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Class 11

Biology

Plant Physiology

Mineral Nutrition

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Write names of essential elements of plants and write their function, availability, form in which absorbed and deficiency symptoms of any four.

Solution: The elements which have been found to occur in all plants and are necessarily required for the normal growth, development and reproduction of plants are called essential elements. The total number of elements essential for plants is seventeen (17). These are C, H, O, P, K, Ca, Mg, S, B, Cu, Fe, Cl, Mo, Mn, Zn and Ni.

Role of macro and micronutrients in plant nutrition:
  • Essential elements participate in different physiological activities of plants.
  • These elements play important role in, regulating the 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, buffer actions, etc.
Potassium (K):
  • It is found in soil insoluble and exchangeable form.
  • It is absorbed in the form of potassium ion ().
  • Potassium is not a component of any biochemical compound and is not a structural element.
  • It plays an important role in four biochemical reactions viz cation neutralization, transport across the membrane, enzyme activation and osmotic potential regulation.
  • The best function of potassium is in regulating the opening of stomata.
  • It also plays an 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 internodes results in 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 () and is absorbed as magnesium ion ().
  • 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 the synthesis of DNA and RNA.
  • It is found in abundant amount in oilseeds as it plays an important role in the formation of oil seeds.
Deficiency symptoms:
  • Magnesium deficiency causes extensive interveinal chlorosis.
  • Leaves show reddish and yellow and orange spots due to the 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 (). Besides this sulphur dioxide () present as an environmental pollutant may also be absorbed by plants.
  • It is helpful in the 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 better 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 the formation of more of thick-walled tissue such as sclerenchyma and xylem.
Iron (Fe):
  • Plants absorb iron from the soil in the form of ferric ions ().
  • It plays an important role in cell division, respiration and different steps of the electron transport system.
  • It is an important constituent of cytochrome and ferredoxin and acts as an 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 (Phytoferritin) in leaves and in the chromatin network of the nucleus.
  • In metabolic reactions, it participates as (Ferrous).
Deficiency symptoms:
  • Young leaves show extensive chlorosis and may become white or yellow-white. The derailment of reactions of photosynthesis, respiration and protein synthesis occurs.
  • Cell division activity is inhibited.
  • Plant growth becomes slow..
Zinc (Zn):
  • This element is absorbed from soil in the form of zinc ions ().
  • Zinc is required for the synthesis of growth hormone indole acetic acid (IAA).
  • It also controls the absorption of phosphorus.
  • Zinc is a component of several types of enzymes especially carboxylases.
Copper (Cu):
  • It is absorbed from soil by roots in the form of divalent cupric ions () or monovalent cuprous ions ().
  • Copper acts as an electron carrier in oxidation-reduction reactions.
  • It is a component of plastocyanin and cytochrome oxidase which acts as an electron carrier in photosynthesis.
Deficiency symptoms:
  • Wilting and curling of leaves occurs.
  • The leaf tip shows chlorosis and becomes discoloured.
  • In Citrus deficiency of copper causes dieback disease of a leaf.
Molybdenum (Mo):
  • This element is absorbed in the form of molybdenum oxide ().
  • 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 a characteristic 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 whiptail disease and is caused due to molybdenum deficiency.
Chlorine (Cl):
  • Chlorine is absorbed from soil in the form of chloride ions ().
  • This element is not a component of any biochemical substance.
  • It plays an important role in carbohydrate metabolism and in balancing anion-cation concentration in the cell.
  • It is helpful in the photolysis of water leading to the evolution of oxygen.
Deficiency symptoms:
  • Leaves show variegated chlorosis followed by necrosis.
  • Fruit formation is reduced and the 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 (). Nickel is neither a component of any biochemical compound nor its function and deficiency are properly understood.
  • It is an essential part of the enzyme urease. It is probably helpful in the transportation of nitrogenous compounds.
Deficiency symptoms:
  • Plants show chlorosis and formation of necrotic tissue in leaves.
  • Plants growing in soil normally do not suffer a 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 as N, P, K, Mg, Zn, Mo.
  • Deficiency symptoms first appear in young leaves due to deficiency of immobile elements such as Cu, S, Fe, Mn. 
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