Plant Physiology

UNIT 4: Plant Physiology

Plant physiology describes physiological processes in flowering plants, focusing on photosynthesis, respiration, and plant growth and development. These processes are explained in molecular terms within the context of cellular activities and organism-level functions, including their relation to the environment.

Chapter 11: Photosynthesis in Higher Plants

Photosynthesis is a physico-chemical process by which green plants use light energy to synthesise organic compounds. It is the primary source of all food on Earth and is responsible for releasing oxygen into the atmosphere. All living forms ultimately depend on sunlight for energy.

What We Know About Photosynthesis

Simple experiments show that chlorophyll, light, and \ce{CO2} are required for photosynthesis.

• Starch formation only occurs in green parts of variegated leaves exposed to light.
• \ce{CO2} is required, as demonstrated by the experiment where a leaf part enclosed with KOH (absorbs \ce{CO2}) does not form starch.

Early Experiments and Discoveries

• Joseph Priestley (1770): Revealed the essential role of air in green plant growth. He observed that a mint plant could restore the air “damaged” by a burning candle or a breathing mouse, which would otherwise extinguish or suffocate in a closed space. He discovered oxygen in 1774.
• Jan Ingenhousz (1730-1799): Showed that sunlight is essential for the plant process that purifies air. His aquatic plant experiment demonstrated small bubbles (identified as oxygen) forming around green parts in bright sunlight, but not in the dark. He concluded that only the green parts release oxygen.
• Julius von Sachs (1854): Provided evidence for glucose production when plants grow, noting it’s usually stored as starch. He located the green substance (chlorophyll) in special bodies (chloroplasts) within plant cells, confirming glucose is made in green parts and stored as starch.
• T.W. Engelmann (1843-1909): Used a prism to split light and illuminated a green alga (Cladophora) with aerobic bacteria. The bacteria accumulated mainly in the blue and red regions of the spectrum, indicating maximal \ce{O2} evolution. This described the first action spectrum of photosynthesis.
• Cornelius van Niel (1897-1985): Based on studies of purple and green bacteria, he demonstrated that photosynthesis is a light-dependent reaction where hydrogen from a suitable oxidisable compound reduces carbon dioxide to carbohydrates. He inferred that the \ce{O2} evolved by green plants comes from \ce{H2O}, not \ce{CO2}. This was later proved by radioisotopic techniques.
  • Overall Equation of Photosynthesis: \ce{6CO2 + 12H2O ->[Light] C6H12O6 + 6H2O + 6O2}.

Where Does Photosynthesis Take Place?

Photosynthesis occurs primarily in the green leaves and other green parts of plants.

• Within leaves, mesophyll cells contain a large number of chloroplasts.
• Chloroplast Structure: Contains a membranous system (grana, stroma lamellae) and a matrix (stroma).
• Division of Labour within Chloroplast:
  • Membrane system (thylakoids, grana): Traps light energy and synthesises ATP and NADPH. These are light reactions (photochemical reactions).
  • Stroma: Enzymatic reactions synthesise sugar, which then forms starch. These are dark reactions (carbon reactions), dependent on ATP and NADPH from light reactions. It’s a misnomer to call them “dark” as they don’t occur in darkness and are light-dependent indirectly.

Pigments Involved in Photosynthesis

Leaf pigments can be separated by paper chromatography into four main types:

1. Chlorophyll a: Bright or blue-green; chief pigment associated with photosynthesis, showing maximum absorption and photosynthesis in blue and red regions.
2. Chlorophyll b: Yellow-green.
3. Xanthophylls: Yellow.
4. Carotenoids: Yellow to yellow-orange.

• Accessory Pigments (Chlorophyll b, xanthophylls, carotenoids): Absorb light and transfer energy to chlorophyll a, enabling a wider range of wavelengths to be utilised and protecting chlorophyll a from photo-oxidation.

Light Reaction (Photochemical Phase)

This phase includes light absorption, water splitting, oxygen release, and formation of ATP and NADPH.

