Unit 2: Plant Physiology

2.1 Water Relation

Diffusion

Diffusion is the passive movement of molecules from a region of higher concentration to a region of lower concentration until equilibrium is reached. It does not require energy input.

Example: When a bottle of perfume is opened, fragrance molecules spread throughout the room by diffusion.

Osmosis

Osmosis is the diffusion of water molecules across a semi‑permeable membrane from a solution of lower solute concentration (higher water potential) to a solution of higher solute concentration (lower water potential).

Types of Solutions

Cell States

• Turgidity: Cell swollen with water, plasma membrane pressed against cell wall; provides rigidity.
• Flaccidity: Cell lacks turgor pressure; occurs in isotonic conditions.
• Plasmolysis: Shrinkage of protoplast away from the cell wall in hypertonic solution. Stages:
  1. Incipient plasmolysis: Just beginning to detach.
  2. Evident plasmolysis: Clear gap visible.
  3. Complete plasmolysis: Protoplast fully retracted.
• Deplasmolysis: Recovery of protoplast when cell is placed in hypotonic solution; water re‑enters.

Ascent of Sap

The upward movement of water from roots to leaves through the xylem is termed ascent of sap. Three main theories explain this process:

1. Root Pressure Theory: Active ion uptake into roots creates a positive pressure that pushes water upward. Effective mainly in small herbs and during night.
2. Cohesion‑Tension Theory (Dixon & Joly, 1895): Transpiration pull creates negative pressure (tension) in the xylem; water columns remain unbroken due to cohesion (hydrogen bonding) and adhesion to xylem walls.
3. Transpiration Pull: The evaporative loss of water from leaf surfaces generates the tension that drives water upward.

Demonstration – Ganong’s Potometer

A Ganong’s potometer measures the rate of water uptake (approximating transpiration rate). A leafy shoot is inserted into a water‑filled tube; an air bubble moves along a graduated scale as water is consumed. The speed of bubble movement indicates transpiration under given environmental conditions.

Transpiration

Transpiration is the loss of water vapor from aerial parts of the plant, mainly through stomata, but also via lenticels and cuticle.

Types

• Stomatal transpiration: Through stomata (≈80‑90% of total).
• Cuticular transpiration: Through the waxy cuticle (minor).
• Lenticular transpiration: Through lenticels in woody stems.

Factors Affecting Transpiration

• Light: Stomata open in light → ↑ transpiration.
• Temperature: Higher temperature ↑ kinetic energy of water molecules → ↑ transpiration.
• Humidity: Low external humidity ↑ gradient → ↑ transpiration.
• Wind: Removes saturated air layer → ↑ transpiration.

Significance

• Cooling of leaves (evaporative cooling).
• Facilitates water uptake and mineral transport from roots.
• Creates transpiration pull essential for ascent of sap.

Guttation

Guttation is the exudation of liquid water from leaf margins or tips through specialized structures called hydathodes. It occurs when root pressure exceeds transpirational loss, typically during high humidity and low transpiration (e.g., early morning).

2.2 Photosynthesis

Introduction

Photosynthesis is the process by which light energy is converted into chemical energy stored as glucose, occurring primarily in the chloroplasts of plant cells.

Overall equation: 6 CO₂ + 6 H₂O + light energy → C₆H₁₂O₆ + 6 O₂

Significance

• Produces atmospheric O₂ essential for aerobic life.
• Synthesizes organic compounds (carbohydrates, proteins, lipids) that form the basis of food chains.
• Fixes inorganic carbon (CO₂) into biomass, mitigating climate change.

Photosynthetic Pigments

Pigments absorb specific wavelengths of light and transfer energy to the reaction centre.

• Chlorophyll a (blue‑green): Primary pigment; directly participates in photochemical reactions.
• Chlorophyll b (yellow‑green): Accessory pigment; broadens absorption spectrum.
• Carotenoids (yellow‑orange): Accessory pigments; also protect against photo‑oxidative damage.
• Phycobilins (red‑blue): Found in red algae and cyanobacteria; absorb green light.

Action Spectrum vs Absorption Spectrum

The action spectrum shows the rate of photosynthesis at different wavelengths, while the absorption spectrum shows light absorption by pigments. Peaks in the action spectrum (blue ~430 nm and red ~660 nm) correspond to chlorophyll absorption peaks.

Mechanism

Photochemical Phase (Light Reaction)

Occurs in the thylakoid membranes; converts light energy to chemical energy (ATP and NADPH).

