Plant Physiology

Learning Objectives

• Understand the fundamental concepts of Plant Physiology
• Apply key formulas and techniques to solve problems
• Practice with exam-level questions to build speed and accuracy

1. Transport in Plants

Transport in plants involves the movement of water, minerals, and organic nutrients over both short distances (cell-to-cell) and long distances (through vascular tissues). Unlike animals, plants lack a circulatory pump; instead, they rely on physical forces such as diffusion, osmosis, transpiration pull, and root pressure to move substances throughout the plant body.

Water Potential

Water potential (Ψ_w) is a measure of the free energy of water per unit volume. It determines the direction of water movement: water always moves from a region of higher water potential to a region of lower water potential. The concept was introduced by Slatyer and Taylor and is fundamental to understanding water relations in plants.

Components of water potential:

• Solute potential (Ψ_s): Also called osmotic potential. Always negative. The presence of solutes lowers the free energy of water. Pure water at standard conditions has Ψ_s = 0.
• Pressure potential (Ψ_p): Can be positive (turgor pressure in a turgid cell) or negative (tension in xylem vessels).
• Matrix potential (Ψ_m): Due to adsorption of water to colloidal surfaces. Important in cell walls and dry seeds.

When a plant cell is placed in a solution, water movement depends on the relative water potentials. In a hypotonic solution (higher Ψ_w outside), water enters the cell, causing it to become turgid. In a hypertonic solution (lower Ψ_w outside), water leaves the cell, causing plasmolysis.

Absorption of Water

Water absorption occurs primarily through root hairs, which are epidermal extensions that greatly increase the surface area for absorption. The pathway of water from root hairs to the xylem can be apoplastic (through cell walls and intercellular spaces) or symplastic (through the cytoplasm via plasmodesmata). The Casparian strip in the endodermis blocks apoplastic flow, forcing water into the symplastic pathway for selective mineral uptake.

Root Pressure

Root pressure is a positive pressure generated in the xylem of roots due to the active absorption of minerals and the subsequent osmotic entry of water. It is most evident at night when transpiration is low and humidity is high. The phenomenon of guttation — the exudation of water droplets from leaf margins through hydathodes — is a direct manifestation of root pressure. Root pressure alone cannot explain water movement in tall trees exceeding 20–30 metres.

Transpiration Pull

The transpiration pull is the primary force driving water movement in plants during the day. The Cohesion-Tension Theory (Dixon & Joly, 1894) explains water ascent: water evaporates from leaf mesophyll cells, creating tension that pulls the continuous water column upward through xylem vessels. Cohesion (H-bonding between water molecules) and adhesion (attraction to xylem walls) maintain the integrity of the water column under tension.

Ascent of Sap

The ascent of sap refers to the upward movement of water and dissolved minerals through the xylem. The Cohesion-Tension Theory is the most widely accepted explanation. Experimental evidence includes:

• Stem girdling: Removing phloem does not stop water flow, proving xylem is the conducting tissue.
• Dye experiments: Coloured solutions like eosin rise through the xylem and can be traced visually.
• Pressure bomb: Scholander’s pressure bomb measures xylem tension (negative pressure) directly.

Transpiration

Transpiration is the loss of water vapour from aerial plant parts. It is described as a necessary evil because it drives water and mineral transport but can cause water deficit. Types include stomatal (80–90%, through stomata), cuticular (5–10%, through the cuticle), and lenticular (1–2%, through lenticels).

Factor	Effect on Transpiration	Mechanism
Light	Increases	Stomata open; increases leaf temperature
Temperature	Increases	Increases evaporation rate and vapour pressure deficit
Humidity	Decreases	Reduces diffusion gradient between leaf and air
Wind	Increases (moderate); Decreases (high)	Removes water vapour; very high wind closes stomata
CO2 Concentration	Decreases	High CO2 induces stomatal closure
Soil Water	Decreases	Water deficit triggers ABA signalling, closing stomata

Stomatal Mechanism

Guard cells regulate stomatal opening and closing through changes in turgor pressure. K⁺ influx into guard cells (driven by H⁺-ATPase proton pumps) lowers water potential, causing water entry and stomatal opening. K⁺ efflux reverses the process. ABA triggers Ca²⁺ signalling that activates K⁺ efflux channels, closing stomata during water stress. Blue light receptors (phototropins) activate the H⁺-ATPase to initiate opening.