• Photosystems (PS I and PS II): Pigments are organised into two light-harvesting complexes (LHCs). They are named in order of discovery.
• LHCs: Made of hundreds of pigment molecules bound to proteins, acting as antennae to absorb different wavelengths of light.
• Reaction Centre: A single chlorophyll a molecule that forms the reaction centre in each photosystem.
  • PS I reaction centre: P700 (absorbs 700 nm red light).
  • PS II reaction centre: P680 (absorbs 680 nm red light).

The Electron Transport (Z-Scheme)

• Electron Excitation: In PS II, chlorophyll a (P680) absorbs 680 nm light, exciting electrons.
• Electron Transfer Chain: Excited electrons are picked up by an electron acceptor, then passed downhill through an electron transport system (ETS) of cytochromes to PS I.
• PS I Excitation: Simultaneously, electrons in PS I (P700) are excited by 700 nm red light and transferred to another acceptor.
• NADPH Formation: These electrons move downhill again to NADP+, reducing it to NADPH + H+.
• Z-Scheme: The entire sequence of electron transfer from PS II, to PS I, and finally to NADP+ forms a characteristic ‘Z’ shape when carriers are placed on a redox potential scale.

Splitting of Water (Photolysis)

• Electron Replacement: PS II continuously supplies electrons by splitting of water (photolysis).
• Process: Water splits into \ce{2H+}, \ce{[O]}, and electrons.
• Products: Creates oxygen (\ce{O2}) as a net product of photosynthesis, along with protons and electrons.
• Location: The water-splitting complex is associated with PS II and is located on the inner side of the thylakoid membrane (lumen side).

Cyclic and Non-cyclic Photophosphorylation

Photophosphorylation is the synthesis of ATP from ADP and inorganic phosphate in the presence of light.

• Non-cyclic Photophosphorylation:
  • Involves both PS II and PS I working in series.
  • Electrons flow from PS II through the electron transport chain to PS I, and then to NADP+.
  • Produces both ATP and NADPH + H+.
• Cyclic Photophosphorylation:
  • Involves only PS I.
  • Electrons are circulated within the photosystem, not passing to NADP+ but cycling back to the PS I complex through the ETS.
  • A possible location is in the stroma lamellae, which lack PS II and NADP reductase enzyme.
  • Produces only ATP, not NADPH + H+.
  • Also occurs when only light of wavelengths beyond 680 nm is available.

Chemiosmotic Hypothesis (ATP Synthesis)

This hypothesis explains ATP synthesis in chloroplasts (and mitochondria in respiration).

• Proton Gradient: ATP synthesis is linked to the development of a proton gradient across the thylakoid membrane, with proton accumulation towards the inside (lumen).
• Causes of Proton Gradient:
  1. Water splitting: Releases protons (\ce{H+}) into the thylakoid lumen.
  2. Electron transport: Primary electron acceptor (outer side of membrane) transfers electrons to an H-carrier, which removes a proton from the stroma. This proton is released into the lumen when the electron is passed to the next carrier on the inner side.
  3. NADP+ reduction: The NADP reductase enzyme is on the stroma side. Protons are removed from the stroma (along with electrons from PS I) to reduce NADP+ to NADPH + H+.
• Result: Protons decrease in stroma, accumulate in lumen, creating a proton gradient and measurable pH decrease in the lumen.
• ATP Synthase: The proton gradient is broken down as protons move across the membrane to the stroma through the transmembrane channel (CF0) of the ATP synthase enzyme.
  • CF0: Embedded in the thylakoid membrane, forms a channel for facilitated diffusion of protons.
  • CF1: Protrudes on the outer surface (stroma side) of the thylakoid membrane.
  • The energy from gradient breakdown causes a conformational change in CF1, leading to ATP synthesis.

