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Kerala Plus One Botany Notes Chapter 8 Mineral Nutrition
Methods to Study the Mineral Requirements of Plants
In 1860, Julius von Sachs a German botanist demonstrated that plants could be grown in a nutrient solution in the complete absence of soil.
Hydroponics and its Importance
- This technique of growing plants in a nutrient solution is known as hydroponics.
- The nutrient solutions must be aerated to obtain optimum growth.
- In this method, essential elements are used and their deficiency symptoms can be studied.
- Hydroponics is used in the commercial production of vegetables such as tomato, seedless cucumber, and lettuce.
Hydroponic plant production. Plants are grown in a tube or trough placed on a slight incline. A pump circulates a nutrient solution from a reservoir to the elevated end of the tube. The solution flows down the tube and returns to the reservoir due to gravity. Inset shows a plant whose roots are continuously bathed in the aerated nutrient solution. The arrows indicate the direction of the flow.
Essential Mineral Elements
Some minerals are not essential to plants
- More than sixty elements are found in different plants.
- Some plant species absorb selenium, some others gold, while some plants growing near nuclear test sites take up radioactive strontium.
Criteria for Essentiality
The criteria for the essentiality of an element are given below:
- The element must be supporting normal growth and production.
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.
- The element must be directly involved in the metabolism of the plant.
Based upon the above criteria 17 elements are essential for plant growth and metabolism. They are
Carbon, hydrogen, oxygen, nitrogen, phosphorous, sulphur, potassium, calcium, and magnesium
They are present in plant tissues in large amounts(in excess of 10 m mole/ Kg of dry matter).
Iron, manganese, copper, molybdenum, zinc, boron, chlorine, and nickel
They are needed in very small amounts (less than 10 m mole /Kg of dry matter).
In addition to the essential elements, sodium, silicon, cobalt, and selenium are required by higher plants. Essential elements are grouped into four broad categories on the basis of their diverse functions.
i. Essential elements as components of biomolecules (e.g., carbon, hydrogen, oxygen, and nitrogen).
ii. Essential elements that are components of energy-related chemical compounds (e.g, magnesium in chlorophyll and phosphorous in ATP).
iii. Essential elements that activate or inhibit enzymes, (Mg2+ is an activator for both ribulose bisphosphate carboxylase oxygenase and phosphoenolpyruvate carboxylase, both of which are critical enzymes in photosynthetic carbon fixation
Zn2+ is an activator of alcohol dehydrogenase and Mo of nitrogenase during nitrogen metabolism.
iv. Essential elements alter the osmotic potential of a cell.
Potassium plays an important role in the opening and closing of stomata.
Role of Macro- and Micro-nutrients
Essential elements participate in various metabolic processes in the plant cells. The various forms and functions of mineral elements are given below.
- It is absorbed mainly as NO3– Some taken up as NO2 or NH4+
- Nitrogen is required in meristematic tissues and the metabolically active cells.
- It is one of the major constituents of proteins, nucleic acids, vitamins and hormones
- It is absorbed in the form of phosphate ions (either as HPO42- or H2PO4–)
- Phosphorus is a constituent of cell membranes, certain proteins, all nucleic acids, and nucleotides.
- It is required for all phosphorylation reactions.
- It is absorbed as a potassium ion (K+).
- It is required in abundant quantities for meristematic tissues, buds, leaves, and root tips.
- Potassium helps to maintain an anion-cation balance in cells
- It is involved in protein synthesis
- It is involved in the opening and closing of stomata and activation of enzymes
- It helps in the maintenance of the turgidity of cells.
- It is absorbed in the form of calcium ions (Ca2+).
- Calcium is required by meristematic and differentiating tissues.
- It is important in the formation of calcium pectate in the middle lamella.
- It is also needed during the formation of the mitotic spindle.
- It activates certain enzymes and plays an important role in regulating metabolic activities.
- It is absorbed by plants in the form of divalent Mg2+
- It activates the enzymes of respiration, photosynthesis, and are involved in the synthesis of DNA and RNA.
- Magnesium is a constituent of the ring structure of chlorophyll
- It helps to maintain the ribosome structure.
- It is absorbed in the form of sulphate (SO42-)ion.
- Sulphur is present in two amino acids – cysteine and methionine
- It is the main constituent of several coenzymes, vitamins (thiamine, biotin, Coenzyme A), and ferredoxin.
- It is absorbed in the form of ferric ions (Fe3+)
- It is an important constituent of proteins involved in the transfer of electrons like ferredoxin and cytochromes.
- It activates the catalase enzyme and is essential for the formation of chlorophyll.
- It is absorbed in the form of manganous ions (Mn2+).
- It activates many enzymes involved in photosynthesis, respiration, and nitrogen metabolism.
- It is also involved in the splitting of water to liberate oxygen during photosynthesis.
- Plants obtain zinc as Zn2+ ions.
- It activates various enzymes, especially carboxylases.
- It is also needed in the synthesis of auxin.
- It is absorbed as cupric ions (Cu2+).
- It is essential for the certain enzymes involved in redox reactions
- It is absorbed as BO33- or B4O72-
- It is required for uptake and utilisation of Ca2+
- it helps in membrane functioning
- it helps pollen germination
- it helps cell elongation and cell differentiation
- it is involved in carbohydrate translocation.
- Plants obtain it in the form of molybdate ions (MoO22+).
- It is a component of nitrogenase and nitrate reductase both of which participate in nitrogen metabolism.