Phloem Transport

Phloem translocates organic solutes (primarily sucrose) from sources (net producers like mature leaves) to sinks (net consumers like roots, fruits, and young leaves). Sieve tube elements (living but enucleate) and companion cells (with nuclei and ribosomes) form the functional unit of phloem.

Pressure Flow Hypothesis

Proposed by Ernst Münch (1930), this hypothesis explains phloem transport: (1) Sucrose is actively loaded into sieve tubes at the source, lowering water potential. (2) Water enters from xylem by osmosis, creating high hydrostatic pressure. (3) Sucrose is unloaded at the sink, raising water potential. (4) Water leaves, creating low pressure. (5) The pressure gradient drives bulk flow from source to sink. Phloem sap typically contains 10–25% sucrose and can flow at 1–2 m h⁻¹.

2. Photosynthesis

Photosynthesis is the process by which plants convert light energy into chemical energy. The overall equation is:

The process occurs in two phases: light reactions (thylakoid membranes, require light) and dark reactions (stroma, do not directly require light). The organelle is the chloroplast, which has a double membrane envelope, thylakoid system (grana and stroma lamellae), and stroma.

Pigments

Pigments absorb specific wavelengths of light. The absorption spectrum shows which wavelengths a pigment absorbs; the action spectrum shows which wavelengths drive the highest rate of photosynthesis.

• Chlorophyll a: Primary pigment (blue-green). Reaction centre in both PS I and PS II. Peaks at 430 nm (blue) and 662 nm (red).
• Chlorophyll b: Accessory pigment (yellow-green). Absorbs at 453 nm and 642 nm. Transfers energy to chlorophyll a.
• Carotenoids: β-carotene (orange) and xanthophylls (yellow). Absorb blue light (400–500 nm). Protect against photooxidation by quenching triplet chlorophyll and scavenging ROS.
• Phycobilins: Phycoerythrin and phycocyanin in cyanobacteria and red algae. Absorb green/yellow light for deep-water survival.

Light Reactions

Light reactions occur in the thylakoid membranes. They produce ATP, NADPH, and O₂ (from water photolysis).

Photosystems

Each photosystem comprises a light-harvesting complex (200–300 pigment molecules) and a reaction centre (special pair of chlorophyll a molecules):

• Photosystem II (P680): Located in granal thylakoids. Involved in water photolysis and initiating electron flow. The oxygen-evolving complex (OEC) contains Mn²⁺, Ca²⁺, and Cl⁻.
• Photosystem I (P700): Located in stromal thylakoids and grana margins. Involved in NADP⁺ reduction via ferredoxin.

Electron Transport

The Z-scheme describes non-cyclic electron flow:

1. Light excites P680 in PS II, ejecting an electron to pheophytin (primary acceptor).
2. The electron passes through plastoquinone (PQ), the cytochrome b₆f complex (proton pump), and plastocyanin (PC) to PS I.
3. Photolysis of water replaces the electron lost by PS II: 2H₂O → 4H⁺ + 4e⁻ + O₂.
4. Light excites P700 in PS I, ejecting another electron that passes through ferredoxin (Fd) and reduces NADP⁺ to NADPH via ferredoxin-NADP⁺ reductase (FNR).

Photophosphorylation:

• Non-cyclic: Involves both PS II and PS I. Linear flow. Produces ATP + NADPH + O₂.
• Cyclic: Involves only PS I. Electrons cycle back from ferredoxin to PQ, generating ATP only. No NADPH or O₂.

ATP is synthesised by chemiosmosis (Peter Mitchell, Nobel 1978): the proton gradient across the thylakoid membrane drives ATP synthase (CF₀-CF₁ complex).

