Plant Physiology

Part I · Botany · Chapter Four

Expect 8–12 questions: water relations (Ψ_w formula, plasmolysis), transpiration & stomatal mechanism, mineral nutrition (deficiency symptoms), photosynthesis (Z-scheme, Calvin, C4 Hatch-Slack, CAM), plant growth regulators (discoverers + functions), photoperiodism (Garner & Allard), vernalisation, and HP-specific angles (apple chill hours, sea buckthorn nitrogen fixation, alpine photosynthesis, Kangra tea). Year-person-discovery facts are heavily tested.

Read · 90 min
Revise · 20 min
MCQs · 28

Syllabus Coverage

Water relations & osmosis • Absorption and transport of water • Transpiration and stomatal physiology • Mineral nutrition and essential elements • Nitrogen metabolism and biological N-fixation • Photosynthesis — light reactions and dark reactions (C3, C4, CAM) • Photorespiration • Respiration (glycolysis, Krebs cycle, ETC) • Plant growth regulators (auxin, GA, cytokinin, ABA, ethylene, brassinosteroids) • Photoperiodism, vernalisation and seed dormancy.

4.1 Plant–Water Relations

Water constitutes 80–90 % of the fresh mass of most plant cells. Life processes—turgor-driven growth, solute transport, enzymatic reactions—depend on its continuous availability and precisely regulated distribution. The physical framework that explains water movement in plants is water potential (Ψ_w), a concept that unifies osmosis, pressure and matric forces under a single thermodynamic variable.

Water Potential (Ψ_w)

Water potential is the chemical potential of water in a system relative to pure water at the same temperature and atmospheric pressure, expressed in units of pressure (MPa or bars). Pure water at atmospheric pressure has Ψ_w = 0. Water always moves passively from a region of higher (less negative) water potential to lower (more negative) water potential.

4.1.1 Components of Water Potential

For a plant cell, water potential is the sum of three major components:

Osmotic (solute) potential Ψ_s — always zero or negative; decreases as solute concentration increases. Also called solute potential.
Pressure potential Ψ_p — usually positive (turgor pressure of the cell wall pushing inward on the protoplast); in xylem vessels under tension it can be negative (tension = negative pressure).
Matric potential Ψ_m — negative; due to adsorption of water to colloids and cell walls (important in seeds and soils, often omitted in leaf-cell calculations).

The combined equation is: Ψ_w = Ψ_s + Ψ_p + Ψ_m

At full turgor: the cell has absorbed maximum water, Ψ_p is at its maximum, and Ψ_w ≈ 0 (if the cell were in equilibrium with pure water). Formally, at full turgor Ψ_p ≈ −Ψ_s. At incipient plasmolysis: turgor is zero (Ψ_p = 0), so Ψ_w = Ψ_s.

Worked example — Water potential calculation

"A cell has an osmotic potential of −0.8 MPa and a pressure potential of +0.3 MPa. What is its water potential? In which direction will water move if this cell is placed adjacent to a cell with Ψ_w = −0.4 MPa?"

Solution: Ψ_w = Ψ_s + Ψ_p = (−0.8) + (+0.3) = −0.5 MPa. The adjacent cell has Ψ_w = −0.4 MPa. Water moves from higher Ψ_w to lower, i.e., from the adjacent cell (−0.4) into this cell (−0.5). Water flows into this cell.

4.1.2 Osmosis

Osmosis is the movement of water molecules through a selectively permeable membrane from a region of lower solute concentration (higher Ψ_w) to a region of higher solute concentration (lower Ψ_w). A cell placed in a hypotonic solution (lower solute outside) gains water and becomes turgid. In a hypertonic solution (higher solute outside) the cell loses water.

4.1.3 Plasmolysis and Deplasmolysis

Plasmolysis is the shrinkage of the protoplast away from the cell wall when a plant cell is placed in a hypertonic solution and loses water. It occurs in stages:

Incipient plasmolysis — the protoplast just begins to pull away from the wall; turgor = 0, Ψ_p = 0.
Full (evident) plasmolysis — the protoplast is clearly retracted, connected to the wall only by thin strands of cytoplasm (Hecht's strands) at plasmodesmata.

Deplasmolysis occurs if the cell is transferred back to a hypotonic or isotonic solution; the protoplast re-expands to refill the wall. This reversibility demonstrates the cell is still alive.

4.1.4 Imbibition

Imbibition is the absorption of water by colloidal particles (cell wall cellulose, seed proteins, dry wood) without the formation of a solution — no membrane is required. It generates enormous imbibition pressure (matric forces) that can crack rocks, germinate seeds buried in soil, and cause swelling of dry wood. Imbibition is responsible for the initial uptake of water during seed germination before a root system develops. Adsorption force is so great that seeds can germinate even against gravity.

Easy to confuse — Diffusion vs Osmosis vs Imbibition

Diffusion

Movement of any molecule (solute or solvent) from high to low concentration. No membrane required. Driven by concentration gradient. Examples: O₂ into mesophyll, CO₂ out of leaf, perfume spreading in air.

Osmosis

Movement of water molecules only through a selectively permeable membrane, from dilute to concentrated solution (high to low Ψ_w). Requires a membrane. Driven by water potential gradient.

Imbibition

Absorption of water by colloids (cellulose, protein, gel). No membrane needed. Driven by matric (adsorption) forces. Can exert huge pressures. Pre-germinative water uptake; wood swelling.

Turgor pressure vs Wall pressure

Turgor pressure (Ψ_p): pressure exerted by cell contents on the cell wall (outward push). Wall pressure: equal and opposite inward push by the rigid wall on the protoplast. They are equal in magnitude at equilibrium.

HP Angle: Seed germination in alpine HP species (e.g., Aconitum, high-altitude cereals) relies heavily on imbibition and is affected by temperature and freeze-thaw cycles. The hard, thick-walled seeds of Hippophae rhamnoides (sea buckthorn) require scarification or prolonged stratification — the seed coat resists imbibition.

4.2 Absorption & Transport of Water

4.2.1 Root Absorption

Water enters the root primarily through root hairs (unicellular extensions of epidermal cells that greatly increase surface area). Once inside the root, water crosses the cortex and endodermis to reach the xylem. Two main pathways:

Apoplast pathway — water moves through the cell walls and intercellular spaces without crossing any membrane (free diffusion in the cell-wall continuum). It is blocked at the endodermis by the Casparian strip (suberin impregnation of the radial and transverse walls of endodermal cells), which forces water to enter the symplast before passing into the stele.
Symplast pathway — water moves from cell to cell through the living cytoplasm, connected by plasmodesmata. This pathway is continuous from root epidermal cells to xylem parenchyma.

Easy to confuse — Symplast vs Apoplast

Symplast

The living continuum of cytoplasm connected by plasmodesmata through all plant cells. Water and small solutes move from cell to cell without crossing a membrane at each junction. Regulated by membrane properties.

Apoplast

The non-living continuum of cell walls and intercellular spaces. Water moves freely by diffusion/capillarity. Blocked at the Casparian strip of the endodermis, forcing entry into the symplast at this point.

4.2.2 Ascent of Sap — Cohesion-Tension Theory

Xylem sap (water + dissolved minerals) must travel from roots to leaves, sometimes 100 m in tall trees. The accepted mechanism is the Cohesion-Tension (transpirational pull) theory proposed by Dixon and Joly (1894) and elaborated by Renner.

Cohesion-Tension theory — Dixon & Joly 1894 · Root pressure — Hales 1727 · Guttation & root pressure demonstration — White 1938 · Pressure-bomb technique — Scholander 1965

Key principles of the theory:

Transpiration pull — evaporation of water from leaf mesophyll cells lowers their Ψ_w, drawing water from adjacent cells and ultimately from xylem elements in the vein.
Cohesion — water molecules are strongly attracted to each other by hydrogen bonds, forming a continuous column in xylem vessels/tracheids. Cohesion prevents the water column from breaking under tension.
Adhesion — water adheres to the hydrophilic xylem walls (lignified but with hydroxyl groups), helping maintain the column and countering gravity.
Continuous water column — the water in xylem is under negative pressure (tension), sometimes as low as −2 to −4 MPa in trees. This tension is the "pull" from above.

Root pressure (a minor, active-transport-driven positive pressure generated in the root stele when transpiration is low) causes guttation — the exudation of liquid water from hydathodes at leaf margins in the morning. Root pressure alone cannot account for ascent in tall trees.

4.2.3 Phloem Transport

Organic solutes (primarily sucrose) move from sources (photosynthetically active leaves, storage organs) to sinks (growing tips, roots, fruits, seeds) via the phloem through sieve tubes. The Mass Flow (Pressure Flow) hypothesis of Münch (1930) proposes that active loading of sucrose into the phloem at the source creates a high Ψ_s (low Ψ_w), drawing water in from xylem, building hydrostatic pressure; at the sink, unloading reduces solute concentration, water moves back to xylem, and the whole column moves by bulk flow down the pressure gradient.

**Table 4.1** Xylem vs Phloem — key comparative facts
Feature	Xylem	Phloem
Conducting elements	Tracheids & vessels (dead at maturity)	Sieve tubes + companion cells (living)
Main solute transported	Mineral ions, low organic matter	Sucrose, amino acids, hormones
Direction of flow	Unidirectional: root → leaf	Bidirectional: source → sink
Driving force	Transpirational pull (cohesion-tension)	Pressure flow (Münch hypothesis)
Pressure	Negative (tension)	Positive (turgor)
Energy requirement	Passive (at macro level)	Active loading/unloading at source and sink

Exam Tip: The Casparian strip is the key "checkpoint" for selective ion uptake. HPRCA questions often ask which pathway is blocked by the Casparian strip (apoplast) and which continues (symplast). Remember: the endodermis is sometimes called the "physiological barrier" of the root.

4.3 Transpiration & Stomatal Physiology

Transpiration is the loss of water vapour from the aerial parts of a plant, primarily through stomata. Though often described as a "necessary evil," it drives the ascent of sap, cools leaf surfaces, and maintains the flow of minerals to shoots. About 97 % of water absorbed by roots is lost by transpiration.

4.3.1 Types of Transpiration

Stomatal transpiration — >90 % of total; occurs through the stomatal pore in the epidermis. Regulated by guard cell movements.
Cuticular transpiration — 3–8 % through the waxy cuticle. Significant in plants with thin cuticles or under drought when stomata close.
Lenticular transpiration — <1 % through lenticels (pores in bark of stems and fruits).

4.3.2 Stomatal Structure and Guard Cells

Each stoma is flanked by two kidney-shaped (in dicots) or dumbbell-shaped (in grasses) guard cells. Guard cells contain chloroplasts (unlike other epidermal cells) and have unevenly thickened walls: the wall facing the pore (ventral) is thicker and less elastic; the dorsal wall is thinner. This structural asymmetry means that when guard cells become turgid, they bow outward, opening the pore.