1. Photosystem II (PSII, P680) absorbs light, excites electrons.
2. Photolysis of water: 2 H₂O → 4 H⁺ + 4 e⁻ + O₂ (releases O₂, provides electrons).
3. Electrons travel through the electron transport chain (ETC) (plastoquinone → cytochrome b₆f → plastocyanine) releasing energy used to pump protons into the thylakoid lumen.
4. Chemiosmosis: Proton gradient drives ATP synthesis via ATP synthase (photophosphorylation).
5. Photosystem I (PSI, P700) re‑excites electrons; they reduce NADP⁺ to NADPH via ferredoxin‑NADP⁺ reductase.

Cyclic and Non‑cyclic Photophosphorylation

• Non‑cyclic: Electrons from H₂O → PSII → ETC → PSI → NADP⁺; produces both ATP and NADPH.
• Cyclic: Electrons from PSI → ETC → back to plastoquinone; generates ATP only, used when NADPH demand is low.

Calvin‑Benson Cycle (Dark Reaction)

Occurs in the stroma; uses ATP and NADPH to fix CO₂ into carbohydrate.

1. Carbon fixation: CO₂ + RuBP → 2 × 3‑PGA (catalyzed by RuBisCO).
2. Reduction: 3‑PGA phosphorylated by ATP → 1,3‑bisphosphoglycerate; reduced by NADPH → G3P (glyceraldehyde‑3‑phosphate).
3. Regeneration of RuBP: Some G3P molecules used to regenerate RuBP (requires ATP).
4. Net output: For every 3 CO₂ fixed, one G3P exits the cycle; two G3P combine to form one glucose molecule.

C₃ and C₄ Plants

Feature	C₃ Plants	C₄ Plants
Primary carboxylation enzyme	RuBisCO (in mesophyll)	PEP carboxylase (in mesophyll)
First stable product	3‑phosphoglycerate (3‑PGA, 3‑C)	Oxaloacetate (OAA, 4‑C)
Kranz anatomy	Absent	Present (bundle‑sheath cells surround vasculature)
Photorespiration	Significant (especially at high O₂/low CO₂)	Negligible (CO₂ concentrated in bundle sheath)
Examples	Rice, wheat, soybean	Maize, sugarcane, sorghum

Photorespiration

When RuBisCO oxygenates RuBP instead of carboxylating it, a 2‑carbon compound (phosphoglycolate) is formed, leading to a loss of fixed carbon and energy. This process is termed photorespiration and reduces photosynthetic efficiency in C₃ plants. C₄ plants minimize photorespiration by spatially separating initial CO₂ fixation (mesophyll) from the Calvin cycle (bundle sheath).

Factors Affecting Photosynthesis

• Light intensity: Increases rate up to saturation point.
• CO₂ concentration: Higher CO₂ ↑ carboxylation rate (until RuBisCO saturated).
• Temperature: Influences enzyme activity; optimal range 20‑30 °C for most C₃ plants.
• Water availability: Stomatal closure under water stress limits CO₂ influx.
• Chlorophyll content: Determines light‑harvesting capacity.

2.3 Respiration

Introduction

Respiration is the catabolic process in which glucose is oxidized to release energy stored as ATP, occurring in all living cells.

Overall aerobic equation: C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + ATP

Significance

• Provides ATP for cellular processes (active transport, biosynthesis, movement).
• Generates carbon skeletons for biosynthesis.
• Produces heat that helps maintain optimal temperature.

Types of Respiration

• Aerobic respiration: Complete oxidation of glucose in presence of O₂; yields maximum ATP.
• Anaerobic respiration (fermentation): Incomplete oxidation in absence of O₂; yields ethanol (yeast, some plants) or lactic acid (some tissues) plus a small amount of ATP.

Mechanism

Glycolysis

Occurs in the cytoplasm; one glucose (6‑C) is split into two pyruvate (3‑C) molecules.

• Investment phase: 2 ATP used.
• Payoff phase: 4 ATP produced + 2 NADH.
• Net: 2 ATP + 2 NADH per glucose.

Krebs Cycle (TCA Cycle)

Occurs in the mitochondrial matrix; each pyruvate is converted to acetyl‑CoA, entering the cycle.

• Per acetyl‑CoA: 3 NADH, 1 FADH₂, 1 ATP (GTP), 2 CO₂ released.
• Per glucose: double the yields.

Electron Transport System (ETS)

Located in the inner mitochondrial membrane; NADH and FADH₂ donate electrons to a series of carriers.

• Energy released pumps protons, creating electrochemical gradient.
• ATP synthase uses this gradient to produce ATP (oxidative phosphorylation).
• Approximate yield: ~34 ATP per glucose (from 10 NADH and 2 FADH₂).

