Topics to be Learn :

  • Plant growth
  • Phases of growth
  • Conditions for growth
  • Growth rate and types of growth
  • Growth curve
  • Differentiation, De-differentiation. Re-differentiation
  • Development
  • Plasticity
  • Growth Hormones
  • Photoperiodism
  • Vernalization
  • Mineral nutrition
  • Nitrogen cycle

Plant Growth

  • Growth is a characteristic feature of living organisms.
  • Two aspects of growth:
    1. Quantitative: Increase in length, size, volume, numbers, body mass, dry weight, etc.
    2. Qualitative: Involves a change in the nature of growth.
      • Development: Orderly progress.
      • Differentiation: Leads to a higher and more complex state.
  • Growth: A permanent, irreversible increase in the bulk of an organism, accompanied by a change of form.
  • Growth in Multicellular/Vascular Plants:
    • Indeterminate growth occurs throughout life indefinitely.
    • It's restricted to specific regions called meristems. 
  • Meristems: Regions where new cells are constantly produced.
    • Three types: a. Apical: Found at the apices of roots and shoots. b. Intercalary: Located at the node or base of the internode of stems. c. Lateral: Located laterally along the axis of dicots and gymnosperms.
  • Apical Meristem:
    • Responsible for growth in length/height and differentiation of cell types.
    • Contributes to primary growth.
  • Intercalary Meristem:
    • Located at the node or base of internode of stem.
    • Responsible for increasing the length of internodes and formation of leaf primordia and lateral buds.
  • Lateral Meristem:
    • Located laterally along the axis of dicotyledons and gymnosperms.
    • Called vascular cambium in dicots.
    • Responsible for an increase in the girth of the stem due to the addition of secondary vascular tissues.
  • Phases of Growth:
    • Three phases: formation phase, elongation phase, and maturation phase.
    • Cells in the meristem divide, enlarge, and get differentiated.

Three Phases of Growth:

(i) Formative Phase (Phase of Cell Division):

  • First phase of growth.
  • Meristematic cells undergo mitosis to produce new cells.
  • Cells are thin-walled, non-vacuolated with prominent nucleus and granular cytoplasm.
  • One cell remains meristematic, while the other undergoes enlargement and differentiation.
  • Growth rate is slower (Lag phase).

(ii) Elongation Phase (Phase of Cell Enlargement):

  • Second phase of growth.
  • Newly formed cells enlarge, increasing size and volume.
  • Cells become vacuolated, osmotically active, and turgid due to water absorption.
  • Enlargement occurs both lengthwise and breadthwise.
  • Considerable increase in size and weight of organs and plant.
  • New wall materials and other substances synthesized.
  • Growth rate accelerates (exponential or Log phase).

(iii) Maturation Phase (Phase of Cell Maturation and Differentiation):

  • Third and last phase of growth.
  • Enlarged cells specialize to perform specific functions and attain maturity.
  • Growth rate slows down and becomes steady (Stationary phase).
Root and Stem Growth:
  • Show indefinite or indeterminate growth.
  • Organs like leaves, flowers, and fruits show determinate growth, growing up to a certain genetically destined size.
  • In unicellular plants, growth is uniform and determinate.

 Characteristics of Growth:
  • Permanent increase in size, weight, shape, volume, and dry weight of a plant.
  • Change is permanent and irreversible.
  • Intrinsic process caused by internal activities.
  • Occurs by cell division and cell elongation followed by cell maturation.
  • Mostly localized to regions with meristematic tissues.
  • Qualitative aspect where development is orderly and differentiation leads to a more complex state.