Where ATP and NADPH are Used (Biosynthetic Phase/Carbon Reactions)

• Products of Light Reaction: ATP, NADPH, and \ce{O2}. \ce{O2} diffuses out, while ATP and NADPH are used in the stroma to drive the synthesis of sugars (food).
• This is the biosynthetic phase, which does not directly depend on light but relies on the products of light reactions, along with \ce{CO2} and \ce{H2O}.
• Melvin Calvin’s Work: Using radioactive \ce{14C} in algal photosynthesis, he discovered that the first \ce{CO2} fixation product was a 3-carbon organic acid, 3-phosphoglyceric acid (PGA). He worked out the complete biosynthetic pathway, known as the Calvin cycle.
• C3 vs. C4 Pathways:
  • C3 pathway: First stable product of \ce{CO2} fixation is a 3-carbon acid (PGA).
  • C4 pathway: First stable product is a 4-carbon acid (Oxaloacetic Acid - OAA).

The Primary Acceptor of \ce{CO2}

• Unexpectedly, the \ce{CO2} acceptor molecule in the Calvin cycle was found to be a 5-carbon ketose sugar, ribulose bisphosphate (RuBP).

The Calvin Cycle (C3 Pathway)

The Calvin cycle operates in all photosynthetic plants. It proceeds in three stages:

1. Carboxylation:
   • Fixation of \ce{CO2} into a stable organic intermediate.
   • \ce{CO2} is utilised for the carboxylation of RuBP.
   • Catalysed by the enzyme RuBP carboxylase-oxygenase (RuBisCO), forming two molecules of 3-PGA.
2. Reduction:
   • A series of reactions leading to glucose formation.
   • For each \ce{CO2} molecule fixed, 2 molecules of ATP are used for phosphorylation and 2 molecules of NADPH for reduction.
   • Six turns of the cycle are required to form one molecule of glucose.
3. Regeneration:
   • Regeneration of the \ce{CO2} acceptor molecule, RuBP, is crucial.
   • Requires one ATP for phosphorylation per RuBP regenerated.

• Energy Requirement: For every \ce{CO2} molecule entering the Calvin cycle, 3 ATP and 2 NADPH are required.
• To make one molecule of glucose (from 6 \ce{CO2}): 18 ATP and 12 NADPH are required.

The C4 Pathway

Plants adapted to dry tropical regions utilise the C4 pathway. They are special because they:

• Have special leaf anatomy (Kranz anatomy).

• Tolerate higher temperatures.

• Show response to high light intensities.

• Lack photorespiration.

• Have greater productivity of biomass.

• Kranz Anatomy: Characterised by particularly large bundle sheath cells surrounding the vascular bundles, forming several layers. These cells have many chloroplasts, thick walls impervious to gaseous exchange, and no intercellular spaces.

• Hatch and Slack Pathway (C4 Cycle): A cyclic process.

  1. Primary \ce{CO2} acceptor: Phosphoenol pyruvate (PEP), a 3-carbon molecule, present in mesophyll cells.
  2. Fixation enzyme: PEP carboxylase (PEPcase) (mesophyll cells lack RuBisCO).
  3. First fixation product: Oxaloacetic acid (OAA), a 4-carbon compound, formed in mesophyll cells.
  4. OAA forms other 4-carbon compounds (malic acid, aspartic acid) which are transported to bundle sheath cells.
  5. In bundle sheath cells, these C4 acids are broken down to release \ce{CO2} and a 3-carbon molecule.
  6. The 3-carbon molecule returns to the mesophyll to regenerate PEP.
  7. The \ce{CO2} released in bundle sheath cells enters the Calvin pathway (C3 pathway), which is common to all plants. Bundle sheath cells are rich in RuBisCO but lack PEPcase.

• Location of Calvin Cycle: In C3 plants, it occurs in all mesophyll cells. In C4 plants, it occurs only in bundle sheath cells.

Photorespiration

• RuBisCO’s Dual Nature: RuBisCO, the most abundant enzyme, can bind to both \ce{CO2} and \ce{O2}. It has a greater affinity for \ce{CO2} when \ce{CO2}:\ce{O2} ratio is nearly equal.
• In C3 plants: Some \ce{O2} binds to RuBisCO, decreasing \ce{CO2} fixation. RuBP binds with \ce{O2} to form one phosphoglycerate and one phosphoglycolate (2-carbon).
• Photorespiratory Pathway:
  • No synthesis of sugars.
  • No synthesis of ATP or NADPH.
  • Results in the release of \ce{CO2} with the utilisation of ATP.
  • Biological function is not known.
• In C4 plants: Photorespiration does not occur. This is because C4 plants have a mechanism that increases \ce{CO2} concentration at the enzyme site in bundle sheath cells, ensuring RuBisCO primarily functions as a carboxylase. This contributes to higher productivity and tolerance to higher temperatures in C4 plants.

