Monday, August 31, 2020

Mineral Nutrition - Notes | Class 11 | Part 1: Essential Mineral Elements

12. MINERAL NUTRITION 

All organisms require macromolecules (carbohydrates, proteins, fats etc.), water & minerals for growth and development.

METHODS TO STUDY THE MINERAL REQUIREMENTS OF PLANTS


The technique of growing plants in a defined nutrient solution (without soil) is known as hydroponics.

It is demonstrated by Julius von Sachs (Germany, 1860).

The nutrient solution is aerated for optimum growth.

Hydroponics is used to identify the essential elements required for plants and their deficiency symptoms.

In this, plant roots are immersed in nutrient solutions and an element is added / removed or given in varied concentration.

Hydroponics is used for commercial production of vegetables such as tomato, seedless cucumber & lettuce.

ESSENTIAL MINERAL ELEMENTS


More than 60 elements are found in different plants.

Some plant species growing near mining sites accumulate selenium, gold etc. Some plants near nuclear test sites take up radioactive strontium.

There are techniques to detect the minerals even at a very low concentration (10^-8 g/mL).

Criteria for Essentiality of an element

  • An element must be needed for normal growth and reproduction. In the absence of the element the plants do not complete their life cycle or set the seeds.
  • The requirement of an element must be specific. i.e., deficiency of an element cannot be met by another element.
  • It must be directly involved in the plant metabolism.
Only 17 elements are absolutely essential for plant growth and metabolism.

Based on quantitative requirements, essential elements are 2 types: Macronutrients & Micronutrients.

i. Macronutrients 

They are present in plant tissues in large amounts (more than 10 mmole Kg–1 of dry matter).

They include carbon, hydrogen, oxygen, nitrogen, phosphorous, sulphur, potassium, calcium & magnesium.

C, H & O are mainly obtained from CO2 & H2O. Others are absorbed from soil as mineral nutrition.

ii. Micronutrients (trace elements) 

They are needed in very small amounts (less than 10 mmole Kg–1 of dry matter).

They include iron, manganese, copper, molybdenum, zinc, boron, chlorine and nickel.

Higher plants also need sodium, silicon, cobalt, selenium etc.

Based on functions, essential elements are 4 categories:
  1. Components of biomolecules & structural elements of cells: E.g. carbon, hydrogen, oxygen & nitrogen.
  2. Components of energy-related chemical compounds: E.g. Mg in chlorophyll and phosphorous in ATP.
  3. Elements that activate or inhibit enzymes: E.g. Mg2+ is an activator for RUBISCO & phosphoenol pyruvate carboxylase (critical enzymes in photosynthetic carbon fixation). Zn2+ is an activator of alcohol dehydrogenase and Mo of nitrogenase during nitrogen metabolism.
  4. Elements that alter the osmotic potential of a cell: E.g. Potassium helps in opening & closing of stomata.

