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State Board · Class 12 · Biology

Lesson 6: Plant Water Relation

7 concepts · 56 practice questions with answers · Free to read

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Water, root and soil water

Link to Std 9–10

Std 11 covered root, stem, leaf and conducting tissues; here we see how water enters roots, rises and is lost from leaves.

From daily life

Roots act like many tiny straws sipping soil moisture.

Simple definition

A root hair = tube-like extension of an epiblema cell that absorbs water and minerals. Rhizosphere = soil microenvironment around the root.

Water and rootWater, root and soil waterFour zonesRoot capMeristematicElongationRoot hair zoneSoil waterGravitational (drains)Capillary = availableHygroscopic, combined = not availableWater 90–95% of cells; solvent; hydrogen bonds; high specific heatCohesion + adhesion + surface tension → capillary riseRoot hair: epiblema extension; 1–10 mm; two-layered wallWall: pectin (outer) + cellulose (inner) = freely permeablePlasma membrane = selectively permeableEpiphytes (orchids): velamen absorbs vapour from airRhizosphere = microenvironment around the root

In the figure : Root, root hair and soil water.

Rule 1 : Properties of water and the root — Water is the ‘elixir of life’; 90–95% of most cells; best solvent; medium for biochemical reactions; keeps cells turgid. Hydrogen bonds give high specific heat, heat of vaporization and fusion ⇒ thermal buffer; cohesion + adhesion and high surface tension ⇒ rise in capillaries. The root is the main absorbing organ; four zones: root cap, meristematic, elongation, root hair / absorption zone. Epiphytes (orchids) absorb vapour from air with velamen.

  • Easy : Water = 90–95%.
  • Medium : Root-hair zone = absorption.
  • Exam-type (CET) : Velamen = orchids.

Rule 2 : Root hair and soil water — Root hair: 1–10 mm; colourless, unbranched, short-lived, delicate; large central vacuole; two-layered wall (outer pectin, inner cellulose) = freely permeable; plasma membrane = selectively permeable. Soil water: gravitational (drains down; not available), capillary (between particles; available to roots), hygroscopic (held on particles; not available), combined (hydrated oxides; not available).

  • Easy : Only capillary water is available.
  • Medium : Pectin outside, cellulose inside.
  • Exam-type (CET) : Membrane = selective.
Remember

Four soil waters: G-C-H-C — only Capillary is absorbed.

Common mistake and CET trap

‘Roots absorb hygroscopic water’ is wrong. Trap: ‘root hairs are multicellular’ is wrong (one-celled).

Shortcut

Memory clue : "Capillary = catchable".

Spot the difference

TypeAvailable?
GravitationalNo
CapillaryYes
HygroscopicNo
CombinedNo

Practice questions

Easy
  1. 1Water % in most cells?
  2. 2Bonds between water molecules?
  3. 3Absorbing zone?
Medium
  1. 4Root hair wall layers?
  2. 5Absorbing tissue of epiphytes?
  3. 6Available soil water?
Challenging
  1. 7What is rhizosphere?
  2. 8Length of a root hair?
Show answers (try them yourself first)
  1. 190–95%.
  2. 2Hydrogen bonds.
  3. 3Root-hair zone.
  4. 4Pectin + cellulose.
  5. 5Velamen.
  6. 6Capillary.
  7. 7Soil zone around roots.
  8. 81–10 mm.

Imbibition, diffusion, osmosis and water potential

From daily life

A wooden door swells and sticks in the monsoon — imbibition at work.

Simple definition

Osmosis = movement of solvent through a semipermeable membrane. Water potential (ψ) = chemical potential of water; pure water has ψ = 0.

Osmosis and ψImbibition, diffusion, osmosis, water potentialImbibitionDiffusionOsmosisSeeds, wood, raisinsHigh → low concentrationSolvent through membraneImbibition: swelling of hydrophilic colloids; imbibant/imbibateDPD = DP pure − DP solution = suction pressure (Meyer 1938)Hypotonic (weak) → isotonic → hypertonic (strong)Endosmosis → turgid; exosmosis → flaccidDPD = OP − TP; fully turgid: TP = WP, DPD = 0Water potential ψ: pure water = 0; solutions negativeWater flows from higher ψ to lower ψ; ψ = ψs + ψpFacilitated diffusion: aquaporins, ion channels (no energy)

In the figure : Three modes of water uptake and DPD.

