Key Notes: Mineral Nutrition - NEET PDF Download

1. IN 1860, JULIUS VON SACHS (GERMAN BOTANIST) → Demonstrated, for the first time, that plants could be grown to maturity in a defined nutrient solution in complete absence of soil.

2. THIS TECHNIQUE OF GROWING PLANTS IN A NUTRIENT SOLUTION IS KNOWN AS HYDROPONICS.

Hydroponics3. Through hydroponics technique, impact of a specific minerals can be studied by adding/substituting or removing that mineral.

4. By this method, essential elements were identified and their deficiency symptoms discovered.

5. Hydroponics technique has been successfully used in commercial production of vegetables such as tomato, seedless cucumber and lettuce.

Essential Mineral Elements

6. Plants obtain their inorganic nutrients from air, water and soil. Plants absorb a wide variety of mineral elements.

7. Not all the mineral elements that they absorb are required by plants.

8. Out of the more than 105 elements discovered so far, less than 21 are essential and beneficial for normal plant growth and development.

9. Criteria for Essentiality

• The element must be absolutely necessary for supporting 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 the element must be specific and not replaceable by another element means no other element can fulfil the requirement of that mineral.
• The element must be directly involved in the metabolism of the plant.

10. VERY FEW MINERALS FULFIL THE CRITERIA OF ESSENTIALITY. These minerals again divided in two groups according to their quantitative requirement.

• MACRONUTRIENTS, AND
• MICRONUTRIENTS

11. Macronutrients

• Macronutrients are generally present in plant tissues in large amounts (in excess of 10 mmole Kg –1 of dry matter).
• THE MACRONUTRIENTS include
• Carbon, Hydrogen, Oxygen, Nitrogen, Phosphorous, Sulphur, Potassium, Calcium And Magnesium.
• Of these, carbon, hydrogen and oxygen are mainly obtained from CO₂ and H₂O, while the others are absorbed from the soil as mineral nutrition.

12. Nitrogen

• Nitrogen is required by plants in the greatest amount.
• It is absorbed mainly as NO₃^– (some are also taken up as NO₂^– or NH₄⁺)
• Nitrogen is required by all parts of a plant, particularly the meristematic tissues and the metabolically active cells.
• It is one of the major constituents of proteins, nucleic acids, vitamins and hormones.

13. Phosphorus

• It is absorbed by the plants from soil in the form of phosphate ions.
• Phosphorus is a constituent of cell membranes, certain proteins, all nucleic acids and nucleotides, and is required for all phosphorylation reactions.

14. Potassium

• It is absorbed as potassium ion (K+).
• It is required in more abundant quantities in the meristematic tissues, buds, leaves and root tips.
• Potassium helps to maintain an anion-cation balance in cells and is involved in protein synthesis, opening and closing of stomata, activation of enzymes and in the maintenance of the turgidity of cells.

15. Calcium

• Plant absorbs calcium from the soil in the form of calcium ions (Ca2+).
• Calcium is required by MERISTEMATIC AND DIFFERENTIATING TISSUES.
• During cell division it is used in the synthesis of cell wall, particularly as calcium pectate in the middle lamella.
• It is also needed during the formation of mitotic spindle.
• It accumulates in older leaves.
• It is involved in the normal functioning of the cell membranes.

16. Magnesium

• It is absorbed by plants in the form of divalent Mg2+.
• It activates the enzymes of respiration, photosynthesis
• Involved in the synthesis of DNA and RNA.
• Magnesium is a constituent of the ring structure of chlorophyll o helps to maintain the ribosome structure.

17. Sulphur

• Plants obtain sulphur in the form of sulphate.
• Sulphur is present in two amino acids –
  - cysteine and
  - methionine
• Main constituent of several coenzymes, vitamins (thiamine, biotin, Coenzyme A) and ferredoxin.