- It is absorbed in the form of chloride anion (Cl–).
- Along with Na+ and 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, a reaction that leads to oxygen evolution.
Deficiency Symptoms of Essential Elements
If the concentration of the essential element below the critical concentration plants shows certain morphological changes. These are indications of deficiency symptoms.
Mobility of element determines deficiency symptoms
Deficiency symptoms in older tissues
Deficiency symptoms also depend on the mobility of the element in the plant. It first appears in the older tissues.
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 and available for mobilising to younger leaves.
Deficiency symptoms in younger tissues
Sometimes the deficiency symptoms appear first in the young tissues. If the elements are immobile, they are not transported from mature organs to younger organs.
For example, Elements like sulphur and calcium are structural components of the cell and hence are not easily released.
The deficiency symptoms are
- stunted plant growth
- premature fall of leaves and 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 like N, K, Mg, S, Fe, Mn, Zn, and Mo.
Necrosis, or 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 causes inhibition of cell division.
Deficiency of elements like N, S, Mo delay flowering
Toxicity of Micronutrients
If the supply of micronutrients at a moderate decreased level shows deficiency symptoms but the moderate increase causes toxicity, i.e the excess of an element inhibits the uptake of another element.
Symptoms and other effects of Manganese toxicity
- Symptom of manganese toxicity is the appearance of brown spots surrounded by chlorotic veins.
- Manganese competes with iron and magnesium for uptake and for binding with enzymes.
- Manganese also inhibits calcium translocation in the shoot apex.
- Symptoms of manganese toxicity induce
- Deficiency symptoms of iron, magnesium, and calcium.
Mechanism of Absorption of Elements
The process of absorption occurs in two main phases-
- Apoplast (passive). The passive movement of ions into the apoplast occurs through ion- channels and the transmembrane proteins.
- Symplast(active) The inward movement of ions into the cells is called influx and the outward movement efflux. This movement occurs by using metabolic energy.
Translocation of Solutes
- Mineral salts are pulled up through the plant by the transpirational pull.
- Analysis of xylem sap shows the presence of mineral salts in it.
- Radioisotopic studies support the xylem transport of mineral elements.
Soil as a Reservoir of Essential Elements
- Soil consist of a variety of minerals, nitrogen-fixing bacteria, and other microbes holds water and supplies air to the roots, and acts as a matrix that stabilises the plant.
- If the amount of nutrients in the soil is decreased, it is supplied from outside as fertilizers in the form of macronutrients (N, P, K, S, etc.) and micronutrients (Cu, Zn, Fe, Mn, etc.)
Metabolism of Nitrogen
Nitrogen is a constituent of amino acids, proteins, hormones, chlorophyll, and many vitamins.
Atmospheric nitrogen consists of two nitrogen atoms joined by a very strong triple covalent bond main nitrogen pools-atmospheric soil, and biomass.
1. N2 Fixation: The process of conversion of atmospheric nitrogen (N2) to ammonia is termed nitrogen fixation.
- Ammonia is converted into nitrate.
- Ammonia is first oxidized to nitrite by Nitrosomonas or Nitrococcus.
- The nitrite is further oxidized to nitrate with the help of the bacterium Nitrobacter
3. Ammonification: Decomposition of organic nitrogen of dead plants and animals into ammonia is called ammonification
4. Denitrification: It is the conversion of soil nitrate into molecular N2 by Thiobacillus and pseudomonas
Formation of nitrogen oxides
- In nature, lightning and UV provide energy to convert nitrogen to nitrogen oxides (NO, NO2, N2O).
- Industrial combustions, forest fires, automobile exhausts, and power generating stations are also sources of atmospheric nitrogen oxides.
Biological Nitrogen Fixation
The nitrogen-fixing microbes are free-living or symbiotic. ‘Free-living nitrogen-fixing aerobic microbes are Azotobacter, Beijernickia Rhodospirillum Bacillus Anabaena Nostoc.
Development of root nodules in soyabean
Development of root nodule sin soyabean:
- Rhizobium bacteria contact susceptible root hair, divide near it.
- Upon successful infection of the root hair cause it to curl.
- Infected thread carries the bacteria to the inner cortex. The bacteria get modified into rod-shaped bacteroids and cause inner cortical and pericycle cells to divide. Division and growth of cortical and pericycle cells lead to nodule formation.
- A mature nodule is complete with vascular tissues continuous with those of the root.
Basic steps are given below
- Rhizobium bacteria attach the root hair.
- Root hair curls.
- Infected thread carries the bacteria to the inner cortex.
The bacteria get modified into rod-shaped bacteroids and cause inner cortical and pericycle cells to divide. Division and growth of cortical and pericycle cells lead to nodule formation, d) A mature nodule is complete with vascular tissues continuous with those of the root.
Overall equation for N2 fixation
N2 + 8e– + 8H+ +16ATP → 2NH3 + H2 + 16ADP + 16Pt
Fate of ammonia
- At first, ammonia protonated to form NH4+.
- This ammonium ion is used to synthesise amino acid in plants
There are two ways for the synthesis of amino acids in plants
1. Reductive animation
In this, ammonium ion reacts with alpha-ketoglutaric acid and forms glutamic acid.
It involves the transfer of an 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 amino groups takes place and other amino acids are formed in the presence of transaminase.
- The important amides are asparagine and aspartate.
- Amide is formed when the hydroxyl group of one amino acid is replaced by an amino group.
- Since amide contains more nitrogen than amino acids. They are transported through xylem vessels.