Fig. 4.1 Stomatal opening mechanism. Blue light activates a plasma-membrane H⁺-ATPase pump in guard cells, expelling protons and creating a negative membrane potential that drives K⁺ influx through channels. Rising K⁺ (+ malate²⁻/Cl⁻) lowers Ψ_w → water enters by osmosis → guard cells become turgid → pore opens. ABA reverses the process by triggering K⁺ efflux and Ca²⁺ signalling.

4.3.3 Mechanism of Stomatal Opening

Blue light is absorbed by phototropin (LOV-domain receptor) in guard cells, activating plasma-membrane H⁺-ATPase.
Proton efflux hyperpolarises the membrane → activates inward-rectifying K⁺ channels → K⁺ accumulates inside guard cells (up to 300 mM from ~50 mM).
Malate²⁻ (synthesised from phosphoenolpyruvate + CO₂ via PEPC) and Cl⁻ serve as counter-ions.
Osmotic potential drops → Ψ_w falls → water enters by osmosis → turgor rises → guard cells bow outward → pore opens.

Stomatal closure occurs in darkness, under water stress, or in response to abscisic acid (ABA): ABA activates outward K⁺ channels → K⁺ leaves → guard cells lose turgor → pore closes. CO₂ accumulation inside leaves (high C_i) also promotes closure.

4.3.4 Factors Affecting Transpiration

External factors that increase transpiration: high temperature, low humidity, high wind speed, intense light. Internal factors: leaf area, number of stomata per unit area, stomatal position (hypostomatous vs amphistomatous leaves), cuticle thickness. Antitranspirants are chemicals applied to reduce transpiration: phenylmercuric acetate (PMA) closes stomata chemically; ABA is a natural antitranspirant; reflectants such as kaolin powder reduce leaf temperature.

HP Angle: In high-altitude apple orchards (Kullu, Shimla), transpiration management is critical during hot summers. Kaolin-based particle films are used as antitranspirants and sun-screen agents. Tea plants in Kangra valley (low elevation, humid) show different stomatal density patterns compared to apple in upper belts — stomatal density and aperture are adaptive characters tested in HPRCA questions framed around HP crop physiology.

4.3.5 Significance of Transpiration

Creates the transpiration pull — the primary driving force for ascent of sap (cohesion-tension).
Cooling — evaporation removes latent heat, keeping leaf temperature below air temperature by 2–5 °C.
Facilitates mineral transport to shoots along with the upward water stream.
Maintains turgor in non-guard cells, supporting cell expansion and growth.

4.4 Mineral Nutrition

Plants require inorganic minerals for structural, enzymatic, and regulatory functions. The science of plant nutrition was built on hydroponics — growing plants in precisely defined mineral salt solutions (pioneered by Sachs 1860 and Knop 1861–65), which allowed systematic omission of individual elements to test essentiality.

Hydroponics (water culture) — Sachs 1860; Knop 1861–65 · Criteria of essentiality — Arnon & Stout 1939 · Nickel as essential element — Brown 1987 · Soil chemistry & ion exchange — Way 1850

4.4.1 Criteria for Essentiality (Arnon & Stout, 1939)

An element is considered essential if:

Its absence prevents the plant from completing its life cycle (reproduction).
Its deficiency cannot be substituted by any other element.
The element must be directly involved in plant metabolism (not merely counteracting a toxic element or improving another element's uptake).

Essential Element

An inorganic mineral element that satisfies all three of the Arnon-Stout criteria: absolute requirement, non-substitutability, and direct metabolic role. Currently 17 elements are recognised as essential: 9 macronutrients + 8 micronutrients (including nickel, added by Brown 1987).

4.4.2 Classification of Essential Elements

Macronutrients (required in large quantities, >10 mM in tissue): C, H, O (from CO₂ and water — not minerals), N, P, K, Ca, Mg, S (9 total including C,H,O). If mineral macronutrients only: N, P, K, Ca, Mg, S (6 elements).

Micronutrients / trace elements (required in small quantities, <0.1 mM): Fe, Mn, Cu, Zn, B, Mo, Cl, Ni (8 elements).

Mnemonic — Essential Elements

Macro (mineral, 6): "Nitrogen Pumps Potassium; Calcium Must Stay" → N, P, K, Ca, Mg, S

Micro (8): "Fe Mn B Zn Cu Mo Cl Ni" — "Friendly Monkeys Bring Zinc, Copper, Molybdenum, Chlorine, Nickel"

Total = 17 = 9 macro + 8 micro (counting C, H, O in macros)

4.4.3 Functions and Deficiency Symptoms

**Table 4.2** Essential mineral elements — functions and deficiency symptoms
Element	Form absorbed	Key functions	Deficiency symptom
N	NO₃⁻, NH₄⁺	Amino acids, proteins, nucleic acids, chlorophyll, ATP	Chlorosis (older leaves first — mobile element); stunted growth; yellowing from base upward
P	H₂PO₄⁻, HPO₄²⁻	ATP, ADP, NADP, DNA, RNA, phospholipids	Purple/reddish colouration (anthocyanin accumulation); poor root growth; delayed maturity
K	K⁺	Guard cell turgor, enzyme activation (70+ enzymes), protein synthesis, phloem loading	Marginal leaf scorch (tip & margin necrosis, older leaves first — mobile); weak stems
Ca	Ca²⁺	Cell wall (middle lamella, Ca-pectate), membrane permeability, cell division, second messenger	Tip burn; growing points die (immobile — young leaves first); blossom-end rot in tomato; bitter pit in apple
Mg	Mg²⁺	Central atom of chlorophyll; enzyme cofactor (Rubisco, kinases)	Interveinal chlorosis (older leaves first — mobile); greenness lost between veins while veins remain green
S	SO₄²⁻	Cysteine, methionine (disulfide bonds in proteins); coenzyme A; ferredoxin	Uniform chlorosis of young leaves (immobile); stunted; reduced seed protein
Fe	Fe²⁺, Fe³⁺	Cytochromes, ferredoxin, Fe-S proteins; chlorophyll synthesis (not in ring)	Interveinal chlorosis of young leaves (immobile); lime-induced chlorosis in calcareous HP soils
Mn	Mn²⁺	Water-splitting complex of PSII (Mn₄CaO₅ cluster); enzyme cofactor	Interveinal chlorosis (young leaves); necrotic spots; reduced O₂ evolution
Zn	Zn²⁺	Auxin synthesis (tryptophan synthesis); carbonic anhydrase; zinc-finger proteins	Little leaf / rosette disease (short internodes, small bunched leaves); reduced auxin → dwarfism
Cu	Cu²⁺, Cu⁺	Plastocyanin (ETC), polyphenol oxidase, laccase	Dieback of shoot tips; bluish-green leaves; wilting
B	B(OH)₃, B(OH)₄⁻	Cell wall synthesis (cross-links pectin); pollen germination; sugar transport	Death of growing points; hollow stem (brown heart in turnip); poor fruit set; heart rot of beet
Mo	MoO₄²⁻	Nitrate reductase (NR) cofactor; nitrogenase in N-fixation; cofactor of XDH	Whiptail of cauliflower (lamina fails to develop); marginal scorch in tomato
Cl	Cl⁻	Photolysis of water (PSII O₂ evolution); stomatal function; osmotic balance	Wilting; chlorosis; necrosis; reduced root growth
Ni	Ni²⁺	Urease (nitrogen recycling from urea); essential for legume N-fixation fully to complete	Urea toxicity (accumulation); leaf tip necrosis; impaired seed germination

HP Angle: Apple orchards in HP commonly show calcium deficiency manifesting as bitter pit (subcortical brown pitting of fruit flesh) in high-yielding seasons when Ca uptake fails to match crop demand — a frequently tested HP-specific agronomic problem. Zinc deficiency (little leaf/khaira-like rosette) is common in alkaline and calcareous soils of Spiti and Kinnaur. Iron chlorosis occurs in high-pH, calcareous apple soils. These are HP-spec. items for teacher examinations.

4.4.4 Criteria for Mobility

Mobile elements (N, P, K, Mg, S) show deficiency symptoms first on older (lower) leaves — the plant remobilises the element to younger growing tissues. Immobile elements (Ca, B, Fe, Mn, Cu, Zn, Mo, Cl) show symptoms first on young (apical) leaves/shoot tips because they cannot be retranslocated from older tissues.

4.5 Nitrogen Metabolism

Nitrogen (N) is a constituent of amino acids, proteins, nucleic acids, chlorophyll, vitamins and hormones. Although 78 % of the atmosphere is N₂, plants cannot use it directly — they depend on inorganic nitrogen (NO₃⁻ or NH₄⁺) absorbed from soil. Biological nitrogen fixation (BNF) and industrial fixation (Haber-Bosch process) replenish soil N; denitrification returns it to the atmosphere, completing the nitrogen cycle.

4.5.1 Biological Nitrogen Fixation (BNF)

The enzyme nitrogenase (a complex of two proteins: dinitrogenase reductase [Fe protein] and dinitrogenase [Mo-Fe protein]) catalyses: N₂ + 8H⁺ + 8e⁻ + 16 ATP → 2 NH₃ + H₂ + 16 ADP + 16 Pi. Nitrogenase is irreversibly inhibited by O₂, so all N-fixing organisms protect it from oxygen by one means or another.

BNF organisms fall into two broad categories:

Symbiotic nitrogen fixers:

Rhizobium / Bradyrhizobium — Gram-negative bacteria forming root nodules in legumes (Fabaceae). Nodules provide O₂-buffering via leghaemoglobin (pink pigment, heme from plant + globin from bacterium). HP legumes: peas, beans, lentils, soybean in terai belt.
Frankia (actinomycete) — forms actinorhizal nodules in non-legume woody plants. Hippophae rhamnoides (sea buckthorn) in cold-desert HP (Lahaul-Spiti, Kinnaur) fixes N through Frankia symbiosis — a critically tested HP-specific example. Also in Casuarina, Alnus, Myrica.
Cyanobacteria — heterocyst-bearing taxa fix N. Anabaena in Azolla leaf cavities (paddy biofertiliser); Nostoc in Cycas root coralloid roots; Anabaena/Nostoc in lichens.

Non-symbiotic (free-living) nitrogen fixers:

Aerobic: Azotobacter (soil); Beijerinckia
Anaerobic: Clostridium pasteurianum
Facultative: Klebsiella pneumoniae
Photosynthetic: Rhodospirillum (purple non-sulphur), Chromatium
Free-living cyanobacteria: Nostoc, Anabaena, Calothrix

HP Angle: Hippophae rhamnoides (sea buckthorn — chharmang in local HP dialect) is the most extensively tested HP-specific example of non-legume biological nitrogen fixation. Its Frankia-mediated actinorhizal nodules enable it to colonise nutrient-poor, eroded, cold-desert soils of Kinnaur, Lahaul-Spiti. The plant is also valued for its berries (rich in vitamin C and omega-7 fatty acids) and as a soil-conservation species. Expect questions like: "Which N-fixing organism is associated with sea buckthorn?" Answer: Frankia (not Rhizobium).