Conditions for Growth:

  • Carbon/Nitrogen ratio affects growth as both are structural elements.
  • Water is essential for cell turgidity during enlargement and as a medium for biochemical reactions.
  • Nutrients, including Macronutrients and micronutrients, are necessary.
  • Optimum temperature: 25 — 35 °C.
  • Light is essential for seed germination and photosynthesis.
  • Oxygen is necessary for respiration.
  • Gravitational force decides the direction of growth.
  • Growth hormones control growth and various physiological aspects.
Growth Rate and Types of Growth

Growth Rate:

  • Definition: Increased growth per unit time, also known as efficiency index.
  • Growth in plants can be measured as an increase in:
    • Number: e.g., Cells
    • Surface area: e.g., Leaf
    • Length: e.g., Pollen tube
    • Volume: e.g., Fruit
    • Girth: e.g., Stem
    • Dry weight

Various Methods for Measurement of Linear Growth:

  • Direct Method: Measurement with a scale.
  • Horizontal Microscope: Useful for measuring growth in fields.
  • Auxanometer: For linear growth of shoot - two types: Arc auxanometer and Pfeffer’s auxanometer.
  • Crescograph: Records primary growth, provides information on growth per second, developed by Sir J. C. Bose.
  • Growth Rate/Efficiency Index: Increased growth per unit time, e.g., increase in area of leaf, size of flower, etc.
  • Absolute Growth Rate (AGR): Ratio of change in cell number (dn) over time interval (dt), i.e., AGR = dn/dt, total growth per unit time.
  • Relative Growth Ratio (RGR): AGR divided by total number of cells present, i.e., growth of given system, RGR = AGR/n, ratio of growth in given time / initial growth.
  • For describing cell growth in culture, AGR and RGR are useful.

Types of Growth:

  • Two Types:
    1. Arithmetic Growth
    2. Geometric Growth

Arithmetic Growth:

  • Characteristics:
    • Rate of growth is constant, hence linear curve.
    • After mitosis, one daughter cell continues to divide, while the other cell undergoes differentiation and maturation.
  • Examples: Elongation of root at a constant rate.
  • Mathematical Expression: Lt = Lo + rt
    • Where: Lt = Length at time ‘t’, Lo = Length at time ‘Zero’, r = Growth rate, t = Time of growth.

Geometric Growth:

  • Characteristics:
    • Cell divides mitotically into two.
    • Both daughter cells continue to divide and redivide repeatedly.
    • Growth rate is slow initially but later there is rapid growth at an exponential rate.
  • Mathematical Expression: W1 = Wo ert
    • Where: W1= Final size, Wo = Initial size, r = Growth rate, t = Time of growth, e = Base of natural logarithm.

Growth Curve:
  • Definition: Graphic representation of total growth against time.
  • Characteristics:
    • Growth rate is low in lag phase.
    • Faster growth rate reaching maximum in exponential or log phase.
    • Gradually slows down in stationary phase.
  • Curve: Sigmoid curve obtained when growth rate plotted against time for all three phases.
  • Grand Period of Growth (GPG): Total period required for all phases (Lag, log, and stationary).

Differentiation, De-Differentiation, Re-Differentiation:

Differentiation:

  • Process of maturation of cells derived from apical meristems.
  • Permanent change in structure and function of cells.
  • Cell undergoes major anatomical and physiological change.
  • e.g., Hydrophytic plants' parenchyma cells develop large schizogenous cavities.

De-differentiation:

  • Process where living differentiated cells regain capacity to divide.
  • Permanent cells become meristematic.
  • e.g., Cork cambium, Parenchyma cells forming interfascicular cambium.

Re-differentiation:

  • Process where cells produced by de-differentiation lose capacity of division.
  • Cells mature to perform specific function.
  • Interfascicular cambium formed by dedifferentiation loses its capacity to divide.
  • Secondary xylem and secondary phloem formed from this cambium.

Development:

  • Progressive changes in shape, form, and degree of complexity in an organism.
  • In plants, includes changes from seed germination to senescence or death of plant.
  • Orderly process including growth, morphogenesis, maturation, and senescence.

 

Plasticity:

  • Capacity of plant to be molded or formed.
  • Ability to develop different kinds of structures in response to environmental factors or stimuli.
  • e.g., Buttercup showing different leaves in terrestrial and aquatic habitats.

Growth Hormones:
  • Definition: Internal factors influencing growth, inhibiting, promoting, or modifying growth.
  • Growth Promoters:
    • Auxins
    • Gibberellins (GA)
    • Cytokinins (CK)
  • Growth Inhibitors:
    • Ethylene
    • Abscisic acid (ABA)
  • Growth Regulators: All phytohormones.
  • Transport: Mainly through phloem parenchyma.