Feature	Non-cyclic	Cyclic
Photosystems	PS II + PS I	Only PS I
Products	ATP + NADPH + O2	ATP only
Electron source	Water (photolysis)	Cyclic flow
Final acceptor	NADP+	Same photosystem
Purpose	Assimilatory power for Calvin cycle	Extra ATP when NADPH is sufficient

Dark Reactions

Dark reactions (Calvin cycle) use ATP and NADPH to fix CO₂ into carbohydrates. They occur in the stroma of the chloroplast.

Calvin Cycle

Discovered by Melvin Calvin (Nobel 1961). Three phases:

1. Carboxylation: CO₂ + RuBP (5C) → 2 × 3-PGA (3C). Catalysed by RuBisCO — the most abundant protein on Earth (30–50% of leaf protein).
2. Reduction: 3-PGA is phosphorylated (ATP) and reduced (NADPH) to form glyceraldehyde-3-phosphate (G3P).
3. Regeneration: 5 out of 6 G3P molecules are used to regenerate RuBP, consuming ATP.

C4 Pathway

The Hatch-Slack pathway is an adaptation in plants growing in hot, dry environments (maize, sugarcane, sorghum). Key features:

• Kranz anatomy: Bundle sheath cells arranged in a wreath-like pattern around vascular bundles.
• In mesophyll cells: CO₂ + PEP (3C) → OAA (4C) via PEP carboxylase (high affinity for CO₂, no oxygenase activity).
• OAA is converted to malate or aspartate and transported to bundle sheath cells.
• In bundle sheath cells: Malate is decarboxylated, releasing CO₂ for the Calvin cycle.
• Pyruvate returns to mesophyll cells and is regenerated to PEP using ATP.

Advantage: Concentrates CO₂ in bundle sheath cells, suppressing photorespiration. C4 plants have higher water-use efficiency and higher optimal temperature (30–45 °C).

CAM Pathway

Crassulacean Acid Metabolism is found in succulent plants (cacti, Agave, Kalanchoe, pineapple). It features temporal separation of carbon fixation:

• Night (stomata open): CO₂ is fixed by PEP carboxylase into OAA, then reduced to malate and stored in the vacuole.
• Day (stomata closed): Malate is decarboxylated, and CO₂ enters the Calvin cycle. This minimises water loss.

Feature	C3 Plants	C4 Plants	CAM Plants
First stable product	3-PGA (3C)	OAA (4C)	OAA (4C, at night)
Primary CO2 acceptor	RuBP	PEP	PEP (at night)
Carboxylating enzyme	RuBisCO	PEP carboxylase	PEP carboxylase (night)
Anatomy	No Kranz	Kranz anatomy	Succulent, no Kranz
Photorespiration	High	Very low	Low
Water-use efficiency	Low	High	Very high
Optimum temperature	15–25 °C	30–45 °C	25–35 °C
Examples	Rice, wheat, soybean, potato	Maize, sugarcane, sorghum	Cactus, Agave, pineapple

Factors Affecting Photosynthesis

Blackman’s Law of Limiting Factors (1905): When a process is affected by multiple factors, its rate is limited by the factor in shortest supply.

• Light intensity: Increases linearly to a saturation point; very high light causes photoinhibition.
• CO₂ concentration: Rate increases up to saturation (~1,000 ppm for C3, higher for C4).
• Temperature: C3 optimum 20–30 °C; C4 optimum 30–45 °C. Above optimum, photorespiration increases in C3.
• Water: Stress closes stomata, reducing CO₂ uptake.
• Oxygen: High O₂ promotes photorespiration in C3 plants (Warburg effect).

3. Respiration in Plants

Respiration is the oxidative breakdown of organic compounds to release ATP. It occurs continuously (day and night) in all living cells. The overall equation is the reverse of photosynthesis:

Respiration has three main stages: glycolysis (cytoplasm), Krebs cycle (mitochondrial matrix), and electron transport chain (inner mitochondrial membrane).

Glycolysis

The Embden-Meyerhof-Parnas (EMP) pathway occurs in the cytoplasm and does not require oxygen. It is common to both aerobic and anaerobic respiration.