4.5.2 Nitrate Reduction

Most plants absorb nitrogen as NO₃⁻, which must be reduced to NH₄⁺ before assimilation. This occurs in two steps:

NO₃⁻ → NO₂⁻: catalysed by nitrate reductase (NR) — a molybdenum-containing enzyme located in the cytoplasm; uses NADH or NADPH.
NO₂⁻ → NH₄⁺: catalysed by nitrite reductase (NiR) — located in the chloroplast (and root plastids); uses reduced ferredoxin as electron donor; 6 electrons involved.

4.5.3 Ammonium Assimilation

NH₄⁺ is toxic in high concentrations and is rapidly assimilated into amino acids via two main routes:

GS-GOGAT pathway (primary route in most plants): Glutamine synthetase (GS) incorporates NH₄⁺ into glutamate → glutamine (ATP required). Glutamate synthase (GOGAT/glutamine oxoglutarate aminotransferase) transfers the amide group to α-ketoglutarate, producing 2 glutamate molecules (uses Fd or NADH).
Reductive amination by GDH: Glutamate dehydrogenase (GDH) catalyses: α-ketoglutarate + NH₄⁺ + NADH → glutamate + NAD⁺. Operates mainly under high-NH₄⁺ conditions.

Glutamate and glutamine are the primary nitrogen donors for all other amino acids (via transamination) and for nucleotide, chlorophyll, and hormone biosynthesis.

**Table 4.3** Important nitrogen-fixing organisms — type and host
Organism	Type	Host / Habitat	HP relevance
Rhizobium	Symbiotic	Legume root nodules	Peas, beans, soybean cultivation
Frankia	Symbiotic (actinorhizal)	Hippophae, Casuarina, Alnus	Sea buckthorn in Lahaul-Spiti — HP-spec.
Anabaena azollae	Symbiotic (cyanobacterium)	Leaf cavities of Azolla	Paddy biofertiliser in lower HP
Nostoc	Symbiotic (cyanobacterium)	Coralloid roots of Cycas; lichens	Forest floor, temperate HP
Azotobacter	Free-living, aerobic	Agricultural soils	Used as biofertiliser
Clostridium	Free-living, anaerobic	Waterlogged soils	Rice paddy soils

4.6 Photosynthesis — Light Reactions

Photosynthesis is the process by which green plants (and some prokaryotes) convert light energy into chemical energy (ATP + NADPH) and use it to fix CO₂ into organic molecules. The overall equation for oxygenic photosynthesis: 6CO₂ + 12H₂O + light energy → C₆H₁₂O₆ + 6O₂ + 6H₂O. Van Niel's contribution (1931) established that oxygen in photosynthesis comes from water, not CO₂.

Priestley 1772 — O₂ produced by plants · Ingenhousz 1779 — light required; only green parts photosynthesize · Senebier 1782 — CO₂ consumed · de Saussure 1804 — H₂O absorbed; quantitative equation · Sachs 1864 — starch as product; chloroplast site · Van Niel 1931 — O₂ from H₂O · Hill 1939 — isolated chloroplasts evolve O₂ (Hill reaction) · Calvin & Benson 1950s — C3 cycle (Nobel 1961) · Hatch & Slack 1966 — C4 pathway · Mitchell 1961 — chemiosmosis (Nobel 1978)

4.6.1 Photosynthetic Pigments

Photosynthetic pigments are located in the thylakoid membranes of chloroplasts, organised into antenna complexes and reaction centres.

**Table 4.4** Photosynthetic pigments — absorption peaks and roles
Pigment	Colour in plant	Absorption peaks (nm)	Role
Chlorophyll a (chl a)	Blue-green	430 nm (violet-blue), 662 nm (red)	Primary pigment; P680 (PSII reaction centre) & P700 (PSI reaction centre)
Chlorophyll b (chl b)	Yellow-green	453 nm, 642 nm	Accessory; transfers energy to chl a; widens absorption range
β-Carotene	Orange	450–480 nm	Accessory; photoprotection; precursor of xanthophylls & abscisic acid
Xanthophylls (e.g. lutein, zeaxanthin)	Yellow	450–490 nm	Accessory; non-photochemical quenching (photoprotection)
Phycoerythrin (red algae, cyanobacteria)	Red/pink	500–565 nm	Allows photosynthesis in dim/deep-water conditions

Easy to confuse — Chl a vs Chl b vs Carotenoids vs Xanthophylls

Chlorophyll a

The only primary photosynthetic pigment. Forms the reaction centres P680 (PSII) and P700 (PSI). Has a magnesium atom at the centre of the porphyrin ring. Peaks at 430 nm and 662 nm. Found in all oxygenic photosynthetic organisms.

Chlorophyll b

Accessory pigment — transfers energy to chl a. Differs from chl a by having a CHO group instead of CH₃ at C-7. Peaks at 453 and 642 nm. Absent in prokaryotic cyanobacteria (they have chl a + phycocyanin only). Ratio chl a:b ≈ 3:1 in higher plants.

Carotenoids (β-carotene)

Orange, 40-C isoprenoid. Absorbs blue-violet light (450–480 nm). Transfers energy to chl a. Plays a major role in photoprotection — quenches singlet oxygen and triplet chl. Precursor of vitamin A (provitamin A).

Xanthophylls (oxygenated carotenoids)

Yellow. Examples: lutein, zeaxanthin, violaxanthin. Zeaxanthin is part of the xanthophyll cycle (de-epoxidation under excess light) → non-photochemical quenching (NPQ). Zeaxanthin also has a signalling role in guard cells / blue-light response.

4.6.2 Photosystems I and II

In higher plants, the two photosystems are located in the thylakoid membrane:

Photosystem II (PSII) — reaction centre contains P680 (chlorophyll a absorbing at 680 nm). Associated with water-splitting (oxygen-evolving complex, OEC): 2H₂O → O₂ + 4H⁺ + 4e⁻. The Mn₄CaO₅ cluster catalyses this reaction. Excited P680 donates electrons to pheophytin → plastoquinone (PQ).
Photosystem I (PSI) — reaction centre contains P700 (chl a absorbing at 700 nm). Excited P700 reduces ferredoxin (Fd) → NADP⁺ reductase (FNR) → NADPH.

4.6.3 The Z-Scheme (Non-cyclic Photophosphorylation)

The Z-scheme describes the path of electrons from water to NADPH in non-cyclic photophosphorylation, passing through both photosystems and the electron transport chain:

Fig. 4.2 The Z-scheme of non-cyclic photophosphorylation. Electrons released from water splitting at PSII (P680) flow through pheophytin → plastoquinone (PQ) → cytochrome b6f complex (generates H⁺ gradient → ATP) → plastocyanin (PC) → PSI (P700*) → ferredoxin (Fd) → FNR → NADPH. Cyclic photophosphorylation (dashed) re-routes electrons from Fd back through cyt b6f, generating only ATP (no NADPH, no O₂).

4.6.4 Photophosphorylation: Cyclic vs Non-cyclic

Easy to confuse — Cyclic vs Non-cyclic Photophosphorylation

Non-cyclic (Z-scheme)

Both PSI and PSII involved. Produces ATP + NADPH + O₂. Electrons do not return to P680 — replaced by electrons from water splitting. This is the main pathway in plants. Stoichiometry: 2H₂O → O₂ + 4H⁺ + 4e⁻; 2NADP⁺ → 2NADPH; ~2–3 ATP synthesised per 2e⁻.

Cyclic

PSI only. Produces ATP only — no NADPH, no O₂. Electrons from excited P700 pass to Fd → cyt b6f → PC → back to P700. Generates extra ATP when the cell needs ATP more than NADPH. Also operates in cells with high NADPH:NADP⁺ ratio (stroma). Bundle-sheath cells of C4 plants use cyclic photophosphorylation.

4.6.5 Chemiosmotic ATP Synthesis

Peter Mitchell's chemiosmotic hypothesis (1961, Nobel 1978) explains ATP synthesis in both chloroplasts and mitochondria. In chloroplasts:

Electron flow through PQ and cyt b6f pumps H⁺ from stroma into the thylakoid lumen.
Water splitting at PSII releases H⁺ into the lumen.
A H⁺ gradient (proton-motive force, pmf) builds across the thylakoid membrane (lumen acidic, stroma basic).
H⁺ flows back through ATP synthase (CF₁F_o) from lumen to stroma, driving ADP + Pi → ATP.

This process is called photophosphorylation. Net products of the light reactions (for 2 NADPH synthesis, i.e., 4 electrons from 2 H₂O): 3 ATP + 2 NADPH + O₂ (the ratio 3ATP:2NADPH is approximate and varies with cyclic flow contribution).

Exam Tip: A very common HPRCA pattern question is: "Which process produces ATP in the absence of NADPH and O₂?" Answer: Cyclic photophosphorylation. Also: "The oxygen evolved in photosynthesis comes from ___" — answer is water (Van Niel, confirmed by Ruben & Kamen with ¹⁸O labelling, 1941). The Hill reaction (isolated chloroplasts evolve O₂ with an artificial electron acceptor, 1939) proved that O₂ release does not require CO₂ fixation.

4.7 Photosynthesis — Dark Reactions (Calvin / C3, C4 / Hatch-Slack, CAM)

The dark reactions (light-independent reactions) use the ATP and NADPH produced by the light reactions to fix CO₂ into stable organic compounds. The reactions occur in the stroma of the chloroplast.

4.7.1 The Calvin Cycle (C3 Pathway)

Discovered by Melvin Calvin and Andrew Benson using ¹⁴CO₂ and paper chromatography (Nobel Prize 1961). The first stable product of CO₂ fixation is a 3-carbon compound, 3-phosphoglycerate (3-PGA) — hence the name C3 cycle.

The cycle has three stages:

Carboxylation: CO₂ + RuBP (ribulose-1,5-bisphosphate, 5C) → 2 × 3-PGA (3C each). Catalysed by RuBisCO (ribulose bisphosphate carboxylase/oxygenase) — the most abundant enzyme on Earth.
Reduction: 3-PGA → glyceraldehyde-3-phosphate (G3P / PGAL). Requires ATP + NADPH (from light reactions). Per CO₂: 2 ATP + 2 NADPH.
Regeneration: G3P molecules are used to regenerate RuBP (5C). Requires ATP. The cycle fixes 3 CO₂ per turn to produce one net G3P (6 turns of the cycle to produce one glucose net equivalent).

Stoichiometry for 1 glucose: 18 ATP + 12 NADPH + 6CO₂ → 1 glucose (C₆H₁₂O₆).