Scientists and Their Work:

  • Charles Darwin: Discovery of auxin with tropism studies of canary grass coleoptile exposure to light.
  • Boysen-Jensen: Observations of bending of coleoptiles with gelatin sheet insertion experiment - effect of auxin.
  • Paal: Observed coleoptile bending due to auxin even in dark.
  • F.W. Went: Successfully isolated natural auxin Avena coleoptile tips in agar blocks - Avena curvature assay.
Auxins:
  • Term Given By: F.W. Went
  • Source:
    • First isolated from human urine.
    • Synthesized in apical meristematic region in plants.
  • IAA (Indole 3 Acetic Acid):
    • Most common natural auxin.
    • Synthesized from amino acid Tryptophan.
  • Synthetic Auxins:
    • IBA (Indole Butyric Acid)
    • NAA (Naphthalene Acetic Acid)
    • 2,4-D (Dichloro Phenoxy Acetic Acid)

Physiological Effects and Applications of Auxins:

  • Cell elongation and cell enlargement.
  • Apical Dominance: Growing apical bud inhibits growth of lateral buds.
  • Stimulation of growth of root and stem.
  • Multiplication of cells hence utilized in tissue culture.
  • Formation of lateral and adventitious roots.
  • 2,4-D: Selective herbicide — kills dicot weeds.
  • Induced parthenocarpy — seedless grapes, banana, lemon, orange.
  • Promote cell division and early differentiation of vascular tissue xylem and phloem.
  • Induces early rooting in cutting method of artificial vegetative propagation.
  • Foliar spray of synthetic auxins:
    • Flowering induced in litchi and pineapple.
    • Prevents early fruit drop of apple, pear, and oranges.
    • Prevents formation of abscission layer.
  • Increase in rate of respiration.
  • Break seed dormancy and promote seed germination.

Gibberellins (GA):
  • Named By: Yabuta and Sumuki
  • Source:
    • First isolated from fungus Gibberella fujikuroi by Kurasawa.
    • Rice seedlings show Bakane disease with stem elongation due to this fungus infestation.
  • Synthesis:
    • From mevalonic acid in young leaves, seeds, root, and stem tips.
  • GA3 (Gibberellic Acid):
    • Most common and biologically active.
    • Contains gibbeane ring.

Physiological Effects and Applications of Gibberellins:

  • Breaking of bud dormancy, seed dormancy.
  • Promoting synthesis of amylase in cereals, stimulating seed germination e.g., Wheat, barley.
  • Increase in length of internodes thereby elongation of stem.
  • Bolting in rosette plants — elongation of internodes before flowering e.g., Cabbage, beet.
  • Parthenocarpy in tomato, apple, pear.
  • Stimulates flowering in long day plants.
  • Increase in fruit size and bunch length e.g., grapes.
  • Overcomes effects of vernalization.
  • Inhibition of root growth, delay senescence and abscission.
  • Production of male flowers on female plants.
  • Convert genetically dwarf plants to phenotypically tall plants e.g., maize.

Cytokinin:
  • Term Coined By: Letham
  • Natural Sources: Banana flowers, apple, and tomato fruits.
  • Discovery:
    • Discovered by Skoog and Miller in Callus culture of Tobacco.
    • Present in herring (fish) sperm DNA — Kinetin.
  • Chemical Structure:
    • Derivatives of adenine, a purine base.
    • Chemically 6-furfuryl amino purine.
  • First Natural Cytokinin: Zeatin, obtained by Letham from maize grain.
  • Synthetic Hormone: 6-benzyl adenine.
  • Importance: Vital in plant tissue culture (callus) for morphogenesis.

Physiological Effects and Applications of Cytokinin:

  • Promote cell division and cell enlargement.
  • Promote shoot formation, buds.
  • Cytokinin and auxin ratio controls morphogenesis.
  • Growth of lateral buds, controls apical dominance.
  • Delay of aging and senescence, also abscission.
  • Formation of interfascicular cambium.
  • Breaks dormancy, promotes germination.
  • Reverse apical dominance effect.
  • Induce RNA synthesis.