Factors Affecting Photosynthesis

The rate of photosynthesis affects plant yield and is influenced by several factors.

• Blackman’s Law of Limiting Factors (1905): If a chemical process is affected by more than one factor, its rate is determined by the factor nearest to its minimal value.

• Internal (Plant) Factors: Number, size, age, and orientation of leaves; mesophyll cells; chloroplasts; internal \ce{CO2} concentration; amount of chlorophyll. These depend on genetic predisposition and plant growth.

• External Factors:

  • Light:
    • Linear relationship between incident light and \ce{CO2} fixation at low light intensities.
    • At higher light intensities, other factors become limiting, and the rate does not increase further.
    • Light saturation occurs at about 10% of full sunlight.
    • Excess light can cause chlorophyll breakdown and decrease photosynthesis.
  • Carbon Dioxide Concentration:
    • The major limiting factor for photosynthesis.
    • Atmospheric \ce{CO2} is very low (0.03-0.04%). Increasing it up to 0.05% can increase fixation rates.
    • C4 plants show saturation at ~360 µlL-1 \ce{CO2}.
    • C3 plants respond to increased \ce{CO2} concentration and saturate beyond 450 µlL-1.
    • Current \ce{CO2} levels are limiting for C3 plants. This is exploited in greenhouses to increase yields (e.g., tomatoes, bell pepper).
  • Temperature:
    • Dark reactions are temperature-controlled (enzymatic).
    • C4 plants respond to higher temperatures and show higher photosynthesis rates; C3 plants have a much lower temperature optimum.
    • Optimum temperature depends on the plant’s habitat (tropical vs. temperate).
  • Water:
    • Effect is indirect, primarily through its impact on the plant.
    • Water stress causes stomata to close (reducing \ce{CO2} availability) and leaves to wilt (reducing surface area and metabolic activity).

Chapter 12: Respiration in Plants

All living organisms need energy for daily activities, obtained by oxidation of macromolecules (food). Green plants prepare their own food via photosynthesis. However, non-green parts of plants, and all animal and microbial cells, rely on this food for energy. Ultimately, all food for respiration comes from photosynthesis.

• Cellular Respiration: The mechanism of breakdown of food materials within the cell to release energy and trap it as ATP.
• Respiratory Substrates: Compounds oxidised during respiration. Primarily carbohydrates (glucose), but also proteins, fats, and organic acids can be used under certain conditions.
• Energy is released in a series of slow, stepwise reactions controlled by enzymes and trapped as ATP (energy currency of the cell).

Do Plants Breathe?

• Yes, plants require for respiration and release .

• They lack specialised respiratory organs but use stomata and lenticels for gaseous exchange.

• Reasons why plants get along without specialised organs:

  1. Each plant part handles its own gas exchange, with little gas transport between parts.
  2. Plants have lower demands for gas exchange compared to animals.
  3. Living cells are located close to the surface, aided by loose packing of parenchyma cells providing an interconnected network of air spaces.

• Overall Equation for complete combustion of glucose: .

• Glucose is oxidised in small steps to prevent all energy from being released as heat, allowing for efficient ATP synthesis.

Glycolysis (EMP Pathway)