Fig. 4.3 The Calvin (C3) cycle. Three turns are needed to produce one net G3P. Six turns produce one hexose equivalent. Stoichiometry: 6CO₂ + 18 ATP + 12 NADPH → 1 glucose. RuBisCO catalyses both carboxylation (CO₂ fixation) and, under high O₂, oxygenation (photorespiration).

4.7.2 C4 Pathway (Hatch-Slack Pathway)

Discovered by Marshall Hatch and Roger Slack (1966) in sugarcane. The first stable product of CO₂ fixation in C4 plants is a 4-carbon compound, oxaloacetate (OAA). C4 plants are adapted to hot, high-light, arid environments and have a higher photosynthetic efficiency than C3 plants under these conditions due to their CO₂-concentration mechanism (CCM) that suppresses photorespiration.

Kranz anatomy: C4 plants have a characteristic leaf anatomy with two distinct cell types surrounding each vascular bundle: (1) mesophyll cells (MC) — peripheral, with numerous chloroplasts (granal), contain PEPC; (2) bundle-sheath cells (BSC) — inner ring, with large chloroplasts (often agranal/reduced grana in NADP-ME type), contain Rubisco and the Calvin cycle.

Fig. 4.4 C3 vs C4 leaf anatomy. In C4 plants (Kranz anatomy) mesophyll cells surround a ring of enlarged bundle-sheath cells (BSC). CO₂ is first fixed by PEPC in MC into OAA (C4) → malate/aspartate → transported to BSC → decarboxylated → CO₂ released at high concentration → enters Calvin cycle via Rubisco in BSC. This CO₂-concentration mechanism suppresses photorespiration.

Steps of the C4 (Hatch-Slack) pathway:

In mesophyll cells: CO₂ + PEP (phosphoenolpyruvate, 3C) → OAA (oxaloacetate, 4C), catalysed by PEPC (phosphoenolpyruvate carboxylase). PEPC has a high affinity for CO₂ and does not react with O₂.
OAA is converted to malate (NADP-malate dehydrogenase) or aspartate (transamination) depending on the C4 subtype.
Malate/aspartate is transported to bundle-sheath cells via plasmodesmata.
In BSC: malate is decarboxylated → pyruvate (3C) + CO₂. CO₂ concentration rises locally → enters Calvin cycle via Rubisco.
Pyruvate returns to mesophyll cells → regenerated to PEP (pyruvate phosphate dikinase, PPDK), using ATP (→ AMP + PPi, equivalent to 2 ATP).

Energy cost of C4 vs C3: C4 requires an additional 2 ATP per CO₂ fixed (to regenerate PEP in MC) compared to C3. But under hot/high-light conditions, the suppression of photorespiration (which is an energy waste) makes C4 more efficient overall.

Examples of C4 plants: Maize (Zea mays), sugarcane (Saccharum officinarum), sorghum (Sorghum bicolor), bajra (Pennisetum glaucum), Amaranthus, Atriplex.

Examples of C3 plants: Wheat, rice, potato, soybean, sunflower, most HP temperate crops.

Easy to confuse — C3 vs C4 vs CAM

C3 Plants

First stable product = 3-PGA (3C). CO₂ fixed by Rubisco directly. No Kranz anatomy. Photorespiration occurs under high temperature/light. Examples: wheat, rice, potato, most temperate crops. HP-relevant: apple, tea, wheat.

C4 Plants

First stable product = OAA (4C), then malate/aspartate. Kranz anatomy. PEPC in MC, Rubisco in BSC. No photorespiration (CCM). 5 ATP per CO₂ (vs 3 in C3). Examples: maize, sugarcane, sorghum, bajra, Amaranthus.

CAM Plants

Temporal separation: stomata open at night, CO₂ fixed by PEPC → malate stored in vacuole. During day: stomata closed, malate decarboxylated → CO₂ → Calvin cycle. Minimises water loss. Examples: Cactus, Agave, Aloe, Kalanchoe, pineapple. CAM = Crassulacean Acid Metabolism.

Photorespiration

Rubisco oxygenase activity: RuBP + O₂ → 3-PGA + 2-phosphoglycolate (2C). 2-phosphoglycolate enters the C2 (glycolate) oxidation cycle (chloroplast → peroxisome → mitochondria). Releases CO₂ and NH₃; wastes ATP and NADPH. Significant in C3 under hot, dry, high-O₂ conditions. Absent in C4 and CAM (CO₂-concentrating mechanisms).

**Table 4.5** Comparative summary — C3 vs C4 vs CAM photosynthesis
Feature	C3	C4	CAM
First stable product	3-PGA (3C)	OAA (4C)	OAA (4C) — at night
CO₂-fixing enzyme (1st step)	RuBisCO	PEPC (MC); Rubisco (BSC)	PEPC (night); Rubisco (day)
Leaf anatomy	No Kranz	Kranz anatomy (MC + BSC)	Usually succulent; no Kranz
Stomata open	Day	Day	Night (CO₂ storage); closed by day
Photorespiration	Present (wasteful)	Absent (CCM suppresses)	Absent (temporal CCM)
Water use efficiency	Low	Moderate	Very high
ATP needed per CO₂	3 ATP	5 ATP	5.5–6.5 ATP
Optimum temperature	15–25 °C	30–40 °C	Variable
Examples	Wheat, rice, potato, most HP crops	Maize, sugarcane, sorghum, Amaranthus	Cactus, Agave, pineapple, Kalanchoe

Worked example — CO₂-concentration mechanism in C4

"A plant growing in a semi-arid region has leaves with enlarged chlorenchymatous cells surrounding each vascular bundle, distinct from the smaller peripheral cells. Identify the photosynthetic pathway and explain how it prevents photorespiration."

Analysis: The description ("enlarged cells surrounding vascular bundle, distinct from peripheral cells") = Kranz anatomy → C4 plant. Prevention of photorespiration: PEPC in mesophyll cells fixes CO₂ (even at low concentrations) into OAA. After transport to bundle-sheath cells and decarboxylation, CO₂ concentration around Rubisco in BSC is 10–120× higher than in ambient air. At such high [CO₂], the oxygenase reaction of Rubisco is outcompeted → photorespiration is suppressed. Examples: maize, sorghum, sugarcane.

4.8 Respiration

Respiration is the process by which organic molecules (primarily glucose) are oxidised to release energy in the form of ATP, which drives all energy-requiring processes in the cell. Unlike photosynthesis, respiration occurs in all living cells at all times. Aerobic respiration occurs in three main stages: glycolysis (cytoplasm), Krebs cycle (mitochondrial matrix), and oxidative phosphorylation via the electron transport chain (inner mitochondrial membrane).

4.8.1 Glycolysis (Embden-Meyerhof-Parnas Pathway)

Glycolysis (from Greek: glykys = sweet; lysis = breakdown) occurs in the cytoplasm and does not require oxygen. Glucose (6C) is converted to 2 pyruvate (3C) in 10 enzymatic steps.

Net products per glucose: 2 pyruvate + 2 ATP (net; 4 produced − 2 consumed) + 2 NADH + 2 H₂O.

Key enzymes: hexokinase, phosphofructokinase-1 (PFK-1, rate-limiting, regulated by ATP/AMP and citrate), pyruvate kinase.

Anaerobic fate of pyruvate:

Alcoholic fermentation (yeast, plant roots in waterlogging): pyruvate → acetaldehyde (pyruvate decarboxylase) → ethanol + CO₂ (alcohol dehydrogenase). Net: 2 ATP per glucose.
Lactic acid fermentation (muscle, bacteria): pyruvate → lactate (lactate dehydrogenase). Net: 2 ATP per glucose.

4.8.2 Krebs Cycle (Tricarboxylic Acid / Citric Acid Cycle)

Discovered by Hans Krebs (1937, Nobel 1953). Occurs in the mitochondrial matrix. Pyruvate is first converted to acetyl-CoA (2C) by the pyruvate dehydrogenase complex (PDC) with release of CO₂ and NADH (oxidative decarboxylation).

Acetyl-CoA (2C) + oxaloacetate (4C) → citrate (6C) → …→ oxaloacetate (cycle).

Per acetyl-CoA (per half of glucose): 3 NADH + 1 FADH₂ + 1 ATP (as GTP) + 2 CO₂ released.

Per glucose (2 turns): 6 NADH + 2 FADH₂ + 2 ATP/GTP + 4 CO₂.

Key intermediates: citrate, isocitrate, α-ketoglutarate, succinyl-CoA, succinate, fumarate, malate, oxaloacetate. Key enzymes: citrate synthase, isocitrate dehydrogenase, α-ketoglutarate dehydrogenase, succinate dehydrogenase (only membrane-bound Krebs enzyme; complex II of ETC).

4.8.3 Oxidative Phosphorylation (ETC + Chemiosmosis)

Occurs on the inner mitochondrial membrane. NADH and FADH₂ from glycolysis and Krebs cycle donate electrons to the electron transport chain (ETC): Complex I (NADH dehydrogenase) → ubiquinone (CoQ) → Complex III (cyt bc1) → cytochrome c → Complex IV (cyt c oxidase, reduces O₂ to H₂O). FADH₂ enters at Complex II. Electron flow pumps H⁺ from matrix to intermembrane space → H⁺ gradient → ATP synthase (Complex V) → ATP.

ATP yield (theoretical): NADH → ~2.5 ATP; FADH₂ → ~1.5 ATP. Total per glucose: ~30–32 ATP (revised estimate using P/O ratios; older textbook value was 36–38 ATP).

Easy to confuse — Aerobic vs Anaerobic Respiration; ATP yields

Aerobic Respiration

Requires O₂. Complete oxidation of glucose. Produces ~30–32 ATP per glucose. Occurs in cytoplasm (glycolysis) + mitochondria (Krebs + ETC). Final e⁻ acceptor = O₂ → H₂O.

Anaerobic Respiration

No O₂ required. Partial oxidation of glucose. Only 2 ATP per glucose (glycolysis only). Products: ethanol + CO₂ (fermentation in yeast/plants) or lactate (in muscle). Regenerates NAD⁺ for glycolysis to continue.

4.8.4 Respiratory Quotient (RQ)

RQ = volume of CO₂ evolved / volume of O₂ consumed.

Carbohydrates: RQ = 1.0 (equal volumes of CO₂ and O₂)
Fats/oils: RQ < 1 (more O₂ needed; e.g. tripalmitin RQ ≈ 0.7)
Organic acids (e.g. oxalic acid): RQ > 1
Proteins: RQ ≈ 0.8
Anaerobic respiration / fermentation: RQ = ∞ (CO₂ without O₂)

Exam Tip: HPRCA questions frequently ask "Which substrate gives an RQ < 1?" (Answer: fats/oils) or "What does an RQ > 1 indicate?" (Answer: organic acids or anaerobic respiration). The Krebs cycle was discovered by Hans Krebs in 1937 (Nobel 1953) — pair the name with the year.