Ethylene:
  • Discovery:
    • Reported by Denny (1924) in fruit ripening.
    • Natural synthesis reported by Gane (1934).
  • Source: Synthesized in roots, shoot apical meristems, and fruits during ripening.
  • Chemical Nature: Unsaturated, colorless, hydrocarbon gas.
  • Commercial Source: Ethephon.
  • Function: Described as a ripening hormone.

Physiological Effects and Applications of Ethylene:

  • Promotes ripening of fruits.
  • Stimulates initiation of lateral roots.
  • Breaks dormancy of buds and seeds.
  • Acceleration of abscission activity by forming abscission layer.
  • Inhibits growth of lateral buds, i.e., apical dominance.
  • Retardation of flowering.
  • Enhancement of senescence.
  • Epinasty — Drooping of leaves and flowers e.g., Pineapple.
  • Degreening effect — Stimulate activity of enzyme chlorophyllase causing loss of green color in fruits of Banana, Citrus.

Abscissic Acid (ABA):
  • Discovery: Responsible for shedding of cotton balls.
  • Chemical Nature:
    • 15-C sesquiterpenoid.
    • Synthesized from mevalonic acid.
  • Source: Leaves, fruits, roots, seeds synthesize ABA.

Physiological Effects and Applications of ABA:

  • Promote abscission of leaves — beneficial for stress — drought.
  • Induces dormancy in buds and seeds.
  • Accelerates senescence of leaves, flowers, and fruits.
  • Delay of cell division, cell elongation, and suppression of cambial activity — Inhibit mitosis.
  • Causes efflux of K+ ions from guard cells and thus closure of stomata — used as antitranspirant.
  • Stress hormone — Overcome stress by inducing dormancy, inhibiting growth thus face adverse environmental conditions.
  • Inhibit flowering in long day plants and stimulate flowering in short day plants.
  • Inhibits growth stimulated by gibberellin.

Photoperiodism:
  • Definition: Influence of light duration on flowering.
  • Coined By: Garner and Allard.
  • Light influences germination, vegetative growth, photosynthesis, etc.
  • Aspects of Light: Quality, Intensity, and Duration.

Duration of Light:

  • Major effect on flowering.
  • Response of plants to relative length of light and dark periods.

Critical Photoperiod: Duration above or below which flowering occurs.

Types of Plants:

Short Day Plants (SDP):

  • Characteristics:
    • Flower under short day length conditions.
    • Also called long night plants.
  • Examples: Dahlia, Xanthlum, Soybean, Aster, Tobacco, Chrysanthemum.
  • Requirements: Require long uninterrupted dark period for flowering.
  • Sensitivity: Flowering affected if dark period interrupted, even by short duration.

Long Day Plants (LDR):

  • Characteristics:
    • Flower only with light period longer than critical photoperiod.
    • Also called short night plants.
  • Examples: Radish, Spinach, Wheat, Poppy, Cabbage, Pea, Sugar beet.
  • Requirements: Require short dark or night period for flowering.

Day Neutral Plants (DNP):

  • Characteristics: Flowering not affected by day length.
  • Examples: Cucumber, Sunflower, Cotton, Balsam, Maize, Tomato.
  • Flowering: Observed throughout the year.

Phytochrome:
  • Discovery: By Hendricks and Borthwick.
  • Function: Pigment system in plants that receives stimulus for photoperiodism.
  • Short Day Plants (SDP):
    • Flowering not observed if dark period interrupted by brief exposure to red light (660 nm).
    • Flowering observed if immediately exposed to far-red light (780 nm).
  • Characteristics:
    • Proteinaceous pigments present in leaves.
    • Exist in two interconvertible forms: Pr and Pfr.
    • Pfr absorbs far-red light, changed to Pr; Pr absorbs red light, changed to Pfr (biologically active form).
    • Situated in cell membrane of chlorophyllous cells of leaves.
  • Role in Flowering:
    • During daytime, Pfr accumulates, stimulates flowering in Long Day Plants (LDP), inhibits in Short Day Plants (SDP).
    • During night (dark), Pfr converted to Pr, stimulates flowering in SDP, inhibits in LDP.
  • Photomorphogenesis:
    • Controls morphogenesis along with light.
    • Photoperiodic stimulus is chemical stimulus called florigen, hormonal in nature, transported through phloem.