Micronutrients

18. MICRONUTRIENTS OR TRACE ELEMENTS, are needed in very small amounts (less than 10 mmole Kg –1 of dry matter).

19. These include Iron, Manganese, Copper, Molybdenum, Zinc, Boron, Chlorine and Nickel.

20. Iron

• Plants obtain iron in the form of ferric ions (Fe3+)
• It is required in larger amounts in comparison to other micronutrients.
• It is an important constituent of proteins involved in the transfer of electrons like ferredoxin and cytochromes.
• It is reversibly oxidised from Fe²⁺ to Fe³⁺ during electron transfer.
• It activates catalase enzyme and is essential for the formation of chlorophyll.

21. Manganese

• It is absorbed in the form of manganous ions (Mn²⁺).
• It activates many enzymes involved in photosynthesis, respiration and nitrogen metabolism.
• It is required in the splitting of water to liberate oxygen during photosynthesis.

22. Zinc

• Plants obtain zinc as Zn²⁺ ions.
• Activates carboxylases enzyme.
• Needed in the synthesis of auxin.

23. Copper

• It is absorbed as cupric ions (Cu2+).
• It is associated with certain enzymes involved in redox reactions and is reversibly oxidised from Cu⁺ to Cu²⁺.

24. Boron

• It is absorbed as BO₃³⁻ or B₄O₇²⁻.
• Boron is required for uptake and utilisation of Ca²⁺ as well as membrane functioning, pollen germination, cell elongation, cell differentiation and carbohydrate translocation.

25. Molybdenum

• Plants obtain it in the form of molybdate ions.
• It is a component of nitrogenase and nitrate reductase both of which participate in nitrogen metabolism.

26. Chlorine

• It is absorbed in the form of chloride anion (Cl–).
• Na+ and K+ & chlorine help in determining the solute concentration and the anion cation balance in cells.
• It is essential for the water-splitting reaction in photosynthesis, a reaction that leads to oxygen evolution.

27. There are some beneficial elements such as sodium, silicon, cobalt and selenium. They are required by higher plants.

28. Deficiency Symptoms of Essential Elements

• critical concentration: The concentration of the essential element below which plant growth is retarded.
• The element is said to be deficient when present below the critical concentration.
• The morphological changes are indicative of certain element deficiencies and are called deficiency symptoms.
• The deficiency symptoms disappear when the deficient mineral nutrient is provided to the plant.
• The deficiency symptoms appear first in the older tissues because of remobilization of element to younger parts from older part.
• For example, the deficiency symptoms of nitrogen, potassium and magnesium are visible first in the senescent leaves. In the older leaves, biomolecules containing these elements are broken down, making these elements available for mobilising to younger leaves.
• When the elements are relatively immobile and part of structural component of cell then they are not transported out of the mature organs, in this condition the deficiency symptoms tend to appear first in the young tissues for example, element like sulphur and calcium are a part of the structural component of the cell and hence are not easily released.
• This aspect of mineral nutrition of plants is of a great significance and importance to agriculture and horticulture as farmers can not delay the supply of structural element in case of deficiency of these elements in soil otherwise the growth will be affected.
• Chlorosis is the loss of chlorophyll leading to yellowing in leaves and it is caused by the deficiency of elements N, K, Mg, S, Fe, Mn, Zn and Mo.
• Necrosis or death of tissue (particularly leaf tissue) is due to the deficiency of Ca, Mg, Cu, K.
• Lack or low level of N, K, S, Mo causes an inhibition of cell division.
• Some elements like N, S, Mo delay flowering if there concentration in plants is low.