4.9 Plant Growth Regulators

Plant growth regulators (PGRs) — formerly called plant hormones or phytohormones — are organic compounds produced in one part of the plant that, in small concentrations, regulate growth and developmental processes in another part. They are distinct from nutrients (which are required in relatively large amounts as structural or metabolic components). The concept of chemical signals controlling plant growth was first clearly demonstrated by the auxin discovery.

Auxin (IAA) — Went 1928 (oat coleoptile agar-block bioassay) · Gibberellin — Kurosawa 1926 (bakanae/foolish seedling of rice; Gibberella fujikuroi); isolated — Yabuta 1935 · Cytokinin — Miller & Skoog 1955 (kinetin from autoclaved herring sperm DNA) · ABA — Addicott 1963 (abscisic acid / dormin); Eagles & Wareing independently · Ethylene — Neljubov 1901 (triple response in dark-grown seedlings); Crocker et al. 1935 (hormone in fruit ripening) · Brassinosteroids — Grove et al. 1979 (from rapeseed pollen)

4.9.1 Auxin

The major naturally occurring auxin is indole-3-acetic acid (IAA), synthesised primarily in shoot apices, young leaves, and developing seeds from tryptophan. Auxin moves predominantly by polar auxin transport (PAT) — basipetally (tip to base) in coleoptiles and shoots, driven by PIN proteins (efflux carriers) that are asymmetrically localised.

Key actions of auxin:

Cell elongation — acidification of cell wall ("acid growth hypothesis": H⁺-ATPase activated → cell wall loosened by expansins). Required for tropic responses.
Phototropism — unequal auxin distribution (more auxin on shaded side) → greater elongation on shaded side → bending toward light.
Geotropism — gravity causes auxin redistribution to lower side → roots (sensitive) inhibited; shoots (less sensitive) stimulated.
Apical dominance — high auxin from the apex suppresses outgrowth of lateral buds. Decapitation → lateral buds grow (cytokinin from root tip promotes lateral bud growth; auxin:cytokinin ratio controls this).
Root initiation — NAA and IBA (synthetic auxins) widely used for rooting cuttings in horticulture.
Parthenocarpy — auxin sprays induce seedless fruit development.
Weed control — 2,4-D (2,4-dichlorophenoxyacetic acid) is a synthetic auxin herbicide selectively lethal to broad-leaved weeds; used in cereal (wheat, maize) fields.
Fruit drop prevention — spraying IAA/NAA delays abscission; used commercially in apple orchards.

4.9.2 Gibberellins (GA)

Gibberellins are a family of diterpenoid acids (>120 known GAs). GA₃ (gibberellic acid) is the most commercially important. Discovered when Kurosawa (1926) found that the pathogenic fungus Gibberella fujikuroi caused bakanae (foolish seedling) disease of rice — infected plants grew abnormally tall and spindly before toppling over.

Key actions of gibberellins:

Stem elongation — promotes cell division and elongation in internodes; corrects genetic dwarfism in some plants (e.g., dwarf pea and maize). Used to study genetic pathways of height.
Bolting — induces flowering in rosette long-day plants by causing internode elongation before flowering.
Breaking dormancy — GA breaks dormancy in seeds that require cold stratification or light for germination; promotes α-amylase synthesis in germinating cereal grains (aleurone layer).
Parthenocarpy — GA sprays produce seedless grapes (elongated rachis, larger berries).
Malting/Brewing — GA₃ used industrially to accelerate barley malting.

HP Angle: Apple orchards in HP use GA sprays to improve fruit size and russeting characteristics (e.g., in certain 'Royal Delicious' strains). GA₃ application to apple before full bloom can cause fruit thinning and improved size of remaining fruits. Wheat varieties grown in HP hills (e.g., Sonalika, HS-507) are semi-dwarf, with reduced GA sensitivity due to Rht genes — a connection between GA biology and Green Revolution genetics that HPRCA examiners have tested.

4.9.3 Cytokinins

Natural cytokinins (e.g., zeatin, isolated from maize endosperm 1963) are adenine derivatives that promote cell division (cytokinesis). Kinetin (6-furfurylaminopurine) was the first cytokinin isolated — by Miller and Skoog (1955) from autoclaved herring sperm DNA; it is not naturally produced by plants but is commonly used in tissue culture.

Key actions of cytokinins:

Cell division — act synergistically with auxin in tissue culture (Skoog-Miller ratio: high cytokinin:auxin → shoot differentiation; low ratio → root differentiation).
Lateral bud growth — promote outgrowth of lateral buds (counter-auxin in apical dominance).
Delay of senescence (Richmond-Lang effect) — cytokinin treatment keeps leaves green by retarding protein breakdown and chlorophyll degradation; they promote nutrient "mobilisation" (sink effect).
Chloroplast development — stimulate chloroplast biogenesis and greening.

4.9.4 Abscisic Acid (ABA)

ABA is a sesquiterpene derived from carotenoid (xanthoxin) cleavage. It is the primary stress hormone of plants, mediating responses to drought, cold, and salinity.

Key actions of ABA:

Stomatal closure — most important anti-stress response; ABA triggers Ca²⁺ signalling in guard cells → K⁺ efflux → guard cells lose turgor → stomata close → water loss reduced.
Seed dormancy — high ABA:GA ratio maintains dormancy; declining ABA / rising GA = germination trigger.
Bud dormancy — ABA promotes dormancy formation in buds; important in perennial plants before winter.
Leaf/fruit abscission — ABA promotes formation of the abscission zone (along with ethylene).
Inhibition of growth — generally antagonises the growth-promoting effects of auxin, GA, and cytokinin; hence called the "inhibitor hormone" or "anti-GA."

4.9.5 Ethylene

Ethylene (C₂H₄) is the only gaseous plant hormone. It is produced from methionine via ACC (1-aminocyclopropane-1-carboxylic acid) through the Yang cycle. ACC synthase (pyridoxal phosphate-dependent) and ACC oxidase are the key enzymes.

Key actions of ethylene:

Fruit ripening — promotes ripening of climacteric fruits (apple, banana, tomato, mango) by stimulating cell wall degradation (polygalacturonase), sugar accumulation, colour changes. Non-climacteric fruits (citrus, grapes, strawberry) respond less. Ethylene production triggers a climacteric rise in respiration during ripening.
Triple response in seedlings (dark-grown): horizontal growth + radial swelling + exaggerated hook. Diagnostic bioassay response (Neljubov 1901).
Abscission — promotes leaf, flower and fruit drop by stimulating abscission zone cellulase activity.
Senescence — promotes leaf and flower senescence.
Epinasty — downward bending of leaves.
Sex expression — promotes female flower development in cucurbits.
Practical use: Ethephon (2-chloroethylphosphonic acid) releases ethylene slowly; used to ripen tomatoes, bananas, and to regulate flowering in pineapple. In HP apple orchards, Ethephon is used for fruit colour improvement and thinning.

4.9.6 Brassinosteroids

First isolated from rape (Brassica napus) pollen by Grove et al. (1979). Brassinolide is the most active form. They are polyhydroxylated steroidal compounds and are now recognised as the 6th class of plant hormone. Functions: promote cell elongation and division, xylem differentiation, photosynthesis enhancement, stress tolerance. Deficiency: dark-green dwarfism, male sterility.

**Table 4.6** Plant growth regulators — discoverer, year, and major functions
PGR	Discovery	Year	Key functions	Inhibits
Auxin (IAA)	F. W. Went (oat bioassay)	1928	Cell elongation, phototropism, apical dominance, rooting, weed control (2,4-D)	Lateral bud growth (high conc.)
Gibberellin (GA₃)	Kurosawa (bakanae)	1926	Stem elongation, bolting, dormancy break, α-amylase induction, parthenocarpy	—
Cytokinin (Kinetin)	Miller & Skoog	1955	Cell division, lateral bud growth, delay of senescence, chloroplast development	Apical dominance (counters auxin)
ABA	Addicott	1963	Stomatal closure, seed dormancy, stress response, abscission	Growth; germination
Ethylene	Neljubov (triple response)	1901	Fruit ripening, abscission, senescence, sex expression	Elongation growth
Brassinosteroids	Grove et al.	1979	Cell elongation/division, xylem differentiation, stress tolerance	—

Easy to confuse — Auxin vs Cytokinin (Apical Dominance)

Auxin (IAA)

Produced at shoot apex. Moves basipetally. At high concentrations in axillary buds, inhibits lateral bud growth → apical dominance. Promotes root elongation at low concentrations. 2,4-D is synthetic auxin herbicide. Promotes apical growth & fruit development.

Cytokinin

Produced mainly in roots. Moves upward. Promotes lateral bud growth — counters auxin in apical dominance. Delays senescence (Richmond-Lang effect). Skoog-Miller ratio: high cytokinin → shoots. Kinetin = first cytokinin (not natural to plants; from DNA hydrolysis).

Mnemonic — PGRs and Discoverers

"Going Away Can Eat Biscuits" = Gibberellin (Kurosawa 1926), Auxin (Went 1928), Cytokinin (Miller-Skoog 1955), Ethylene (Crocker 1935), Brassinosteroids (Grove 1979)

ABA = Abscisic acid = Anti-growth, Bad news for open stomata, Addicott 1963

4.10 Photoperiodism, Vernalisation & Dormancy

4.10.1 Photoperiodism

The response of plants to the relative length of day and night (photoperiod) is called photoperiodism. Discovered by W. W. Garner and H. A. Allard (1920) while studying a mutant tobacco (Maryland Mammoth) and soybean. They found that flowering was triggered by specific daylength, not temperature or nutrition.

Photoperiodism — Garner & Allard 1920 · Florigen hypothesis — Chailakhyan 1936 (transmissible flowering signal) · Phytochrome — Borthwick & Hendricks 1952 (discovered by red/far-red reversible responses) · FT protein as florigen — Corbesier et al., Tamaki et al. 2007 · Critical day length concept — Hamner & Bonner 1938 (night length is critical, not day length)

Classification of plants by photoperiodic response:

**Table 4.7** Photoperiodic groups — examples and HP relevance
Group	Requirement	Critical day length	Examples	HP relevance
Short Day Plants (SDP)	Day shorter than critical / night longer than critical	~12–14 h (vary by species)	Rice, soybean, tobacco (Maryland Mammoth), chrysanthemum, Xanthium, sugarcane, potato	Rice varieties in lower HP zones flower in late season (short day)
Long Day Plants (LDP)	Day longer than critical / night shorter than critical	~12–14 h (vary)	Wheat, barley, oat, radish, spinach, Hyoscyamus niger, Henbane	Wheat and barley in HP hills — LDP; critical for high-altitude cultivation timing
Day-Neutral Plants (DNP)	No photoperiod requirement	—	Tomato, cotton, sunflower, Cucurbita, maize, Pisum (some varieties)	Tomato cultivated in varied HP microclimates year-round
Intermediate Day Plants	Flower only within a narrow range of daylength	Intermediate (~12 h)	Mikania spp., some sugarcane varieties	Rare; mainly tropical

It is the length of the dark period (night) that is the actual critical stimulus — demonstrated by Hamner and Bonner (1938) using interrupted night experiments: a brief flash of light during a long dark period prevented flowering in SDP (like Xanthium). Red light (660 nm) was most effective in interrupting the dark period; far-red light (730 nm) reversed this effect — evidence for phytochrome.