• Definition: The breakdown of glucose to pyruvic acid without the help of oxygen.
• Origin: From Greek words ‘glycos’ (sugar) and ‘lysis’ (splitting).
• Discovered by: Gustav Embden, Otto Meyerhof, and J. Parnas (EMP pathway).
• Occurrence: In the cytoplasm of the cell, present in all living organisms.
• Process: Glucose undergoes partial oxidation to form two molecules of pyruvic acid.
  • Glucose (from sucrose or storage carbohydrates) is converted to glucose-6-phosphate, then fructose-6-phosphate, which is phosphorylated again to fructose 1,6-bisphosphate.
  • This 6-carbon sugar splits into two 3-carbon molecules (dihydroxyacetone phosphate and 3-phosphoglyceraldehyde (PGAL)).
• Energy Balance:
  • ATP utilised: 2 molecules (glucose to glucose-6-P, fructose-6-P to fructose 1,6-bisP).
  • NADH + H+ formed: 2 molecules (from PGAL conversion to 1,3-bisphosphoglycerate (BPGA)).
  • ATP synthesised directly (substrate-level phosphorylation): 2 molecules (BPGA to 3-PGA) + 2 molecules (PEP to pyruvic acid) = 4 ATP.
  • Net ATP gain from glycolysis directly: 2 ATP (4 produced - 2 utilised).

Metabolic Fate of Pyruvic Acid

Pyruvic acid, the key product of glycolysis, can be handled in three ways depending on cellular needs and oxygen availability:

1. Lactic acid fermentation
2. Alcoholic fermentation
3. Aerobic respiration (requires O2).

Fermentation

• Conditions: Occurs under anaerobic conditions (incomplete oxidation of glucose).
• Types:
  • Alcoholic Fermentation (e.g., yeast): Pyruvic acid is converted to CO2 and ethanol by pyruvic acid decarboxylase and alcohol dehydrogenase.
  • Lactic Acid Fermentation (e.g., some bacteria, animal muscle cells during exercise): Pyruvic acid is reduced to lactic acid by lactate dehydrogenase.
• NADH + H+ reoxidation: In both, NADH + H+ is reoxidised to NAD+.
• Energy Yield: Very little energy is released (less than 7% of glucose energy), and not all is trapped as ATP.
• Net ATP: Only 2 ATP molecules per glucose are gained.
• Toxicity: Products like acid or alcohol are hazardous; yeast can be poisoned at ~13% alcohol concentration.

Aerobic Respiration

• Conditions: Requires oxygen (O2) for complete oxidation of organic substances.

• Products: Releases CO2, water, and a large amount of energy.

• Location: Occurs within the mitochondria in eukaryotes.

• Crucial Events:

  1. Complete oxidation of pyruvate by stepwise removal of hydrogen atoms, leaving CO2. This occurs in the mitochondrial matrix.
  2. Passing electrons (from hydrogen atoms) to molecular O2 with simultaneous ATP synthesis. This occurs on the inner membrane of the mitochondria.

• Oxidative Decarboxylation of Pyruvate:

  • Pyruvate (from glycolysis in cytosol) enters mitochondrial matrix.
  • Undergoes oxidative decarboxylation by pyruvic dehydrogenase complex (requires NAD+, Coenzyme A).
  • Reaction: .
  • Produces 2 molecules of NADH (from one glucose, yielding two pyruvic acids).
  • Acetyl CoA then enters the Krebs’ cycle.

Tricarboxylic Acid Cycle (TCA Cycle/Krebs’ Cycle/Citric Acid Cycle)

• Discovered by: Hans Krebs.
• Location: Mitochondrial matrix.
• Process:
  • Starts with Acetyl CoA condensing with oxaloacetic acid (OAA) and water to yield citric acid (catalysed by citrate synthase).
  • Involves two successive decarboxylation steps (releasing CO2).
  • GTP (later converted to ATP) is synthesised at one point (substrate-level phosphorylation).
  • Produces 3 NADH + H+ and 1 FADH2 per acetyl CoA (therefore, 6 NADH + H+ and 2 FADH2 per glucose molecule, as two acetyl CoA molecules are formed).
• Requirements: Requires continued replenishment of OAA and regeneration of NAD+ and FAD+.
• At this stage, glucose has been fully broken down to CO2. The remaining energy is in NADH + H+ and FADH2.