• Glucose (6C) is phosphorylated twice (using 2 ATP) to form fructose-1,6-bisphosphate.
• The 6C sugar is split into two 3C molecules: DHAP and G3P (interconvertible).
• G3P is oxidised (NAD⁺ → NADH) and substrate-level phosphorylation yields ATP.
• Net products per glucose: 2 Pyruvate + 2 ATP (net) + 2 NADH.

Krebs Cycle

The TCA cycle (citric acid cycle) takes place in the mitochondrial matrix. Before entering the cycle, pyruvate is converted to acetyl-CoA by the pyruvate dehydrogenase complex (releasing 1 CO₂ and producing 1 NADH per pyruvate).

• Acetyl-CoA (2C) + oxaloacetate (4C) → citrate (6C).
• Through a series of reactions: citrate → α-ketoglutarate → succinate → fumarate → malate → oxaloacetate.
• Products per turn: 3 NADH + 1 FADH₂ + 1 GTP + 2 CO₂.
• Per glucose: 6 NADH + 2 FADH₂ + 2 GTP.

Electron Transport Chain

The ETC is located in the inner mitochondrial membrane and consists of four protein complexes (I–IV) plus ATP synthase (Complex V).

• Complex I (NADH dehydrogenase): Accepts e⁻ from NADH, pumps 4 H⁺.
• Complex II (Succinate dehydrogenase): Accepts e⁻ from FADH₂, does not pump H⁺.
• Complex III (Cytochrome bc₁): Pumps 4 H⁺.
• Complex IV (Cytochrome c oxidase): Transfers e⁻ to O₂ (forming H₂O), pumps 2 H⁺.
• ATP synthase (Complex V): Uses the proton gradient to synthesise ATP by chemiosmosis.

ATP yield: Each NADH produces ~2.5 ATP; each FADH₂ produces ~1.5 ATP. Total: ~30–32 ATP per glucose.

Stage	Location	Products per glucose
Glycolysis	Cytoplasm	2 ATP + 2 NADH + 2 Pyruvate
Link reaction (×2)	Matrix	2 NADH + 2 CO2
Krebs cycle (×2)	Matrix	2 GTP + 6 NADH + 2 FADH2 + 4 CO2
ETC + Chemiosmosis	Inner membrane	~26–28 ATP + H2O
Total		~30–32 ATP

Fermentation

Fermentation is the anaerobic pathway that allows glycolysis to continue when O₂ is unavailable (regenerates NAD⁺). Only 2 ATP per glucose are produced.

• Alcoholic fermentation (yeast, some plant roots): Pyruvate → Acetaldehyde + CO₂ → Ethanol. Enzyme: alcohol dehydrogenase.
• Lactic acid fermentation (animals, some bacteria): Pyruvate → Lactate. Enzyme: lactate dehydrogenase.

Respiratory Quotient

• Carbohydrates: RQ = 1.0 (C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O)
• Fats: RQ ~ 0.7 (require more O₂ per CO₂ released)
• Proteins: RQ ~ 0.8–0.9
• Organic acids: RQ > 1 (e.g., oxalic acid RQ = 4)
• Anaerobic: RQ = ∞ (no O₂ consumed)

4. Plant Growth & Development

Plant growth is an irreversible increase in size, volume, and weight. Development includes all changes from germination to senescence.

Phases of Growth

• Meristematic phase: Cells in apical and lateral meristems divide actively. Rich in protoplasm, large nuclei, thin cell walls.
• Elongation phase: Cells enlarge and elongate. Vacuoles form and coalesce. Most increase in plant size occurs here.
• Maturation phase: Cells differentiate into specialised types (xylem, phloem, fibres, etc.).

Growth rates: Arithmetic (one daughter cell continues dividing — root elongation) and Geometric (both daughters divide — exponential, J-shaped). The grand period of growth follows an S-shaped (sigmoid) curve. Growth requires water, oxygen, nutrients, light, and appropriate temperature.

Plant Hormones

Phytohormones are organic compounds produced in low concentrations that regulate physiological processes. They are classified as promoters (auxins, gibberellins, cytokinins) and inhibitors (ABA, ethylene).