Vernalization:
  • Definition: Influence of low temperature on flowering in plants.
  • Coined By: Lysenko (1928).
  • Discovery: Klippart (1918) observed low temperature or chilling treatment stimulates early flowering.
  • Process:
    • Seeds or seedlings exposed to low temperatures (1 - 6°C) for about a month.
    • Shoot apical meristem receives stimulus in seedlings.
    • Effective in seed stage for annual plants.
  • Chemical Substance: Vernalin, proved by grafting experiment by Melcher.
  • Devernalization: Reversal of vernalization by high temperature treatment.

Advantages of Vernalization:

  • Crops can be produced earlier.
  • Cultivation possible where they do not occur naturally.

Mineral Nutrition:
  • Sources:
    • Atmosphere: Carbon (as Carbon dioxide), Oxygen.
    • Water: Hydrogen, Oxygen.
  • Chief Source: Soil, solid, inorganic materials from earth’s crust.
  • Absorption: Minerals absorbed in dissolved form through roots.
  • Major Elements: Carbon, Hydrogen, Oxygen, non-mineral structural components.

Classification of Minerals:

  • Essential Minerals:
    • Without which life cycle of plants cannot be completed.
    • Important structural and functional role.
    • Their unavailability causes major deficiency symptoms. e.g., C, H, O, N, P.
  • Non-Essential Minerals:
    • Not indispensable for completion of life cycle.
    • Do not cause major deficiency.
    • Needed only at specific times during growth. e.g., Bo, Si, Al.

Based on Quantity Requirement:

  • Microelements:
    • Required in traces, mainly have catalytic role as co-factors or activators of enzymes.
    • Needed for specific activities in the plant life cycle. e.g., B for pollen germination, Si for stress conditions.
    • Important micronutrients: Mn, B, Cu, Zn, Cl.
  • Macroelements:
    • Required in large amounts, play nutritive and structural roles.
    • Major elements: C, H, O, P, Mg, N, K, S, and Ca.
    • e.g., Ca pectate cell wall component, Mg component of chlorophyll.
    • C, H, O obtained from air and water e.g., CO2, H2O.

Symptoms of Mineral Deficiency:

  • Definition: Any visible deviation from normal structure and function.
  • Critical Concentration: Required amount below which plant growth is affected.
  • Indication of Deficiency:
    • Morphological changes.
    • Related to mobility of element in plant body.

Important Symptoms:

  • Stunting: Growth retardation, condensed and short stem.
  • Chlorosis: Loss or non-development of chlorophyll, yellowing of leaves.
  • Necrosis: Localized death of tissue in leaves.
  • Mottling: Appearance of green and non-green patches on leaves.
  • Abscission: Premature fall of flowers, fruits, and leaves.

Roles of Mineral Elements in Plants:

  1. Nitrogen (NO3-, NO2-, NH4+):

    • Region Required: Everywhere, particularly in meristematic tissues.
    • Functions: Constituent of proteins, nucleic acids, vitamins, hormones, coenzymes, ATP, chlorophyll.
    • Deficiency Symptom: Stunted growth, chlorosis.

  2. Phosphorus (H2PO4-, HPO42-):

    • Region Required: Younger tissues, obtained from older, metabolically less active cells.
    • Functions: Constituent of cell membrane, certain proteins, all nucleic acids, nucleotides, required for phosphorylation reactions.
    • Deficiency Symptom: Poor growth, dull green leaves.

  3. Potassium (K+):

    • Region Required: Meristematic tissues, buds, leaves, root tips.
    • Functions: Determines anion-cation balance, involved in protein synthesis, formation of cell membrane, stomata regulation, turgidity maintenance.
    • Deficiency Symptom: Yellow edges to leaves, premature death.

  4. Calcium (Ca2+):

    • Region Required: Meristematic and differentiating tissues, accumulates in older leaves.
    • Functions: Selective permeability of cell membranes, enzyme activation, stem and root apex development, calcium pectate in cell wall.
    • Deficiency Symptom: Stunted growth.