Electron Transport System (ETS) and Oxidative Phosphorylation

• Purpose: To release and utilise the energy stored in NADH + H+ and FADH2.
• Mechanism: They are oxidised as electrons pass from one carrier to another through the ETS, ultimately reaching O2, which is reduced to H2O.
• Location: Inner mitochondrial membrane.
• Components (Complexes I-IV):
  • Complex I (NADH dehydrogenase): Oxidises NADH, transfers electrons to ubiquinone.
  • Complex II: Receives electrons from FADH2 (generated during succinate oxidation in TCA cycle), transfers to ubiquinone.
  • Complex III (cytochrome bc1 complex): Ubiquinol oxidised, transfers electrons to cytochrome c.
  • Cytochrome c: A small, mobile protein carrier between Complex III and IV.
  • Complex IV (cytochrome c oxidase complex): Contains cytochromes a, a3, and two copper centres, transfers electrons ultimately to oxygen.
• ATP Synthesis: As electrons pass through the ETS, they are coupled to ATP synthase (Complex V) for ATP production from ADP and inorganic phosphate.
• ATP Yield:
  • Oxidation of one NADH gives 3 ATP.
  • Oxidation of one FADH2 gives 2 ATP.
• Role of Oxygen: Vital, acts as the final hydrogen acceptor, driving the whole process by removing hydrogen from the system.
• Oxidative Phosphorylation: ATP synthesis driven by the energy of oxidation-reduction reactions, generating a proton gradient.
• ATP Synthase (Complex V):
  • F1 headpiece: Peripheral membrane protein, site for ATP synthesis.
  • F0: Integral membrane protein, forms the channel through which protons cross the inner membrane.
  • The passage of 4 H+ through F0 from intermembrane space to matrix is coupled to the catalytic site of F1 to produce one ATP.

The Respiratory Balance Sheet (Theoretical)

• Assumptions for Calculation:
  • Orderly, sequential pathway (glycolysis, TCA, ETS).
  • NADH from glycolysis transferred to mitochondria and undergoes oxidative phosphorylation.
  • No intermediates are utilised for other compounds.
  • Only glucose is respired.
• Reality: These assumptions are not entirely valid in living systems; pathways work simultaneously, intermediates are withdrawn/entered as needed, ATP is used as needed, and enzymatic rates are controlled.
• Theoretical Net Gain: Up to 38 ATP molecules during aerobic respiration of one glucose molecule.

Comparison: Fermentation and Aerobic Respiration

Amphibolic Pathway

• Respiration is traditionally considered a catabolic process (breakdown of substrates).
• However, the respiratory pathway is also involved in synthesis (anabolism).
  • For example, fatty acids can be broken down to acetyl CoA to enter the pathway, but acetyl CoA can also be withdrawn from the pathway to synthesise fatty acids.
  • Similarly, respiratory intermediates link in protein breakdown and synthesis.
• Because the respiratory pathway participates in both catabolism and anabolism, it is better considered an amphibolic pathway.

Respiratory Quotient (RQ)

• Definition: The ratio of the volume of CO2 evolved to the volume of O2 consumed in respiration. RQ = (Volume of CO2 evolved) / (Volume of O2 consumed).
• Dependence: Depends on the type of respiratory substrate used.
  • Carbohydrates: RQ = 1 (e.g., glucose: 6CO2/6O2 = 1).
  • Fats (e.g., Tripalmitin): RQ < 1 (e.g., 0.7).
  • Proteins: RQ ≈ 0.9.
• In living organisms, respiratory substrates are often a mix, not pure proteins or fats.

Chapter 13: Plant Growth and Development

Development is the sum of growth and differentiation. It is a precise and highly ordered succession of events from a zygote to a mature plant, producing various organs and eventually leading to death. The first step is seed germination, which requires favourable environmental conditions. These processes are controlled by both intrinsic (internal) and extrinsic (external) factors.

Growth

• Definition: An irreversible, permanent increase in size of an organ, its parts, or an individual cell.

• Accompanied by metabolic processes (anabolic and catabolic) that consume energy.

• Indeterminate Growth: Plants have the unique capacity for unlimited growth throughout their life due to the presence of meristems (regions of constantly dividing cells that can self-perpetuate). This is called the open form of growth.

  • Primary Growth: Root apical meristem and shoot apical meristem are responsible for elongation along the plant axis.
  • Secondary Growth: In dicotyledonous plants and gymnosperms, lateral meristems (vascular cambium and cork-cambium) appear later, causing an increase in girth.