Hormone	Site of Synthesis	Major Functions	NEET Key Fact
Auxin (IAA)	Young leaves, shoot apex	Cell elongation, apical dominance, phototropism, root initiation	2,4-D is a synthetic auxin used as herbicide
Gibberellins (GA)	Young leaves, roots, seeds	Stem elongation, seed germination, fruit growth, bolting	GA3 induces α-amylase in barley malting
Cytokinins	Root tips	Cell division, shoot initiation, delay senescence	Kinetin is a synthetic cytokinin
ABA	Mature leaves, stressed roots	Stomatal closure, seed dormancy, stress tolerance	Called the stress hormone
Ethylene (C2H4)	Senescing tissues, fruits	Fruit ripening, senescence, abscission, epinasty	Only gaseous hormone; ethephon is synthetic source

Auxins

IAA is the most common natural auxin. It is synthesised in shoot tips and young leaves and transported basipetally via polar transport. Key effects: cell elongation (acid-growth hypothesis: H⁺-ATPase activation loosens cell walls), apical dominance (apex suppresses lateral buds), phototropism (asymmetric auxin distribution causes bending), and adventitious root formation. Synthetic auxin 2,4-D is used as a selective herbicide against dicot weeds.

Gibberellins

GAs are diterpenoid acids (GA₃ is gibberellic acid). They stimulate stem elongation (subapical meristem), bolting in rosette plants, seed germination (induction of α-amylase in barley aleurone cells — used in malting), and fruit growth (parthenocarpic fruits like seedless grapes). Dwarf mutants often lack functional GA synthesis.

Cytokinins

Cytokinins (zeatin, kinetin) are adenine derivatives synthesised in root tips. They promote cell division (synergistically with auxins), shoot initiation in tissue culture, delay senescence (Richmond-Lang effect), and mobilise nutrients. The auxin:cytokinin ratio determines root vs shoot formation in callus culture.

ABA

Abscisic acid is a sesquiterpenoid (15C) from the carotenoid pathway. It is the stress hormone. Functions: induces stomatal closure via Ca²⁺-dependent K⁺ efflux, promotes seed dormancy (ABA:GA ratio determines dormancy vs germination), induces desiccation tolerance proteins (LEA proteins), and promotes abscission.

Ethylene

Ethylene is a gaseous hormone synthesised from methionine (Yang cycle). Key effects: fruit ripening (climacteric rise, starch → sugar conversion), senescence and abscission of leaves, flowers, and fruits, epinasty (downward leaf bending), and the triple response in etiolated seedlings (inhibited stem elongation, stem thickening, horizontal growth). Commercially, ethephon is used to release ethylene for uniform fruit ripening.

Photoperiodism

Photoperiodism is the response of plants to relative lengths of day and night. The photoperiodic stimulus is perceived by leaves, and a transmissible signal — florigen (the FT protein) — moves to the shoot apex to induce flowering.

• Short-day plants (SDP): Flower when day length is shorter than a critical period (e.g., Chrysanthemum, Soybean, Rice, Tobacco). They actually require a continuous dark period longer than a critical length. A flash of light during the dark period prevents flowering.
• Long-day plants (LDP): Flower when day length exceeds a critical period (e.g., Wheat, Spinach, Radish, Sugar Beet). A long night inhibits flowering.
• Day-neutral plants (DNP): Flowering is unaffected by day length (e.g., Tomato, Sunflower, Cucumber, Maize). They flower when they reach a certain maturity.

The phytochrome system detects photoperiods. Phytochrome exists in two forms: P_r (inactive, absorbs red light at 660 nm) and P_fr (active, absorbs far-red light at 730 nm). Red light converts P_r → P_fr; far-red light converts P_fr → P_r. The ratio of P_fr/P_r determines the flowering response.

Vernalisation

Vernalisation is the induction of flowering by prolonged cold exposure (0–5 °C for several weeks). It is perceived by the shoot apical meristem (not leaves). Cold treatment induces epigenetic silencing of the flowering repressor FLC (FLOWERING LOCUS C) through histone modification (H3K27me3). This allows the expression of FT (florigen) under favourable (spring) conditions. Examples: winter wheat, sugar beet, cabbage, and many biennials.