  5. Magnesium (Mg2+):

    • Region Required: Leaves, withdrawn from ageing leaves, exported to developing seeds.
    • Functions: Activates enzymes in phosphate metabolism, constituent of chlorophyll, maintains ribosome structure.
    • Deficiency Symptom: Chlorosis.

  6. Sulphur (SO42-):

    • Region Required: Stem and root tips, young leaves.
    • Functions: Constituent of certain proteins, vitamins, Ferredoxin.
    • Deficiency Symptom: Chlorosis.

  7. Iron (Fe3+):

    • Region Required: Everywhere, carried along leaf veins.
    • Functions: Constituent of ferredoxin, cytochrome, activates catalase.
    • Deficiency Symptom: Chlorosis.

  8. Manganese (Mn2+): (Trace)

    • Region Required: Leaves, seeds.
    • Functions: Activates certain enzymes (carboxylases).
    • Deficiency Symptom: Chlorosis, grey spots on leaves.

  9. Molybdenum (MoO22+): (Trace)

    • Region Required: Everywhere, particularly in roots.
    • Functions: Activates certain enzymes in nitrogen metabolism.
    • Deficiency Symptom: Slight growth retardation.

  10. Boron (BO3-3, B4O72-): (Trace)

    • Region Required: Leaves, seeds.
    • Functions: Required for Ca2+ uptake, pollen germination, cell differentiation, carbohydrate translocation.
    • Deficiency Symptom: Brown heart disease.

  11. Copper (Cu2+): (Trace)

    • Region Required: Everywhere.
    • Functions: Activates certain enzymes.
    • Deficiency Symptom: Die-back of shoots.

  12. Zinc (Zn2+): (Trace)

    • Region Required: Everywhere.
    • Functions: Activates various enzymes, carbonic anhydrase, dehydrogenases.
    • Deficiency Symptom: Malformed leaves.

  13. Chlorine (Cl-):

    • Region Required: Everywhere.
    • Functions: Helps solute concentration, anion-cation balance, oxygen evolution in photosynthesis.
    • Deficiency Symptom: Poor growth.

Toxicity of Micronutrients:

  • Micronutrients are required in small quantities by plants.
  • A moderate decrease causes deficiency symptoms, while a moderate increase causes toxicity.
  • Toxicity is defined as a reduction in dry tissue weight by 10% due to excess minerals.
  • Identifying toxicity symptoms isn't always easy; often, excess of one element inhibits the uptake of another, causing deficiency symptoms.
  • For example, manganese competes with iron and magnesium for uptake, leading to chlorosis with grey spots on leaves, which may be mistaken for deficiency symptoms of Fe, Mg, and Ca.

Mineral Salt Absorption: In soil, minerals exist as charged particles, and absorption is independent of water absorption.

Passive Absorption:

  • Movement of mineral ions into root cells by diffusion without energy expenditure.
  • Can occur through direct or indirect ion exchange, mass flow, or Donnan equilibrium.

Donnan Equilibrium:

  • Some anions accumulate on the inner side of cell membranes.
  • Additional cations are needed to balance these, leading to increased cation concentration.
  • This transport against concentration gradients for neutralization is Donnan equilibrium.

Active Absorption:

  • Absorption of minerals against concentration gradients, requiring metabolic energy.
  • ATP energy from root cell respiration is used.
  • Oxygen scarcity decreases mineral absorption.
  • Ions accumulate in root hair against concentration gradients, pass into cortical cells, reach xylem, then are redistributed.
  • Membrane proteins pump ions into the cytoplasm.

Nitrogen Cycle:

  • Definition: The cyclic movement of nitrogen between atmosphere, biosphere (organisms), and soil in natural processes.
  • Nitrogen available to plants from the environment is inert and needs to be converted into reactive forms, mainly nitrate ions, for utilization in synthetic processes.
  • Nitrogen is a limiting element affecting productivity.
  • It is made available to plants through biological and physical fixation.