• Growth is Measurable: Since protoplasm increase is hard to measure directly, growth is quantified by parameters like fresh/dry weight, length, area, volume, or cell number.

• Phases of Growth: Generally divided into three phases, observable at root tips:

  1. Meristematic Phase: Cells at the root/shoot apex, constantly dividing, rich in protoplasm, large nuclei, thin cellulosic primary walls with abundant plasmodesmatal connections.
  2. Elongation Phase: Cells proximal to the meristematic zone, characterised by increased vacuolation, cell enlargement, and new cell wall deposition.
  3. Maturation Phase: Cells further proximal, attain maximal size with wall thickening and protoplasmic modifications.

• Growth Rates: Increased growth per unit time.

  • Arithmetic Growth: After mitotic division, only one daughter cell continues to divide while the other differentiates. Exemplified by a root elongating at a constant rate, yielding a linear curve (Lt = L0 + rt).
  • Geometric Growth: Both progeny cells retain the ability to divide. Initial growth is slow (lag phase), then increases rapidly (exponential/log phase). With limited nutrients, growth slows (stationary phase). Produces a typical sigmoid (S-curve), characteristic of living organisms in a natural environment (W1 = W0 ert).
  • Absolute Growth Rate: Total growth per unit time.
  • Relative Growth Rate: Growth per unit time expressed on a common basis, like per unit initial parameter.

• Conditions for Growth:

  • Water: Essential for cell enlargement (turgidity), provides medium for enzymatic activities.
  • Oxygen: Helps release metabolic energy for growth.
  • Nutrients (macro and micro essential elements): Required for protoplasm synthesis and as energy sources.
  • Optimum Temperature Range: Essential for growth; deviations are detrimental.
  • Environmental Signals: Light and gravity also affect certain phases/stages.

Differentiation, Dedifferentiation, and Redifferentiation

• Differentiation: The process where cells derived from meristems mature to perform specific functions. Involves structural changes in cell walls and protoplasm (e.g., tracheary elements losing protoplasm and developing strong, lignocellulosic walls).
• Dedifferentiation: The phenomenon where living differentiated cells regain the capacity to divide under certain conditions (e.g., formation of interfascicular cambium and cork cambium from parenchyma cells).
• Redifferentiation: Dedifferentiated meristems divide and produce cells that again lose the capacity to divide but mature to perform specific functions.
• Open Differentiation: Differentiation in plants is open, meaning cells/tissues from the same meristem can have different structures at maturity, often determined by their location within the plant (e.g., root cap cells vs. epidermis cells from root apical meristem).

Development

• Definition: A term that encompasses all changes an organism undergoes during its life cycle, from seed germination to senescence.
• Plasticity: Plants exhibit plasticity, following different pathways in response to the environment or life phases to form different kinds of structures (e.g., heterophylly – different leaf shapes in juvenile vs. mature plants, or in aquatic vs. aerial environments in the same plant, like buttercup).
• Control: Plant development (growth and differentiation) is controlled by both intrinsic factors (intracellular/genetic, and intercellular/chemical like Plant Growth Regulators) and extrinsic factors (light, temperature, water, oxygen, nutrition).

Plant Growth Regulators (PGRs)

PGRs are small, simple molecules of diverse chemical composition, also known as plant hormones or phytohormones.

• Chemical Types:

  • Indole compounds (e.g., indole-3-acetic acid, IAA).
  • Adenine derivatives (e.g., N6-furfurylamino purine, kinetin).
  • Derivatives of carotenoids (e.g., abscisic acid, ABA).
  • Terpenes (e.g., gibberellic acid, GA3).
  • Gases (e.g., ethylene, C2H4).

• Functional Groups:

  • Plant Growth Promoters: Involved in growth-promoting activities like cell division, cell enlargement, pattern formation, tropic growth, flowering, fruiting, and seed formation (e.g., auxins, gibberellins, cytokinins).
  • Plant Growth Inhibitors: Play roles in plant responses to wounds and stresses, and in growth-inhibiting activities like dormancy and abscission (e.g., abscisic acid (ABA)).
  • Ethylene: Can fit either group but is largely an inhibitor of growth activities.