4.10.2 Phytochrome and the Photo-Reversible Pigment System

Phytochrome is a photoreversible blue-green biliprotein pigment existing in two interconvertible forms:

Pr (phytochrome red; P660) — absorbs red light (660 nm) → converts to Pfr. Biologically inactive form. Accumulates in darkness (slow reversion Pfr → Pr in dark).
Pfr (phytochrome far-red; P730) — absorbs far-red light (730 nm) → converts back to Pr. Biologically active form. Promotes germination, de-etiolation, LDP flowering. In SDP, Pfr must be absent (converted to Pr) during the long dark period for flowering.

Formula: Pr ⇌ (Red light) ⇌ Pfr. Pfr is the "active form" — when present, it promotes LDP flowering; when absent (during long night), SDP flower.

Worked example — Predicting photoperiodic response

"A plant flowers only when the day length is 14 hours. When a 15-minute light break was given in the middle of a 16-hour dark period, the plant flowered. Classify the plant and explain the mechanism."

Analysis: Flowers when day = 14 h (i.e., its critical day length is 14 h or less). The fact that a night-break promoted flowering means the plant needs the dark period to be broken — it is a Long Day Plant (LDP). In LDP, Pfr accumulation (from the night-break light flash) promotes flowering. The brief light pulse converts Pr → Pfr, signalling "short night" conditions. Examples: wheat, Hyoscyamus, spinach.

4.10.3 Florigen

Chailakhyan (1936) proposed that a transmissible flowering hormone — which he named florigen — is produced in leaves under appropriate photoperiod and transported to the shoot apex where it triggers flowering. Grafting experiments across LDP and SDP provided strong indirect evidence. Molecular identification: FLOWERING LOCUS T (FT) protein (phloem-mobile protein, not a hormone in the classic sense) produced in leaves under inductive photoperiod and transported to the shoot apical meristem — confirmed in 2007 by multiple groups as the long-sought florigen.

4.10.4 Vernalisation

Vernalisation (from Latin vernus = of spring) is the process by which exposure to a prolonged period of cold temperature (usually 1–10 °C for several weeks) promotes or accelerates subsequent flowering. It was studied systematically by T. D. Lysenko (1928) (the Soviet agronomist who coined the term in 1928) and later by Pierre Chouard (France). Earlier work by Klippart (1857) showed that winter wheat germinated in autumn would flower in the following season if chilled.

Key characteristics:

The vernalisation stimulus is perceived by the shoot apical meristem (or embryo of seeds).
It can be transmitted to grafted scions (suggesting a transmissible signal, vernalin).
Devernalisation: heat treatment after vernalisation reverses the effect. Repeated cycles of vernalisation and devernalisation suggest an epigenetic mechanism.
Molecular basis: cold exposure activates FLC (FLOWERING LOCUS C) repressor silencing via Polycomb Group (PcG) chromatin marks → allows flowering genes (FT, SOC1) to be expressed.
Requires temperatures of 0–7 °C (or in some species up to 15 °C) for 4–8 weeks depending on species.

Crop applications in HP:

Winter wheat — requires vernalisation before it can flower; spring wheat does not. HP wheat varieties (Sonalika, HS-507) grown in higher altitudes behave as spring wheat (less vernalisation requirement due to vrn gene mutations).
Apple — requires a certain number of chilling hours (exposure to 0–7 °C) to break bud dormancy and flower normally. Most high-chill apple varieties in HP require 800–1200 chilling hours below 7 °C. Without sufficient chilling: erratic bud-break, delayed bloom, poor fruit set. This is a critically important HP-spec. physiological concept tested in HPRCA exams.
Biennial plants (carrot, cabbage, celery) flower in their second year after vernalisation during the cold winter months of the first year.

HP Angle — Apple Chilling Requirement: Due to climate change, the decrease in chilling hours in lower apple-growing zones of HP (below 1800 m) is causing increasing physiological disorders (delayed foliation, erratic bloom, reduced yields) in high-chill apple varieties like 'Royal Delicious'. This has prompted introduction of low-chill varieties. The standard chilling requirement is 800–1200 hours below 7 °C for most HP apple varieties — a number directly asked in HPRCA-pattern questions. Also note: Garner & Allard discovered photoperiodism in 1920; Lysenko coined "vernalisation" in 1928.

4.10.5 Seed Dormancy

Dormancy is a state in which a viable seed (or bud) fails to germinate (or grow) even when provided with favourable conditions of water, temperature, and oxygen. It is a survival strategy that prevents germination during transient favourable conditions that may not last long enough for seedling establishment.

Easy to confuse — Dormancy vs Quiescence; Primary vs Secondary Dormancy

Dormancy

Seed fails to germinate even in apparently favourable conditions (water, O₂, temperature present). Caused by internal blocks: hard seed coat, immature embryo, germination inhibitors (ABA), requirement for light or cold. Must be broken by specific treatments (scarification, stratification, light, GA).

Quiescence

Seed fails to germinate simply because one or more external conditions are unfavourable (too dry, too cold, no light). Germination occurs immediately once the missing factor is provided. No internal block. Reversible by supplying the limiting factor.

Types of dormancy:

Primary dormancy — innate; present before seed is shed from the parent plant; imposed by the seed itself during development.
Secondary dormancy — induced after shedding by unfavourable conditions (e.g., prolonged darkness, high temperature), even in a non-dormant seed.

Causes of dormancy:

Hard, impermeable seed coat (physical dormancy) — prevents water/O₂ entry. E.g., legumes, Nelumbo, Hippophae.
Immature embryo at seed dispersal — embryo must complete development in soil.
High ABA concentration — inhibits germination metabolically.
Germination inhibitors in seed coat/fruit (coumarins, phenols).
Light requirement (photoblastic seeds): positively photoblastic (lettuce, tobacco — need light to germinate via Pfr formation); negatively photoblastic (pumpkin, onion — inhibited by light).

Breaking dormancy (treatments):

Scarification — mechanical (filing, nicking) or chemical (acid, hot water) treatment to break hard seed coat.
Stratification — moist cold storage (0–5 °C, weeks to months). Mimics winter conditions. Breaks dormancy in apple, peach, plum, Aconitum seeds.
GA treatment — exogenous GA₃ can replace cold requirement (substitutes for vernalisation/stratification in some species).
Light (red light) — converts Pr → Pfr → breaks photodormancy in positively photoblastic seeds.
Nitrate (KNO₃) — breaks dormancy in some weed seeds.
Washing — removes inhibitors from seed coat.

4.11 Quick-Reference Tables

**Table 4.8** Calvin cycle stoichiometry — per glucose produced
Parameter	Per CO₂ fixed	Per glucose (6 CO₂)
RuBP consumed	1 (5C)	6
3-PGA produced	2 (3C each)	12
ATP (reduction)	2	12
NADPH (reduction)	2	12
ATP (regeneration)	1	6
Total ATP	3	18
Total NADPH	2	12
G3P produced (net)	1/3	2 (net; used for 1 glucose)

Exam Tip: "How many ATP and NADPH are needed to fix one CO₂ in the Calvin cycle?" — Answer: 3 ATP + 2 NADPH. For one glucose (6 CO₂): 18 ATP + 12 NADPH. The ratio 3:2 (ATP:NADPH) is the stoichiometric demand of the Calvin cycle alone; the light reactions must supply this ratio. Cyclic photophosphorylation supplements ATP when the ratio is insufficient.

Quick Recap

Ψ_w = Ψ_s + Ψ_p + Ψ_m; water moves from high to low Ψ_w. Pure water = 0; solutions always negative Ψ_s.
Plasmolysis (hypertonic) → incipient → full. Deplasmolysis on hypotonic transfer. Imbibition = colloidal absorption, no membrane needed.
Ascent of sap = Dixon-Joly cohesion-tension theory (1894). Apoplast blocked at Casparian strip; symplast continues. Phloem: Münch pressure-flow hypothesis.
Stomata open: blue light → H⁺-ATPase → K⁺ influx → Ψ_w falls → water enters → guard cells turgid. ABA causes K⁺ efflux → closure.
Transpiration: stomatal (>90 %) + cuticular + lenticular. Antitranspirants: PMA, ABA, kaolin.
17 essential elements: 9 macro (C,H,O,N,P,K,Ca,Mg,S) + 8 micro (Fe,Mn,B,Zn,Cu,Mo,Cl,Ni). Arnon-Stout criteria 1939.
Mobile elements (N,P,K,Mg,S) → deficiency in old leaves first. Immobile (Ca,Fe,Mn,B,Zn) → young leaves/tips first.
Nitrogen fixation: Rhizobium (legume nodules), Frankia (Hippophae, sea buckthorn — HP angle), Anabaena-Azolla, Azotobacter, Clostridium. Nitrogenase: O₂-sensitive. NO₃⁻ → NR → NO₂⁻ → NiR → NH₄⁺; assimilation via GS-GOGAT.
Photosynthesis light reactions: PSII (P680, water splitting) → PQ → cyt b6f → PC → PSI (P700) → Fd → FNR → NADPH. Non-cyclic: ATP + NADPH + O₂. Cyclic (PSI only): ATP only.
Calvin (C3) cycle: carboxylation (Rubisco: RuBP + CO₂ → 2×3-PGA) + reduction (G3P; 2ATP + 2NADPH per CO₂) + regeneration (1ATP per CO₂). Total per glucose: 18 ATP + 12 NADPH.
C4 (Hatch-Slack 1966): Kranz anatomy; PEPC in MC; first product OAA (4C); Rubisco in BSC; suppresses photorespiration. Examples: maize, sugarcane. CAM: temporal separation; night CO₂ fixation; e.g., cactus, pineapple, Agave.
PGRs: Auxin/IAA (Went 1928) — elongation, apical dominance, 2,4-D. GA (Kurosawa 1926) — elongation, bolting, dormancy break. Cytokinin (Miller-Skoog 1955) — cell division, lateral buds, delay senescence. ABA (Addicott 1963) — stomatal closure, dormancy. Ethylene — fruit ripening, abscission.
Photoperiodism: Garner-Allard 1920. SDP (rice, chrysanthemum) need short day; LDP (wheat, barley) need long day. Night length is critical. Phytochrome: Pr ↔ Pfr. Florigen = FT protein.
Vernalisation: cold treatment to induce flowering. Lysenko 1928. Apple chill requirement = 800–1200 h below 7 °C (HP-spec.). Dormancy broken by scarification, stratification, GA, light (Pfr).