 Nitrogen Fixation: Physical Nitrogen Fixation:
  • Occurs in the atmosphere and soil.
  • Electric discharge (Lightning):
    • Nitrogen combines with oxygen to form nitric oxide.
      • N2 + O2 → Electric discharge (Lightning) → 2NO
    • Nitric oxide oxidizes to nitrogen peroxide in the presence of oxygen.
      • 2NO + O2 → Oxidation → 2NO2 (Nitrogen peroxide)
    • Nitrogen peroxide combines with rainwater to form nitrous and nitric acid, coming down as acid rain.
      • 2NO2 + Rainwater → HNO2 + HNO3
    • Alkali radicals react with nitric acid on the ground to produce nitrites and nitrates, absorbable forms for plants.
  • Industrial Nitrogen Fixation (Haber-Bosch process):
    • N2 + 3H2 → 450°C, 200atm → 2NH3 (Ammonia)
    • Ammonia is converted to urea, which is less toxic.
    • About 80% of nitrogen in human tissues originates from the Haber-Bosch process.

Biological Nitrogen Fixation:

  • Carried out by living organisms, mainly prokaryotic bacteria in the soil.
  • Nitrogen Fixers:
    • Diazotrophs or nitrogen fixers fix about 70% of nitrogen.
    • Can be free-living bacteria or symbiotic with higher plants (e.g., Rhizobium).
    • Cyanobacteria have specialized cells, heterocysts, aiding nitrogen fixation.
  • Energy Requirement:
    • High-energy process, requiring 16 ATP molecules for fixing one molecule of nitrogen to ammonia.

Nitrification: Soil bacteria like Nitrosomonas and Nitrobacter convert ammonia to nitrate.

  • Nitrification Steps:
    • 2NH3 + 3O2 → Nitrosomonas, Nitrosococcus → 2HNO2 + 2H2O
    • 2HNO2 + O2 → Nitrobacter → 2HNO3
  • Symbiotic N2 Fixation: Rhizobium forms root nodules in plants of the Fabaceae family (e.g., beans, gram, groundnut).

Ammonification:

  • After plant and animal death, fungi, actinomycetes, and some ammonifying bacteria decompose tissues.
  • They convert organic nitrogen into amino acids, then into ammonia (NH4+), which returns to the ecosystem.
  • Ammonia is used by plants and micro-organisms for growth.

Nitrogen Assimilation:

  • Soil contains nitrogen in nitrate, nitrite, and ammonia (NH4-) forms.
  • Plants take up these forms and convert them into amino acids and nucleic acids through assimilation.
  • Nitrogen moves through the food chain and reaches decomposers.
  • Amino acids are used to synthesize proteins.

Amino Acid Synthesis:

  • Reductive Amination:

    • Ammonia reacts with α-ketoglutaric acid to form glutamic acid.
    • It's a reduction reaction.

  • Transamination:

    • Glutamic acid reacts with oxaloacetic acid (OAA) to form aspartic acid.
    • It involves the transfer of an amino group to another carboxylic acid.

Amides:

  • Ammonia can be absorbed by amino acids to produce amides (amidation).
  • Amides are amino acids with two amino groups.
  • An extra amino group is attached to the acidic group (-COOH) in the presence of ATP.
  • Glutamic acid + NH4+ + ATP → alpha glutamine + ADP
  • Aspartic acid + NH4+ + ATP → Asparagine + ADP
  • Amides like asparagine and glutamine are formed from glutamic acid and aspartic acid by adding another amino group to each.
  • Amides are transported through xylem vessels.

De-nitrification:

  • It's the process where anaerobic bacteria convert soil nitrates back into nitrogen gas.
  • Denitrifying bacteria remove fixed nitrogen (nitrates) from the ecosystem, returning it to the atmosphere in an inert form.
  • Denitrifying bacteria include Bacillus spp., Paracoccus spp., and Pseudomonas denitrificans.
  • They transform nitrates to nitrous and nitric oxides, then to gaseous nitrogen.
  • 2NO3 → 2NO2 → 2NO → N2

Sedimentation:

  • Soil nitrates are washed away to the sea or leached deep into the earth with percolating water.
  • They accumulate as sediments and remain locked away from free circulation.