Total ATP Yield

Aerobic respiration yields approximately 38 ATP per glucose molecule (2 from glycolysis, 2 from Krebs cycle, ~34 from ETS).

Factors Affecting Respiration

• Temperature: Rate increases with temperature up to an optimum; beyond that, enzymes denature.
• O₂ concentration: Essential for aerobic pathway; low O₂ shifts to fermentation.
• Light: Indirect effect via photosynthesis‑produced sugars.
• Water content: Severe dehydration inhibits metabolic enzymes.

2.4 Plant Hormones

Auxins

Produced chiefly at shoot tips and young leaves; primary natural auxin is indole‑3‑acetic acid (IAA).

• Promotes cell elongation via acid growth hypothesis (activation of plasma‑membrane H⁺‑ATPase → wall loosening).
• Regulates apical dominance (suppresses lateral bud growth).
• Stimulates root initiation and development.
• Mediates phototropism and gravitropism (differential auxin distribution).

Gibberellins (GAs)

A group of diterpenoid acids; GA₁ is the most bioactive.

• Stimulate stem elongation (especially internodes).
• Break seed dormancy and promote germination.
• Induce flowering in long‑day plants and promote fruit growth.

Cytokinins

Derived from adenine; zeatin is a common natural cytokinin.

• Promote cell division (cytokinesis).
• Delay senescence (nutrient mobilization).
• Stimulate shoot formation in tissue culture.
• Interact with auxins to regulate differentiation (high auxin/cytokinin → roots; low auxin/high cytokinin → shoots).

Abscisic Acid (ABA)

Known as a stress hormone.

• Induces stomatal closure during water stress.
• Promotes seed and bud dormancy.
• Inhibits growth under adverse conditions.

Ethylene

A gaseous hormone.

• Promotes fruit ripening (e.g., banana, tomato).
• Induces leaf and flower abscission.
• Involved in stress responses and senescence.

2.5 Plant Growth and Movement

Seed Germination

The process by which a quiescent seed resumes metabolic activity and develops into a seedling.

1. Imbibition: Seed absorbs water, causing swelling and activation of enzymes.
2. Activation of enzymes: Mobilization of stored nutrients (starch, lipids, proteins).
3. Emergence of radicle: First root tip penetrates seed coat.
4. Emergence of plumule: Shoot apex develops, followed by cotyledons and true leaves.

Dormancy

A temporary suspension of growth that enables survival under unfavorable conditions (e.g., cold, drought). Dormancy can be seed‑coat imposed, embryo‑based, or environmentally induced.

Photoperiodism

Response of plants to the relative length of day (light) and night (dark) periods for flowering.

• Short‑day plants (SDP): Flower when day length < critical length (e.g., Poinsettia, rice, soybean).
• Long‑day plants (LDP): Flower when day length > critical length (e.g., wheat, spinach, barley).
• Day‑neutral plants (DNP): Flowering unaffected by photoperiod (e.g., tomato, cucumber, pepper).

Vernalization

Exposure to low temperatures (typically 0‑10 °C for weeks) induces flowering in certain biennial and perennial plants (e.g., winter wheat, barley). The cold period modifies gene expression (e.g., vernalization‑independent flowering locus C).

Senescence

The programmed aging and death of plant parts, especially leaves. Nutrients (especially nitrogen and phosphorus) are remobilized from senescing tissues to growing points or storage organs.

Plant Movements

Movements are categorized as tropic (directional) or nastic (non‑directional).

Tropic Movements

• Phototropism: Growth toward (positive) or away from (negative) light; mediated by auxin redistribution.
• Geotropism (Gravitropism): Roots show positive (downward) growth; shoots show negative (upward) growth.
• Hydrotropism: Growth toward moisture; important for root orientation.
• Chemotropism: Growth toward or away from chemical stimuli (e.g., pollen tube growth toward ovule).
• Thigmotropism: Growth in response to touch or contact (e.g., tendril coiling around a support).

Nastic Movements

• Seismonasty: Rapid movement in response to touch (e.g., leaf folding in Mimosa pudica).
• Thigmonasty: Movement triggered by mechanical stimulus irrespective of direction (e.g., Venus flytrap closure).
• Nyctinasty: Sleep movements; leaves open in daylight and close at night (e.g., legumes).

Summary

This chapter has integrated the core physiological processes that enable plants to acquire resources, convert energy, grow, and respond to their environment. Mastery of these concepts provides a foundation for advanced studies in plant biology, agriculture, and ecological sciences.