Chapter 4 Cheatsheet

Water Relations

Ψ_w = Ψ_s + Ψ_p (+Ψ_m). Pure water = 0.
Osmosis: through membrane, high→low Ψ_w
Imbibition: colloids, no membrane, matric force
Ascent: Dixon-Joly cohesion-tension 1894
Casparian strip blocks apoplast; symplast continues
Root pressure → guttation from hydathodes
Phloem: Münch pressure-flow (source → sink)

Mineral Nutrition

17 essential: 9 macro + 8 micro (Arnon-Stout 1939)
Macro (mineral): N,P,K,Ca,Mg,S
Micro: Fe,Mn,B,Zn,Cu,Mo,Cl,Ni
Interveinal chlorosis: Mg (old leaves), Mn (young)
Little leaf / rosette: Zn (auxin ↓)
Tip burn: Ca (immobile, young leaves)
Whiptail: Mo (cauliflower)
Bitter pit in HP apple = Ca deficiency

Photosynthesis

Chl a peaks: 430 nm & 662 nm
Hill reaction 1939: isolated chloroplasts evolve O₂
Z-scheme: PSII(P680)→PQ→cytb6f→PC→PSI(P700)→Fd→NADPH
Cyclic: PSI only → ATP only, no O₂, no NADPH
Calvin: 18 ATP + 12 NADPH per glucose
C4 PEPC: high CO₂ affinity, no O₂ reaction
CAM: stomata open at night, CO₂ → vacuole (malate)

PGRs — Discovery Key Facts

Auxin (Went 1928): elongation, 2,4-D weed killer
GA (Kurosawa 1926 bakanae): elongation, bolting
Cytokinin (Miller-Skoog 1955): division, delay senescence
ABA (Addicott 1963): stress, stomata closure, dormancy
Ethylene (Neljubov 1901): fruit ripening, abscission
Brassinosteroids (Grove 1979): elongation, xylem diff.
High auxin:cytokinin → roots; high cytokinin:auxin → shoots

Photoperiodism & Vernalisation

Garner & Allard 1920 — photoperiodism
SDP: rice, chrysanthemum, Xanthium (long night)
LDP: wheat, barley, Hyoscyamus (long day)
Night length is CRITICAL (Hamner & Bonner 1938)
Pfr = active form → promotes LDP flowering
Florigen = FT protein (Chailakhyan 1936 concept)
Vernalisation: Lysenko 1928; cold → flowering
HP apple: 800–1200 chill hours below 7 °C needed

HP-Specific Items

Hippophae rhamnoides (sea buckthorn) — Frankia N-fixation — cold desert Spiti/Kinnaur
Apple bitter pit = Ca deficiency; chilling = 800–1200 h ≤ 7 °C
Kangra tea = C3 plant; shade & humidity adapted
Alpine photosynthesis: low pCO₂, high UV — C3 adaptation
Lahaul-Spiti: Zn & Fe chlorosis in alkaline soils
GA application in HP apple: improves fruit size (Royal Delicious)
Winter wheat HP varieties: semi-dwarf, reduced GA sensitivity

Practice Questions

The water potential of pure water at atmospheric pressure is: HPRCA-pat.

+1 MPa
−1 MPa
0 MPa
Depends on temperature

Answer: C — 0 MPa

Pure water at standard temperature and atmospheric pressure is set as the reference state with Ψ_w = 0. Any dissolved solute lowers Ψ_s (makes it negative), and therefore lowers Ψ_w below zero.

Casparian strip is found in: HPRCA-pat.

Cortex
Endodermis
Pericycle
Epidermis

Answer: B — Endodermis

The Casparian strip is a band of suberin impregnating the radial and transverse walls of endodermal cells, blocking the apoplastic pathway and forcing solutes to enter the symplast before reaching the stele.

The cohesion-tension theory for ascent of sap was proposed by:

Sachs and Knop
Dixon and Joly
Münch
Hales and White

Answer: B — Dixon and Joly

H. H. Dixon and J. Joly (1894) proposed the cohesion-tension (transpirational pull) theory. Münch (1930) proposed the pressure-flow hypothesis for phloem transport.

Which of the following is an essential micronutrient added to the list in 1987? HPRCA-pat.

Molybdenum
Chlorine
Nickel
Boron

Answer: C — Nickel

Nickel was the 17th essential element, added by Brown (1987). It is the metal cofactor of urease, necessary for urea metabolism and for full expression of nitrogen-fixing symbioses in legumes.

Interveinal chlorosis of older leaves is a characteristic symptom of deficiency of: HPRCA-pat.

Nitrogen
Calcium
Magnesium
Boron

Answer: C — Magnesium

Magnesium is the central atom of chlorophyll and is mobile; deficiency first appears in older leaves as interveinal chlorosis (yellowing between veins while veins remain green). Calcium deficiency affects young leaves (Ca is immobile).

The nitrogen-fixing symbiont of sea buckthorn (Hippophae rhamnoides) is: HP-spec.

Rhizobium
Azotobacter
Frankia
Anabaena

Answer: C — Frankia

Hippophae rhamnoides (sea buckthorn) forms actinorhizal nodules with the actinomycete Frankia. This is not a legume, so Rhizobium is incorrect. This is a key HP-specific example of non-legume biological nitrogen fixation in cold-desert soils of Lahaul-Spiti and Kinnaur.

The first stable product of CO₂ fixation in C3 plants is: HPRCA-pat.

Oxaloacetate
3-Phosphoglycerate
Glyceraldehyde-3-phosphate
Phosphoenolpyruvate

Answer: B — 3-Phosphoglycerate

In the Calvin cycle (C3 pathway), RuBisCO catalyses the carboxylation of RuBP + CO₂ to produce two molecules of 3-phosphoglycerate (3-PGA, a 3-carbon compound). Calvin and Benson identified 3-PGA as the first stable product using ¹⁴CO₂ labelling.

The Z-scheme of photosynthesis was elucidated to explain: HPRCA-pat.

Cyclic photophosphorylation only
Non-cyclic photophosphorylation
The dark reactions
Photorespiration

Answer: B — Non-cyclic photophosphorylation

The Z-scheme (Hill and Bendall, 1960) describes the electron flow from water (via PSII) through the electron transport chain to NADP⁺ reduction (via PSI), producing O₂, ATP, and NADPH — this is non-cyclic photophosphorylation. The "Z" shape comes from the energy diagram of the zigzag electron pathway.

Which enzyme catalyses the first CO₂ fixation step in C4 plants in mesophyll cells? HPRCA-pat.

RuBisCO
PEPCK
PEPC (PEP Carboxylase)
Pyruvate phosphate dikinase

Answer: C — PEPC (PEP Carboxylase)

In C4 plants, CO₂ is first fixed by phosphoenolpyruvate carboxylase (PEPC) in mesophyll cells. PEPC has a higher affinity for CO₂ than RuBisCO and does not react with O₂. RuBisCO is present only in bundle-sheath cells where the Calvin cycle operates.

Bakanae (foolish seedling) disease of rice is caused by a toxin produced by:

Rhizobium leguminosarum
Gibberella fujikuroi
Agrobacterium tumefaciens
Xanthomonas oryzae

Answer: B — Gibberella fujikuroi

Bakanae disease is caused by the fungus Gibberella fujikuroi (anamorph: Fusarium moniliforme), which secretes gibberellins. Kurosawa (1926) isolated the culture filtrate responsible for the excessive elongation, leading to the discovery of gibberellins.

Photoperiodism was first reported by: HPRCA-pat.

Chailakhyan and Lysenko
Garner and Allard
Borthwick and Hendricks
Hamner and Bonner

Answer: B — Garner and Allard

W. W. Garner and H. A. Allard (USDA, 1920) discovered photoperiodism while studying the 'Maryland Mammoth' tobacco variety, which only flowered in short days. They showed that flowering was controlled by the relative length of day and night (photoperiod).

The chilling requirement of most high-chill apple varieties grown in Himachal Pradesh is: HP-spec.

200–400 hours below 7 °C
400–600 hours below 7 °C
800–1200 hours below 7 °C
1500–2000 hours below 7 °C

Answer: C — 800–1200 hours below 7 °C

High-chill apple varieties commonly grown in HP (e.g., 'Royal Delicious', 'Red Chief') require 800–1200 chilling hours (exposure to temperatures of 0–7 °C) during winter dormancy to break bud dormancy and produce a regular bloom and fruit set the following season. Insufficient chilling leads to erratic bud-break and poor yield.

The enzyme nitrate reductase contains which metal cofactor essential for its activity? HPRCA-pat.

Iron
Copper
Molybdenum
Zinc

Answer: C — Molybdenum

Nitrate reductase (NR) uses molybdenum as an essential cofactor (the molybdenum cofactor, MoCo, at its active site). This is why molybdenum deficiency causes impaired nitrate reduction, and the characteristic symptom is "whiptail" of cauliflower (lamina fails to develop) due to nitrate accumulation.

Which of the following plants has CAM photosynthesis?

Sugarcane
Wheat
Pineapple
Rice

Answer: C — Pineapple

Pineapple (Ananas comosus, Bromeliaceae) is a classic CAM plant — it opens stomata at night to fix CO₂ via PEPC into malate (stored in vacuoles) and closes stomata during the day when malate is decarboxylated and CO₂ feeds the Calvin cycle. Sugarcane is C4; wheat and rice are C3.

Assertion (A): Blue light promotes stomatal opening in plants.
Reason (R): Blue light activates H⁺-ATPase in guard cells, causing K⁺ influx and a decrease in guard cell water potential. HPRCA-pat.

Both A and R are true and R is the correct explanation of A
Both A and R are true but R is not the correct explanation of A
A is true but R is false
A is false but R is true

Answer: A — Both true; R explains A

Blue light (absorbed by phototropin/zeaxanthin in guard cells) activates the plasma-membrane H⁺-ATPase → proton efflux → membrane hyperpolarisation → K⁺ influx via channels → osmotic potential decreases (Ψ_s falls) → water enters → turgor increases → stomata open.

Assertion (A): Cyclic photophosphorylation does not produce O₂ or NADPH.
Reason (R): In cyclic photophosphorylation, photosystem II is not involved and water is not split. HPRCA-pat.

Both A and R are true and R is the correct explanation of A
Both A and R are true but R is not the correct explanation of A
A is true but R is false
A is false but R is true

Answer: A — Both true; R explains A

In cyclic photophosphorylation, only PSI operates. Electrons cycle from P700* → Fd → cyt b6f → PC → back to P700. Because PSII (water-splitting) is not involved, neither O₂ nor NADPH is produced; only ATP is generated via the proton gradient across the thylakoid membrane.

Assertion (A): C4 plants exhibit higher photosynthetic efficiency than C3 plants at high temperatures.
Reason (R): C4 plants have a CO₂-concentration mechanism that suppresses the oxygenase activity of RuBisCO and prevents photorespiration.