Rule 1 : Imbibition and diffusion — Imbibition: hydrophilic colloids (pectin, cellulose, proteins) swell by adsorbing water; substance = imbibant, water = imbibate; no solution forms; continues to equilibrium; e.g. soaking seeds, swelling raisins. Diffusion: molecules move from higher to lower concentration (kinetic energy) until equilibrium. Diffusion pressure (DP) is higher for pure solvent. DPD = DP (pure) − DP (solution) = suction pressure (Meyer 1938) = the cell's ‘thirst’. Facilitated diffusion: via aquaporins and ion channels, without energy.

  • Easy : Imbibition = swelling.
  • Medium : DPD = thirst.
  • Exam-type (CET) : Aquaporin = water channel.

Rule 2 : Osmosis, TP and water potential — A root-hair cell has more concentrated sap → water enters from weaker outside solution (endosmosis). Solutions: hypotonic (low OP), hypertonic (high OP), isotonic (equal). Endosmosis → turgid; exosmosis → flaccid. Turgor pressure (TP) and wall pressure (WP) act oppositely; in a fully turgid cell TP = WP and DPD = 0. Formula: DPD = OP − TP. Flaccid cell: TP = 0 ⇒ DPD = OP. Modern terms: DPD → water potential (ψ), OP → osmotic potential (ψs), TP → pressure potential (ψp); ψ = ψs + ψp. Pure water ψ = 0; solutions are negative. Water flows from higher ψ to lower ψ.

  • Easy : DPD = OP − TP.
  • Medium : Turgid: DPD = 0.
  • Exam-type (CET) : Pure water ψ = 0.
Remember

TP pushes out, OP pulls in: DPD = OP − TP.

Common mistake and CET trap

‘Water flows from low to high ψ’ is wrong. Trap: ‘a turgid cell has high DPD’ is wrong (0).

Shortcut

Memory clue : "DPD = dry plant desire (thirst)".

Spot the difference

ProcessExample
ImbibitionSoaking seeds
DiffusionSpreading of scent
OsmosisWater into root hair

Practice questions

Easy
  1. 1What is an imbibant?
  2. 2Another name for DPD?
  3. 3DPD formula?
Medium
  1. 4DPD of a flaccid cell?
  2. 5DPD of a turgid cell?
  3. 6Effect of endosmosis?
Challenging
  1. 7ψ of pure water?
  2. 8Direction of water flow?
Show answers (try them yourself first)
  1. 1Substance that adsorbs water.
  2. 2Suction pressure.
  3. 3OP − TP.
  4. 4= OP.
  5. 50.
  6. 6Turgid.
  7. 70.
  8. 8Higher ψ → lower ψ.

Plasmolysis and absorption of water

From daily life

Vegetables shrink in salty water and swell in plain water.

Simple definition

Plasmolysis = protoplast pulling away from the wall by exosmosis in a hypertonic solution. Deplasmolysis = becoming turgid again in a hypotonic solution.

Plasmolysis and absorptionPlasmolysis and water absorptionTurgidPlasmolysedHypertonicApoplastwalls + spacesSymplastplasmodesmataPlasmolysis: exosmosis in hypertonic solution; protoplast shrinks; TP = 0Deplasmolysis: turgid again in hypotonic solutionPath: epiblema → cortex → endodermis (Casparian strip) → pericycle → xylemCasparian strip (suberin): forces water through symplastPassive absorption ~98%: by day, transpiration pull; no ATPActive: at night; root pressure; ATP; osmotic / non-osmotic (Kramer–Thimann 1959)Favourable: capillary water, soil 20–30 °C, aeration; high salts reduce uptake

In the figure : Plasmolysis and pathways of water.