5. Mineral Nutrition

Mineral nutrition deals with the uptake and functions of essential mineral elements. Arnon and Stout’s criteria (1939): An element is essential if (1) it is necessary for the plant to complete its life cycle, (2) its function cannot be replaced by another element, and (3) it is directly involved in plant metabolism.

Essential Elements

Macronutrients (required in large amounts, >10 mmol kg⁻¹ dry matter): C, H, O, N, P, K, Ca, Mg, S. Micronutrients (trace amounts, ≤10 mmol kg⁻¹): Fe, Mn, Cu, Zn, Mo, B, Cl, Ni.

Element	Absorbed Form	Major Functions	Deficiency Symptoms
N	NO3−, NH4+	Proteins, nucleic acids, chlorophyll, ATP, hormones	Chlorosis of older leaves, stunted growth
P	H2PO4−	ATP, nucleic acids, phospholipids, phosphorylation	Stunted growth, dark green/purple leaves
K	K+	Stomatal opening, enzyme activation, osmotic balance	Marginal chlorosis, necrotic spots, weak stems
Ca	Ca2+	Cell wall (pectate), membrane integrity, signalling	Inhibited bud growth, necrosis of young leaves
Mg	Mg2+	Chlorophyll centre, enzyme cofactor (RuBisCO)	Interveinal chlorosis of older leaves
S	SO42−	Cysteine, methionine, CoA, vitamins	Chlorosis of young leaves, stunted growth
Fe	Fe2+	Cytochromes, ferredoxin, chlorophyll synthesis	Interveinal chlorosis of young leaves
Mn	Mn2+	Photolysis of water (OEC), enzyme activator	Interveinal chlorosis, grey necrotic spots
Zn	Zn2+	Enzyme cofactor, auxin synthesis	Malformed leaves, little leaf disease, stunting
B	H3BO3	Cell wall cross-linking, pollen tube growth	Inhibited shoot/root growth, brown heart

Deficiency Symptoms

Deficiency symptoms depend on element mobility. Mobile elements (N, P, K, Mg, Zn, Mo) are translocated from older to younger tissues; deficiency appears first in older leaves. Immobile elements (Ca, B, Fe, S, Cu) cannot be readily retranslocated; deficiency appears in younger leaves and growing points.

• Chlorosis: N, Mg, Fe, Mn, S deficiency
• Necrosis: Ca, K, Mg, Cu deficiency
• Stunted growth: N, P, K, Zn, Ca deficiency
• Purple pigmentation: P deficiency (anthocyanin accumulation)
• Toxicity: Excess of one element can induce deficiency of another (e.g., excess Mn induces Fe deficiency)

Nitrogen Metabolism

Nitrogen is the most limiting nutrient. Although the atmosphere is 78% N₂, plants cannot use it directly; it must be fixed into NH₃ or NO₃⁻.

Nitrogen Fixation

Biological nitrogen fixation is carried out by prokaryotes possessing the nitrogenase enzyme:

• Symbiotic: Rhizobium in legume root nodules. Bacteria enter via infection threads, differentiate into bacteroids surrounded by the peribacteroid membrane. Leghaemoglobin (pink) maintains low O₂ for O₂-sensitive nitrogenase.
• Free-living: Azotobacter (aerobic), Clostridium (anaerobic), Anabaena (cyanobacteria, used as biofertiliser with Azolla in rice paddies).

Nitrate assimilation: NO₃⁻ → NO₂⁻ (nitrate reductase, cytoplasm) → NH₄⁺ (nitrite reductase, chloroplast). Ammonia is incorporated into amino acids via the GS-GOGAT pathway.

Nitrification: NH₃ → NO₂⁻ (Nitrosomonas) → NO₃⁻ (Nitrobacter). Denitrification: NO₃⁻ → N₂ (Pseudomonas) — returns nitrogen to the atmosphere.

Practice Questions