Toxicity of Micronutrients

• Any mineral ion concentration in tissues that reduces the dry weight of tissues by about 10 per cent is considered toxic.
• Toxicity concentration varies for different micronutrients.
• Many a times excess of an element may inhibit the uptake of another element.
• For example, the prominent symptom of manganese toxicity is the appearance of brown spots surrounded by chlorotic veins.
• Manganese competes with iron and magnesium for uptake
• Manganese also competes with magnesium for binding with enzymes.
• Manganese also inhibit calcium translocation in shoot apex.
• Therefore, excess of manganese may induce deficiencies symptoms of iron, magnesium and calcium(even they are present in soil in sufficient amount).
• Thus, what appears as symptoms of manganese toxicity may actually be the deficiency symptoms of iron, magnesium and calcium. On the basis of their diverse functions essential elements can also be grouped into four broad categories. These categories are:
• Structural Elements of Cells
  - Essential elements as components of biomolecules and hence (e.g., carbon, hydrogen, oxygen and nitrogen).
• COMPONENTS OF ENERGY-RELATED CHEMICAL COMPOUNDS
  - e.g., magnesium in chlorophyll and phosphorous in ATP.
• ESSENTIAL ELEMENTS THAT ACTIVATE OR INHIBIT ENZYMES
  - for example, Mg2+ is an activator for both ribulose bisphosphate carboxylase oxygenase and phosphoenol pyruvate carboxylase, both of which are critical enzymes in photosynthetic carbon fixation.
  - Zn²⁺ is an activator of alcohol dehydrogenase.
  - Mo of nitrogenase during nitrogen metabolism.
• ESSENTIAL ELEMENTS WHICH ALTER THE OSMOTIC POTENTIAL OF A
  CELL
  - Sodium and chloride ions and potassium.
  - Potassium plays an important role in the opening and closing of stomata.

Mechanism of Absorption of Elements

• The process of absorption → two main phases.
• In the first phase à an initial rapid uptake of ions into the ‘free space’ or ‘outer space’ of cells – the APOPLAST, is passive.
  - The passive movement of ions into the apoplast usually occurs through ion-channels, the trans-membrane proteins that function as selective pores.
• In the second phase of uptake → the ions are taken in slowly into the inner space which is called THE SYMPLAST of the cells.
  - The entry or exit of ions to and from the symplast requires the expenditure of metabolic energy, which is an ACTIVE PROCESS.
  - The movement of ions is usually called flux; the inward movement into the cells is influx and the outward movement, efflux.

Nitrogen Metabolism

• Plants compete with microbes for the limited nitrogen that is available in soil.
• Nitrogen is a limiting nutrient for both natural and agricultural eco-systems.
• Abundant nitrogen (79%) is present in atmosphere but plant absorb it in nitrate form but some are also taken up it as NO₂^– or NH₄⁺.
• In nature, lightning and ultraviolet radiation provide enough energy to convert nitrogen to nitrogen oxides (NO, NO₂, N₂O).
• Industrial combustions, forest fires, automobile exhausts and powergenerating stations are also sources of atmospheric nitrogen oxides.
• The process of conversion of nitrogen (N₂) to ammonia is termed as nitrogen fixation.
• This conversion of ammonia is done by biological fixation and industrial N₂ fixation.
• Decomposition of organic nitrogen of dead plants and animals into ammonia is called AMMONIFICATION.
• Some of this ammonia volatilises and re-enters the atmosphere but most of it is converted into nitrate by soil bacteria.
• Conversion of ammonia into nitrate is called NITRIFICATION.
• Nitrification is completed in two steps.
  - First, ammonia is oxidized into nitrites by NITROSOMONAS and/or NITROCOCCUS bacteria.
  - Second, nitrite is oxidized into nitrate by NITROBACTAR bacteria.
• THESE NITRIFYING BACTERIA ARE CHEMOAUTOTROPHS.
• Then nitrate is absorbed by plants and is transported to the leaves.
• In leaves, it is reduced to form ammonia that finally forms the amine group of amino acids.
• Denitrification
  - Nitrate present in the soil is also reduced to nitrogen by the process of denitrification.
  - Denitrification is carried by bacteria Pseudomonas and Thiobacillus.
• BIOLOGICAL NITROGEN FIXATION
  (i) REDUCTION OF NITROGEN TO AMMONIA BY LIVING ORGANISMS IS CALLED BIOLOGICAL NITROGEN FIXATION.
  (ii) The enzyme, nitrogenase which is capable of nitrogen reduction is present exclusively in prokaryotes.
  (iii) Biological fixation is property of prokaryotic. Such microbes are called N2- fixers.
  (iv) The nitrogen-fixing microbes could be free-living or symbiotic.
  (v) Free-living nitrogen-fixing aerobic microbes are Azotobacter and Beijernickia.
  (vi) Rhodospirillum is anaerobic and Bacillus free-living.
  (vii) In addition, a number of cyanobacteria such as Anabaena and Nostoc are also free-living nitrogen-fixers.