Live figure

Plasmolysis and absorption of water

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Rule 1 : Plasmolysis — When a living cell is put in a hypertonic solution, exosmosis shrinks the protoplast, leaving a gap between wall and protoplast that fills with outer solution. In a plasmolysed cell TP = 0. In a hypotonic solution endosmosis ⇒ deplasmolysis (turgid again). In a fully turgid cell TP = OP ⇒ DPD = 0. Applications: salting pickles/fish for preservation; excess fertiliser wilting plants.

  • Easy : Plasmolysis: hypertonic.
  • Medium : Deplasmolysis: hypotonic.
  • Exam-type (CET) : Plasmolysed: TP = 0.

Rule 2 : Pathway of water and modes of absorption — Water enters root hairs by osmosis, then a gradient of DPD across the cortex (epiblema → cortex → endodermis (passage cells) → pericycle → xylem). Two pathways: apoplast (through walls and spaces; up to the endodermis) and symplast (cell to cell through plasmodesmata; transmembrane). The Casparian strip (suberin) blocks the apoplast at the endodermis ⇒ all water goes through symplast. Passive absorption (~98%): by day, transpiration pull; no ATP; DPD gradient. Active absorption: at night (stomata closed), root pressure; ATP: osmotic (Atkins–Priestley 1922) and non-osmotic (Kramer–Thimann 1959). Favourable factors: capillary water, soil 20–30 °C, good aeration; high soil salts reduce uptake.

  • Easy : Passive = 98%; daytime.
  • Medium : Casparian strip = suberin.
  • Exam-type (CET) : Active = night, ATP.
Remember

Apoplast = walls; symplast = through living cells.

Common mistake and CET trap

‘Most absorption is active’ is wrong (passive). Trap: ‘Casparian strip blocks symplast’ is wrong (apoplast).

Shortcut

Memory clue : "Casparian strip cuts the apoplast".

Spot the difference

TypeTimeEnergy
PassiveDay (~98%)None
ActiveNightATP

Practice questions

Easy
  1. 1Plasmolysis occurs in?
  2. 2TP in plasmolysed cell?
  3. 3Deplasmolysis occurs in?
Medium
  1. 4What is apoplast?
  2. 5Symplast goes through?
  3. 6Where is the Casparian strip?
Challenging
  1. 7Passive absorption %?
  2. 8When is active absorption?
Show answers (try them yourself first)
  1. 1Hypertonic.
  2. 20.
  3. 3Hypotonic.
  4. 4Path through walls and spaces.
  5. 5Plasmodesmata.
  6. 6Endodermis.
  7. 7~98%.
  8. 8At night.

Translocation of water (ascent of sap)

From daily life

Sipping through a straw lifts juice; in a tree, transpiration does the pulling.

Simple definition

Ascent of sap = upward movement of water and minerals from roots to leaves against gravity through xylem (tracheids, vessels).

Ascent of sapTranslocation of water (ascent of sap)Leaf: transpiration pullRoot: root pressureDixon–Jolly 1894Cohesion–tension (transpiration pull)Widely accepted theoryCohesion + adhesion + tensionXylem = path: ringing experiment (tracheids + vessels)Root pressure (Priestley): proof = sap oozing from cut stem; guttationLimits: ~0 in tall trees; absent during active transpirationCapillarity: narrow tube rises higher; limit: closed ends of tracheidsCohesion–tension: DPD in leaf → xylem under tension → water risesLimit: air bubbles (vessels more efficient)Ascent occurs even in leafless trees: living cells involved

In the figure : Root pressure and cohesion–tension.

Rule 1 : Root pressure and capillarity — Ringing experiment: transport is through xylem. Root pressure (vital theory, Priestley): hydrostatic pressure from living root cells pushes water into xylem; proof: sap oozing from a cut stem, guttation. Limits: ≈ 0 in tall gymnosperms; absent during active transpiration; xylem sap is normally under tension (negative pressure). Capillarity: water rises higher in narrow tubes; limits: tracheids have closed ends; the root cortex is a barrier; tall trees have wide vessels.

