cases, abundance of Mn
    leads to Fe deficiency.
    4. It is necessary for the evolution of O2 during photosynthesis.
    5. 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.

    3. Copper (Cu)
    Source :
    • Copper is found in smaller quantity 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.
    Physiological Roles
    1. It acts as a catalyst and regulator
    2. It is a constituent of several oxidizing enzymes like ascorbic oxidase, lactase, tyrosinase, phenoloxidase, plastocyanin etc.
    3. It is essential for photosynthesis, respiration and to maintain carbon/ nitrogen balance.
    4. Its higher concentration is toxic to plants.
    5. It is immobile in the plant tissues.

    4. 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 large or prolonged supply of phosphate fertilizers.
    • It is generally found to be toxic in the neighborhood of zinc deposits.
    Physiological Role
    1. It is a component of enzymes like carbonic anhydrase, alcohol dehyrogenase, glutamic dehydrogenase, lactic dehydrogenase, alkaline phosphatase and carboxy pepsidase.
    2. It is essential for the evolution and utilization of CO2, carbohydrate and phosphorus metabolism.
    3. It is also essential for the biosynthesis of the growth hormone, Indole-3-acetic acid (IAA) and also for the synthesis of RNA.
    4. It is readily mobile within the plant tissues.
    5. It is closely involved in the chlorophyll formation.
    5. 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
    1. It is associated with the prosthetic group of enzyme, nitrate reductase and thus involved in nitrate metabolism.
    2. It acts as an activator of some dehydrogenases and phosphatases and as cofactors in synthesis of ascorbic acid.
    3. It is necessary in the formation of nodules in legumes for the fixation of atmospheric nitrogen.

    6. 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
    1. It is necessary for the translocation of sugars within the plant system
    2. It is involved in reproduction and germination of pollens (tube)
    3. It is concerned with water reactions in cells and regulates intake of water into the cell
    4. It keeps Ca in soluble form within the plant and may act as a regulator of K ratios (K/Ca etc.)
    5. It is also concerned with the nitrogen metabolism and with oxidation and reduction equilibrium in cells.
    6. It is immobile in the plant tissues.

    7. Chlorine
    • 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 nonessential elements by counteracting the toxicity of certain elements present in soil.

Last modified: Wednesday, 6 June 2012, 5:19 PM