Both A and R are true and R is the correct explanation of A
Both A and R are true but R is not the correct explanation of A
A is true but R is false
A is false but R is true

Answer: A — Both true; R explains A

At high temperatures, O₂ solubility decreases relative to CO₂, favouring RuBisCO's oxygenase activity and increasing photorespiration in C3 plants. C4 plants concentrate CO₂ in bundle-sheath cells via PEPC and Kranz anatomy, keeping the CO₂:O₂ ratio high around RuBisCO, suppressing photorespiration entirely.

Assertion (A): Abscisic acid is called the "stress hormone" of plants.
Reason (R): ABA levels rise under drought, cold, and salt stress and promote stomatal closure to conserve water.

Both A and R are true and R is the correct explanation of A
Both A and R are true but R is not the correct explanation of A
A is true but R is false
A is false but R is true

Answer: A — Both true; R explains A

ABA is synthesised and transported to guard cells during water stress. It triggers Ca²⁺-mediated K⁺ efflux from guard cells → turgor loss → stomatal closure → reduced transpiration. ABA also induces stress-response gene expression (LEA proteins, dehydrins) contributing to tolerance.

A: Kinetin is a naturally occurring cytokinin produced in all plant roots.
R: Kinetin was first isolated from autoclaved herring sperm DNA by Miller and Skoog (1955) and is not a natural plant product.

Both A and R are true and R is the correct explanation of A
Both A and R are true but R is not the correct explanation of A
A is true but R is false
A is false but R is true

Answer: D — A false, R true

Kinetin (6-furfurylaminopurine) is an artifact — produced by the degradation of adenine in autoclaved DNA. It is not synthesised naturally in plants. The first natural cytokinin, zeatin, was isolated from immature maize kernels (endosperm) in 1963 by Letham. Kinetin is widely used in tissue culture but is not an endogenous plant hormone.

Match the photosynthesis discoverer with the year and contribution: HPRCA-pat.

Column I (Scientist)	Column II (Discovery)
(a) Priestley	(i) CO₂ consumed in photosynthesis
(b) Ingenhousz	(ii) Starch as photosynthetic product; chloroplast site
(c) Senebier	(iii) O₂ produced by plants
(d) Sachs	(iv) Light required; only green parts photosynthesize

a-iii, b-iv, c-i, d-ii
a-iv, b-iii, c-ii, d-i
a-i, b-ii, c-iii, d-iv
a-iii, b-i, c-iv, d-ii

Answer: A — a-iii, b-iv, c-i, d-ii

Priestley (1772): O₂ produced; Ingenhousz (1779): light + green parts; Senebier (1782): CO₂ consumed; Sachs (1864): starch produced in chloroplasts. These are among the most commonly tested photosynthesis history facts in HPRCA-pattern papers.

Match the mineral element with its characteristic deficiency symptom: HPRCA-pat.

Column I (Element)	Column II (Deficiency)
(a) Zinc	(i) Whiptail of cauliflower
(b) Molybdenum	(ii) Tip burn; young growing points die
(c) Calcium	(iii) Little leaf / rosette disease
(d) Boron	(iv) Heart rot; death of growing points; hollow stem

a-iii, b-i, c-ii, d-iv
a-i, b-iii, c-iv, d-ii
a-iii, b-iv, c-i, d-ii
a-iv, b-i, c-iii, d-ii

Answer: A — a-iii, b-i, c-ii, d-iv

Zinc: little leaf/rosette (due to reduced auxin — Zn needed for tryptophan → IAA). Molybdenum: whiptail of cauliflower. Calcium: tip burn, young-tissue death (Ca immobile). Boron: heart rot, hollow stem, death of growing points (B involved in pectin cross-linking and cell wall synthesis).

Match the plant growth regulator with its primary discovery:

Column I (PGR)	Column II (Discovery event)
(a) Auxin	(i) Kinetin isolated from autoclaved herring sperm DNA
(b) Gibberellin	(ii) Oat coleoptile agar-block bioassay
(c) Cytokinin	(iii) Foolish seedling (bakanae) of rice
(d) ABA	(iv) Abscission acceleration in cotton fruit explants

a-ii, b-iii, c-i, d-iv
a-iii, b-ii, c-iv, d-i
a-i, b-iv, c-iii, d-ii
a-ii, b-iv, c-i, d-iii

Answer: A — a-ii, b-iii, c-i, d-iv

Auxin: Went's 1928 oat coleoptile bioassay. Gibberellin: Kurosawa's 1926 bakanae (foolish seedling) observation. Cytokinin: Miller & Skoog's 1955 kinetin isolation. ABA: Addicott (1963) found it accelerated abscission in cotton fruit explants (independently isolated by Wareing as "dormin").

Match the photosynthetic pathway/process with the correct location:

Column I (Process)	Column II (Location)
(a) Water splitting (PSII)	(i) Stroma of chloroplast
(b) Calvin cycle (C3)	(ii) Thylakoid membrane (grana)
(c) C4 initial carboxylation	(iii) Bundle-sheath cell cytoplasm/chloroplast
(d) C4 Rubisco (Calvin cycle)	(iv) Mesophyll cell chloroplast (cytoplasm)

a-ii, b-i, c-iv, d-iii
a-i, b-ii, c-iii, d-iv
a-ii, b-i, c-iii, d-iv
a-iv, b-iii, c-ii, d-i

Answer: A — a-ii, b-i, c-iv, d-iii

PSII (water splitting) is on the thylakoid membrane (granal stacks). Calvin cycle occurs in the stroma. In C4 plants, initial PEPC-mediated CO₂ fixation occurs in mesophyll cell cytoplasm (some sources say chloroplast). The Calvin cycle (with Rubisco) in C4 is restricted to bundle-sheath cells.

Consider the following statements about photoperiodism:

Garner and Allard discovered photoperiodism in 1920.
The actual critical factor is the length of the night (dark period), not the day.
Phytochrome Pfr is the biologically inactive form that inhibits flowering in LDP.
Florigen is produced in leaves and transported to the shoot apex to induce flowering.

Which of the above are correct?

I, II and IV only
I and II only
I, III and IV only
All four

Answer: A — I, II and IV only

Statement III is incorrect: Pfr is the biologically active form — it promotes LDP flowering and seed germination. In SDP, Pfr must be absent (converted back to Pr during the long dark period) for flowering. Statements I (Garner-Allard 1920), II (critical night length — Hamner-Bonner 1938), and IV (florigen/FT produced in leaves) are all correct.

Consider the following statements about the Calvin cycle:

The enzyme catalysing CO₂ fixation to RuBP is RuBisCO.
One turn of the Calvin cycle produces one molecule of glucose.
The reduction of 3-PGA to G3P requires both ATP and NADPH.
Six turns of the Calvin cycle are needed to produce one net glucose molecule.

Which are correct?

I, III and IV only
I, II and III only
II and IV only
All four

Answer: A — I, III and IV only

Statement II is incorrect: one turn of the Calvin cycle fixes one CO₂ and produces one net G3P (triose phosphate), not one glucose. Six turns are required (fixing 6 CO₂) to produce enough G3P (12 total, 10 recycled to regenerate 6 RuBP, leaving 2 net G3P = 1 glucose equivalent). Statements I, III, and IV are correct.

Which of the following are correctly matched pairs (nitrogen-fixing organism : association type)?

Rhizobium : symbiotic with legume roots; nodules with leghaemoglobin
Frankia : free-living in soil; aerobic heterotroph
Anabaena azollae : symbiotic in leaf cavities of Azolla
Azotobacter : free-living aerobic soil bacterium

I, III and IV only
I and III only
II and IV only
All four

Answer: A — I, III and IV only

Statement II is incorrect: Frankia is not free-living — it is an actinomycete that forms obligate symbiotic actinorhizal nodules in non-legume hosts like Hippophae, Casuarina, and Alnus. Statements I (Rhizobium-legume symbiosis with leghaemoglobin), III (Anabaena-Azolla), and IV (Azotobacter free-living aerobe) are all correct.

Arrange the following discoveries in correct chronological order: HPRCA-pat.

Calvin cycle elucidated (Calvin & Benson)
Auxin discovered (Went)
Photoperiodism described (Garner & Allard)
Gibberellin discovered (Kurosawa / bakanae)

III → IV → II → I
II → III → I → IV
IV → III → II → I
I → II → III → IV

Answer: A — III → IV → II → I

Garner & Allard: 1920 (photoperiodism). Kurosawa: 1926 (bakanae/GA). Went: 1928 (auxin). Calvin: 1950s, Nobel 1961. Chronological order: 1920 → 1926 → 1928 → 1950s.

Which of the following is the odd one out in terms of being a C4 plant? HPRCA-pat.

Maize (Zea mays)
Sugarcane (Saccharum officinarum)
Wheat (Triticum aestivum)
Sorghum (Sorghum bicolor)

Answer: C — Wheat (Triticum aestivum)

Maize, sugarcane, and sorghum are all C4 plants with Kranz anatomy. Wheat is a C3 plant — its CO₂ is fixed directly by RuBisCO in mesophyll cells, it undergoes photorespiration, and it lacks bundle-sheath cells with concentrated CO₂. Wheat is the most widely cultivated C3 cereal in temperate regions including HP.

The process of vernalisation was first systematically described and the term coined by: HPRCA-pat.

Pierre Chouard
T. D. Lysenko
Garner and Allard
Klippart

Answer: B — T. D. Lysenko

T. D. Lysenko (Soviet agronomist) coined the term vernalisation in 1928 and systematically studied the role of cold treatment in promoting flowering of winter crops. Klippart (1857) made an earlier practical observation about spring wheat; Chouard (France) later studied the mechanism scientifically. Garner-Allard studied photoperiodism, not vernalisation.

In which plant tissue is the Skoog-Miller ratio (high cytokinin:auxin) used to regenerate shoots in tissue culture? HPRCA-pat.

Roots; ratio directs root meristem activation
Shoot apical meristem; ratio promotes apical dominance
Undifferentiated callus; high cytokinin:auxin → shoot differentiation
Seed endosperm; cytokinin promotes storage protein synthesis

Answer: C — Undifferentiated callus; high cytokinin:auxin → shoot differentiation

Skoog and Miller (1957, following up their 1955 kinetin work) demonstrated that the ratio of cytokinin to auxin in nutrient medium controls the type of organ differentiation from callus: a high cytokinin:auxin ratio promotes shoot bud differentiation; a low ratio (high auxin:cytokinin) promotes root formation. This principle underlies all plant tissue culture micropropagation, including apple propagation extensively used in HP horticulture.

End of Chapter 4 · Plant Physiology. HPRCA-pat. indicates HPRCA / state-TGT pattern questions; literal past-paper items will be flagged with year when official papers are sourced.

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