  • Easy : Root pressure = vital theory.
  • Medium : Guttation = evidence.
  • Exam-type (CET) : Capillarity is not the sole mechanism.

Rule 2 : Cohesion–tension theory — Dixon and Jolly (1894); the most widely accepted today. Principles: cohesion (attraction between water molecules) + adhesion (water to lignified xylem walls) ⇒ a continuous water column from root to leaf; transpiration pull: water vapour lost from leaves → mesophyll DPD rises → tension in xylem → water is passively pulled up. Limits: temperature changes may let air bubbles break the column (vessels are more efficient than tracheids); ascent occurs even in leafless or vaseline-smeared plants ⇒ living cells also contribute.

  • Easy : Cohesion + adhesion + pull.
  • Medium : Dixon–Jolly 1894.
  • Exam-type (CET) : Pull is passive.
Remember

Root = push (pressure); leaf = pull (transpiration).

Common mistake and CET trap

‘Water rises through phloem’ is wrong (xylem). Trap: ‘root pressure is the main force in tall trees’ is wrong.

Shortcut

Memory clue : "Cohesion = water to water; adhesion = water to wall".

Spot the difference

TheoryForce
Root pressurePushing pressure from roots
CapillaritySurface tension
Cohesion–tensionLeaf pull + cohesion

Practice questions

Easy
  1. 1Ascent of sap occurs in?
  2. 2Evidence of root pressure?
  3. 3What is cohesion?
Medium
  1. 4What is adhesion?
  2. 5Dixon–Jolly year?
  3. 6Transpiration pull arises from?
Challenging
  1. 7Most accepted theory?
  2. 8A limitation?
Show answers (try them yourself first)
  1. 1Xylem.
  2. 2Sap exuding from cut stem / guttation.
  3. 3Attraction between water molecules.
  4. 4Attraction of water to xylem wall.
  5. 51894.
  6. 6Water loss from leaves.
  7. 7Cohesion–tension.
  8. 8Air bubbles.

Transport of minerals and food

From daily life

Mineral roads run upward only, but food roads run in both directions.

Simple definition

Translocation of food = movement of prepared food (sucrose) from source (leaf) to sink (root, fruit, bud) through phloem.

Minerals and foodTransport of minerals and foodXylem: minerals upwardPhloem: sucrose both waysMinerals as ions; pulled up; ~36–40 essential elementsMacro: C, H, O, N, P, S, Mg; micro: Cu, Co, Mn, B, ZnRemobilisation: P, S, N, K readily; Ca notFood: sucrose; source (leaf) → sink; sieve tubes + companion cellsRadial: phloem → pith; tangential: phloem → cortexMunch pressure flow (mass flow): high OP at sourceLimit: bidirectional flow unexplained; purely physical

In the figure : Xylem and phloem transport.

Rule 1 : Transport of minerals — Soil is the main source; minerals are absorbed as ions independently of water, then carried up with xylem sap by transpiration pull. About 36–40 elements are essential; macro: C, H, O, N, P, S, Mg; micro: Cu, Co, Mn, B, Zn. They are unloaded through fine veins where needed (tips, young leaves, flowers, fruits). Remobilisation: before leaf fall P, S, N, K move to young leaves; structural Ca is not remobilised. Nitrogen travels in xylem as ions and amino acids.

  • Easy : Remobilised: P, S, N, K.
  • Medium : Ca is not.
  • Exam-type (CET) : Minerals = xylem.

Rule 2 : Food transport: Munch hypothesis — Food moves as sucrose through phloem (sieve tubes + companion cells); evidence: ringing, ¹⁴C isotope. Vertical: leaf → stem/root (down) and buds/fruits (up); lateral: radial (phloem → pith) and tangential (phloem → cortex). Munch (pressure flow / mass flow): leaf cells hold much sugar ⇒ high OP ⇒ water enters ⇒ turgor drives sap to the sink; at the sink sugar is used/stored and water returns to xylem. Limits: does not explain bidirectional flow; purely physical.

  • Easy : Food = sucrose; phloem.
  • Medium : Source → sink.
  • Exam-type (CET) : Munch = pressure flow.
Remember

Xylem = water up; phloem = food both ways.