• Discovery of PGRs (Accidental):

  • Auxins: Charles and Francis Darwin observed phototropism in canary grass coleoptiles; F.W. Went isolated auxin from oat coleoptile tips. First isolated from human urine.
  • Gibberellins: E. Kurosawa reported symptoms of ‘bakanae’ disease (foolish seedling) in rice caused by a fungal filtrate, which led to the identification of gibberellic acid.
  • Cytokinins: F. Skoog and co-workers observed callus proliferation in tobacco stems with certain extracts; Miller et al. (1955) identified and crystallised kinetin. Zeatin was isolated from corn-kernels and coconut milk.
  • Abscisic Acid (ABA): Three independent researches in mid-1960s reported inhibitors (inhibitor-B, abscission II, dormin) that were chemically identical and named abscisic acid.
  • Ethylene: H.H. Cousins (1910) confirmed a volatile substance from ripened oranges hastened the ripening of stored unripened bananas; later identified as ethylene.

• Physiological Effects of PGRs:

  • Auxins (e.g., IAA, IBA, NAA, 2,4-D):
    • Initiate rooting in stem cuttings for propagation.
    • Promote flowering (e.g., pineapples).
    • Prevent early fruit/leaf drop, but promote abscission of older ones.
    • Apical dominance: Growing apical bud inhibits lateral bud growth; decapitation removes this inhibition.
    • Induce parthenocarpy (seedless fruits, e.g., tomatoes).
    • Used as herbicides (e.g., 2,4-D kills dicot weeds but not monocots).
    • Control xylem differentiation and aid cell division.
  • Gibberellins (GAs, e.g., GA3):
    • Increase length of axis (e.g., grape stalks) and fruit size/shape (e.g., apple).
    • Delay senescence, extending market period.
    • Speed up malting process in brewing industry.
    • Increase sugarcane yield by increasing stem length.
    • Hasten maturity in juvenile conifers, leading to early seed production.
    • Promote bolting (internode elongation) in rosette plants like beet, cabbages.
  • Cytokinins (e.g., Kinetin, Zeatin):
    • Synthesised in regions of rapid cell division (root apices, shoot buds, young fruits).
    • Specific effects on cytokinesis (cell division).
    • Promote new leaves, chloroplasts in leaves, lateral shoot growth, adventitious shoot formation.
    • Help overcome apical dominance.
    • Promote nutrient mobilisation, delaying leaf senescence.
  • Ethylene (C2H4, e.g., Ethephon as source):
    • A simple gaseous PGR, synthesised in senescing and ripening tissues.
    • Influences horizontal growth of seedlings, axis swelling, and apical hook formation in dicot seedlings.
    • Promotes senescence and abscission of plant organs (leaves, flowers).
    • Highly effective in fruit ripening, enhancing respiration rate (respiratory climactic).
    • Breaks seed and bud dormancy, initiates germination (peanut seeds), sprouting (potato tubers).
    • Promotes rapid internode/petiole elongation in deep water rice.
    • Promotes root growth and root hair formation, increasing absorption surface.
    • Used to initiate flowering and synchronise fruit-set in pineapples, and induce flowering in mango.
    • Ethephon is widely used to hasten fruit ripening (tomatoes, apples) and accelerate abscission (cotton, cherry, walnut thinning), and promote female flowers in cucumbers.
  • Abscisic Acid (ABA):
    • Acts as a general plant growth inhibitor and inhibitor of plant metabolism.
    • Inhibits seed germination.
    • Stimulates closure of stomata and increases tolerance to various stresses, hence called the stress hormone.
    • Plays an important role in seed development, maturation, and dormancy, helping seeds withstand desiccation.
    • Acts as an antagonist to Gibberellins in most situations.

• PGR roles can be complimentary, antagonistic, individualistic, or synergistic. Multiple PGRs often interact in plant events (e.g., dormancy, abscission, apical dominance).

• PGRs, along with genomic control and extrinsic factors like temperature and light, significantly control plant growth and development (e.g., vernalisation, flowering, dormancy, seed germination, plant movements).