Role of Macro- and Micro-nutrients


Essential elements have role in metabolic processes such as
  • Permeability of cell membrane.
  • Maintenance of osmotic concentration of cell sap.
  • Electron transport systems.
  • Buffering action.
  • Enzymatic activity.
  • Constituents of macromolecules and co-enzymes.
Nitrogen:
  • This is required by plants in the greatest amount.
  • It is absorbed mainly as NO3–. Some are also taken up as NO2– or NH4+.
  • It is essential for all plant parts, particularly the meristematic tissues and the metabolically active cells.
  • It is the major constituents of amino acids, proteins, nucleic acids, chlorophyll, vitamins and hormones.
Phosphorus:
  • It is absorbed by plants from soil as phosphate ions (as H2PO4− or HPO42−).
  • It is a constituent of cell membranes, certain proteins, all nucleic acids and nucleotides.
  • It is required for all phosphorylation reactions.
Potassium:
  • It is absorbed as potassium ion (K+).
  • Essential in meristematic tissues, buds, leaves & root tips.
  • It maintains an anion-cation balance in cells.
  • It is involved in protein synthesis, opening & closing of stomata, activation of enzymes and in the maintenance of the turgidity of cells.
Calcium:
  • It is absorbed from the soil as calcium ions (Ca2+).
  • It is required by meristematic and differentiating tissues.
  • During cell division, it is used in the synthesis of cell wall, particularly as calcium pectate in middle lamella. It is also needed during the formation of mitotic spindle.
  • It accumulates in older leaves.
  • It is involved in the functioning of the cell membranes.
  • It activates some enzymes and regulates metabolic activities.
Magnesium:
  • It is absorbed by plants as divalent Mg2+.
  • It activates enzymes of respiration & photosynthesis.
  • It is involved in the synthesis of DNA and RNA.
  • It is a constituent of the ring structure of chlorophyll.
  • It helps to maintain the ribosome structure.
Sulphur:
  • Plants obtain it as sulphate (SO)42−.
  • It is present in 2 amino acids (cysteine & methionine).
  • It is the constituent of several coenzymes, vitamins (thiamine, biotin, Coenzyme A) and ferredoxin.
Iron:
  • Plants obtain iron as ferric ions (Fe3+).
  • It is required in larger amounts in comparison to other micronutrients.
  • It is a main constituent of proteins involved in the transfer of electrons like ferredoxin and cytochromes.
  • It is reversibly oxidized from Fe2+ to Fe3+ during electron transfer.
  • It activates catalase enzyme, and is essential for the formation of chlorophyll.
Manganese:
  • It is absorbed as manganous ions (Mn2+).
  • It activates many enzymes involved in photosynthesis, respiration and nitrogen metabolism.
  • The best defined function of manganese is in the splitting of water to liberate O2 during photosynthesis.
Zinc:
  • Plants obtain zinc as Zn2+ ions.
  • It activates various enzymes, especially carboxylases.
  • It is needed in the synthesis of auxin.
Copper:
  • It is absorbed as cupric ions (Cu2+).
  • It is essential for the overall metabolism in plants.
  • Like iron, it is associated with some enzymes in redox reactions and is reversibly oxidised from Cu+ to Cu2+.
Boron:
  • It is absorbed as BO3 3− or B4O72−.
  • It is required for uptake & utilisation of Ca2+, membrane functioning, pollen germination, cell elongation, cell differentiation & carbohydrate translocation.
Molybdenum:
  • Plants obtain it as molybdate ions (MoO)22+.
  • It is a component of many enzymes such as nitrogenase & nitrate reductase. These enzymes participate in nitrogen metabolism.
Chlorine:
  • It is absorbed as chloride anion (Cl–).
  • Along with Na+ & K+, it helps in determining the solute concentration and the anion-cation balance in cells.
  • It is essential for the water-splitting reaction in photosynthesis that leads to oxygen evolution.

Deficiency Symptoms of Essential Elements


Deficiency of an essential element causes retarded growth.

Concentration of an essential element below which plant growth is retarded is called critical concentration. The element is said to be deficient when present below the critical concentration.

The morphological changes due to deficiency or absence of an element are called deficiency symptoms.

Deficiency symptoms vary from element to element.

The plant parts that show the deficiency symptoms depend on the mobility of the element. For elements that are actively mobilized and exported to young developing tissues, the deficiency symptoms appear first in the older tissues. E.g. deficiency symptoms of nitrogen, potassium and magnesium are visible first in the senescent leaves.

In older leaves, biomolecules containing these elements are broken down. It makes these elements available for mobilizing to younger leaves.

If the elements are relatively immobile and are not transported out of the mature organs, the deficiency symptoms appear first in the young tissues. E.g. S and Ca are part of the structural component of the cell and hence are not easily released.

This aspect of mineral nutrition has great significance in agriculture and horticulture.

The deficiency symptoms include chlorosis, necrosis, stunted growth, premature fall of leaves & buds and inhibition of cell division.

Chlorosis is the loss of chlorophyll leading to yellowing in leaves. It is due to the deficiency of elements N, K, Mg, S, Fe, Mn, Zn and Mo.

Necrosis is the death of tissue, particularly leaf tissue. It is due to the deficiency of Ca, Mg, Cu, K.

Lack or low level of N, K, S & Mo inhibits cell division. 

Low concentration of N, S, Mo etc. delay flowering.

Deficiency of different elements may cause same symptoms. Hence, to identify the deficient element, all the symptoms are studied. Also, different plants respond differently to the deficiency of the same element.

Toxicity of Micronutrients


A moderate increase in micronutrients causes toxicity.

Any mineral ion concentration in tissues that reduces the dry weight of tissues by about 10% is considered toxic. Such critical concentrations vary widely among different micronutrients.

The toxicity symptoms are difficult to identify. Toxicity levels for an element also vary for different plants.

Excess of an element may inhibit the uptake of another element. E.g. Excess of Mn induces deficiencies of Fe, Mg & Ca because it competes with Fe & Mg for uptake and with Mg for binding with enzymes. Mn also inhibits Ca translocation in shoot apex. Thus symptoms of Mn toxicity may actually be the deficiency symptoms of Fe, Mg & Ca. Main symptom of manganese toxicity is the appearance of brown spots surrounded by chlorotic veins.
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