Common mistake and CET trap

‘Food moves in xylem’ is wrong (phloem). Trap: ‘Ca is readily remobilised’ is wrong.

Shortcut

Memory clue : "Phloem = food; xylem = water".

Spot the difference

PointXylemPhloem
CarriesWater + mineralsSucrose
DirectionUpwardBoth
Driving forceTranspiration pullPressure flow

Practice questions

Easy
  1. 1Form in which minerals are absorbed?
  2. 2Macro element example?
  3. 3Element not remobilised?
Medium
  1. 4Food moves as?
  2. 5What is the source?
  3. 6What is a sink?
Challenging
  1. 7Munch hypothesis is?
  2. 8Radial translocation?
Show answers (try them yourself first)
  1. 1Ions.
  2. 2N / P / Mg.
  3. 3Calcium.
  4. 4Sucrose.
  5. 5Leaf, where food is made.
  6. 6Part that uses/stores food.
  7. 7Pressure flow.
  8. 8Phloem → pith.

Transpiration

From daily life

Wet clothes dry quickly in sun; leaves likewise lose water as vapour.

Simple definition

Transpiration = loss of water as vapour from aerial parts. Guttation = loss of water in liquid form (through hydathodes).

TranspirationTranspirationStomatal 90–93%Cuticular 8–10%Lenticular (little)Of absorbed water 5% used; 95% surplus → 99% vapour, 1% liquidIn liquid form: guttation (hydathodes / water stomata)Stomatal: only by day (desert plants an exception)Cuticular: cutin; thicker cuticle = lessLenticular: stems, woody fruits; day and nightStomatal frequency = stomata per unit area; stomatal indexEpistomatic (hydrophytes), hypostomatic (Nerium), amphistomatic (grass)Significance: cooling, pull, mineral transport; cost: water loss

In the figure : Types of transpiration.

Rule 1 : How much water, by which routes? — Of absorbed water only 5% is used for growth; 95% is surplus — 99% of it is lost as vapour (transpiration), 1% as liquid (guttation, hydathodes/water stomata). Three kinds of transpiration: stomatal (90–93%; only by day; desert plants an exception), cuticular (8–10%; cutin; thicker cuticle = less; by simple diffusion), lenticular (lenticels; stems and woody fruits; very little; day and night). Mostly foliar (through leaves).

  • Easy : 95% surplus; 5% used.
  • Medium : Stomatal 90–93%.
  • Exam-type (CET) : Cuticular 8–10%.

Rule 2 : Stomatal distribution and significance — Stomatal frequency = stomata per unit area; stomatal index = ratio of stomata to epidermal cells. Distribution: epistomatic (upper side; hydrophytes), hypostomatic (lower side; Nerium, xerophytes), amphistomatic (both; grass, mesophytes). Significance: cooling by evaporation, transpiration pull moves water and minerals; cost: heavy water loss (hence adaptations in xerophytes).

  • Easy : Epi = upper; hypo = lower.
  • Medium : Amphi = both.
  • Exam-type (CET) : Frequency ≠ index.
Remember

Stomata = day shift; lenticels = 24×7; thick cuticle = less.

Common mistake and CET trap

‘Guttation is vapour loss’ is wrong (liquid). Trap: ‘stomatal transpiration is higher at night’ is wrong.

Shortcut

Memory clue : "Epi = elevated (upper); hypo = hidden (lower)".

Spot the difference

Type%
Stomatal90–93
Cuticular8–10
LenticularVery small

Practice questions

Easy
  1. 1What is transpiration?
  2. 2Guttation occurs through?
  3. 3Surplus water %?
Medium
  1. 4Stomatal transpiration share?
  2. 5Cuticular share?
  3. 6Epistomatic leaves in?
Challenging
  1. 7Amphistomatic example?
  2. 8A benefit of transpiration?
Show answers (try them yourself first)
  1. 1Loss of water vapour.
  2. 2Hydathodes.
  3. 395%.
  4. 490–93%.
  5. 58–10%.
  6. 6Hydrophytes.
  7. 7Grass.
  8. 8Cooling / pull.