Symbiotic Biological Nitrogen Fixation

Rhizobium and legumes symbiotic relation

• Species of rod-shaped Rhizobium has such relationship with the roots of several legumes such as alfalfa, sweet clover, sweet pea, lentils, garden pea, broad bean, clover beans, etc.
• These nodules are small outgrowths on the roots.
• Rhizobium and Frankia are free-living in soil, but as symbionts, can fix atmospheric nitrogen.
• FRANKIA PRODUCES ROOT NODULE IN NON-LEGUMINOUS PLANT.
• Nodule Formation: stages in the nodule formation:
  (i) Rhizobia multiply and colonise the surroundings of roots and get attached to epidermal and root hair cells.
  (ii) The root-hairs curl and the bacteria invade the root-hair.
  (iii) An infection thread is produced carrying the bacteria into the cortex of the root, where they initiate the nodule formation in the cortex of the root.
  (iv) Then the bacteria are released from the thread into the cells which leads to the differentiation of specialised nitrogen fixing cells.
  (v) The nodule establishes a direct vascular connection with the host for exchange of nutrients.
  (vi) The nodule contains enzyme nitrogenase and leghaemoglobin.
  (vii) THE ENZYME NITROGENASE IS A MO-FE PROTEIN AND CATALYSES THE CONVERSION OF ATMOSPHERIC NITROGEN TO AMMONIA (THE FIRST STABLE PRODUCT OF NITROGEN FIXATION).
  (viii) The enzyme nitrogenase is highly sensitive to the molecular oxygen and it requires anaerobic conditions.
  (ix) To protect these enzymes, the nodule contains an oxygen scavenger called leg-haemoglobin.
  (x) Rhizobium and Frankia microbes live as aerobes under free-living conditions but during nitrogen-fixing events (in symbiotic association), they become anaerobic to protect the nitrogenase enzyme.
  (xi) The energy require in nitrogen fixation by symbiotic bacteria is fulfilled by host plant.
  (xii) 8 ATP is required to produce one ammonia molecule.
  (xiii) At physiological pH, the ammonia is protonated to form NH₄⁺ (ammonium) ion.
  (xiv) Most of the plants can assimilate nitrate as well as ammonium ions.
  (xv) Ammonium ions are toxic to plants and cannot accumulate in plant so the NH4 + is used to synthesise amino acids in plants.
• THERE ARE TWO MAIN WAYS IN WHICH THIS CAN TAKE PLACE.
  (i) REDUCTIVE AMINATION: In these processes, ammonia reacts with αketoglutaric acid and forms glutamic acid and the enzyme for this reaction is GLUTAMATE DEHYDROGENASE.
  (ii) TRANSAMINATION: It involves the transfer of amino group from one amino acid to the keto group of a keto acid. Glutamic acid is the main amino acid from which the transfer of NH2, the amino group takes place and other amino acids are formed through transamination. The enzyme transaminase catalyses all such reactions.
  (iii) The two most important amides – asparagine and glutamine – found in plants are a structural part of proteins.
  (iv) They are formed from two amino acids, namely aspartic acid and glutamic acid, respectively, by addition of another amino group to each.
  (v) in this reaction, the hydroxyl part of the acid is replaced by another NH2 radicle.
  (vi) Amides contain more nitrogen than the amino acids, they are transported to other parts of the plant via xylem vessels.
  (vii) The nodules of some plants like soybean export the fixed nitrogen as ureides. these compounds also have a particularly high nitrogen to carbon ratio.