Stomata and their movement

From daily life

Doors open by day and close at night; stomata do the same.

Simple definition

A stoma = elliptical pore bounded by two guard cells; exchange of gases and vapour.

StomataStomata and their movementTwo guard cellsKidney (dicot)Dumbbell (grass)Inner wall thickOuter wall thinLevitt 1974: by day starch → malic acid → H⁺ + malateK⁺ in (+ Cl⁻); K-malate ↑ OP → endosmosis → turgid → stoma opensNight: ABA stops K⁺ → exosmosis → flaccid → stoma closesGuard cells: living, nucleated; few chloroplastsSubsidiary cells take H⁺, give K⁺Cyperus: both kinds of guard cellsLight, low CO₂, water → open; water stress → close

In the figure : Guard cells and stomatal movement.

Rule 1 : Structure of the stomatal apparatus — A stomatal apparatus = two guard cells + stoma + subsidiary (accessory) cells. Guard cells: living, nucleated, few chloroplasts; inner wall thick and inelastic, outer wall thin and elastic. Dicots: kidney-shaped; monocots (grasses): dumbbell-shaped; Cyperus: both. Guard cells change shape to open/close the stoma.

  • Easy : Dicot = kidney; grass = dumbbell.
  • Medium : Inner wall thick.
  • Exam-type (CET) : Cyperus: both.

Rule 2 : How do stomata open and close? — Levitt (1974) proton transport / K⁺ theory: by day starch → malic acid → malate + H⁺. H⁺ goes to subsidiary cells, K⁺ enters guard cells (with Cl⁻) ⇒ potassium malate ⇒ OP rises ⇒ endosmosis ⇒ guard cells turgid ⇒ stoma opens. At night abscisic acid (ABA) blocks K⁺/Cl⁻ uptake ⇒ exosmosis ⇒ flaccid ⇒ stoma closes. Light, low CO₂ and adequate water open stomata; water stress closes them.

  • Easy : K⁺ in = opens.
  • Medium : ABA = closes.
  • Exam-type (CET) : Turgid = open.
Remember

K⁺ in = open; ABA = bolts the stoma shut.

Common mistake and CET trap

‘The stoma opens when guard cells turn flaccid’ is wrong. Trap: ‘ABA opens stomata’ is wrong.

Shortcut

Memory clue : "Turgid → open; flaccid → closed".

Spot the difference

StateGuard cellsStoma
DayK⁺ in; turgidOpen
Night / ABAK⁺ out; flaccidClosed

Practice questions

Easy
  1. 1Guard cell shape in dicots?
  2. 2In grasses?
  3. 3Inner wall of guard cell?
Medium
  1. 4By day starch forms?
  2. 5Ions entering guard cells?
  3. 6Why close at night?
Challenging
  1. 7Levitt's year?
  2. 8Role of subsidiary cells?
Show answers (try them yourself first)
  1. 1Kidney.
  2. 2Dumbbell.
  3. 3Thick, inelastic.
  4. 4Malic acid.
  5. 5K⁺ and Cl⁻.
  6. 6ABA blocks K⁺ → flaccid.
  7. 71974.
  8. 8Take H⁺, give K⁺.

Quick revision of the whole lesson

ConceptKey fact
Root / soilOnly capillary water available
OsmosisDPD = OP − TP; pure water ψ = 0
AbsorptionPassive 98%; Casparian strip
AscentCohesion–tension (Dixon–Jolly 1894)
Minerals / foodXylem up; phloem sucrose, Munch
Transpiration95% surplus; stomatal 90–93%
StomataK⁺ in = open; ABA = closed

🎯 Most important points for the exam

  • Root hair and soil water.
  • Osmosis, DPD, water potential.
  • Plasmolysis, apoplast/symplast.
  • Cohesion–tension theory.
  • Munch pressure flow.
  • Types of transpiration.
  • Stomatal movement.
Study mantra

Understand the concept → draw the figure → memorise the key points → solve questions yourself.