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Water & Plant Cells: Chapter 3Water & Plant Cells: Chapter 3

Why is water important?Why is water important?1.1. 1g organic material requires 500g water.1g organic material requires 500g water.2.2. Roles of water in plants (97% is lost)Roles of water in plants (97% is lost)

A.A. Solvent Solvent -- Fluid medium for cytoplasmFluid medium for cytoplasmB.B. PhotosynthesisPhotosynthesisC.C. TurgorTurgor pressurepressureD.D. Transpiration & Heat dissipationTranspiration & Heat dissipationE.E. Transport dissolved mineralsTransport dissolved minerals

3.3. ProductivityProductivity

Properties of WaterProperties of Water1.1. Hydrogen bondingHydrogen bonding2.2. Excellent solvent Excellent solvent –– shells of hydrationshells of hydration3.3. Specific heat = 4.187kJ kgSpecific heat = 4.187kJ kg--11 (1c/g)(1c/g)4.4. Latent heat of vaporization = 44kJ molLatent heat of vaporization = 44kJ mol--11

5.5. Cohesion, adhesion & capillarityCohesion, adhesion & capillarity6.6. Surface tensionSurface tension7.7. Tensile strength > Tensile strength > --30MPa30MPa

1 1 MPaMPa = 1 = 1 newtonnewton mm--2 2

1 1 MPaMPa = 1 Joule m= 1 Joule m--3 3

1 1 MPaMPa = 9.9 atmospheres= 9.9 atmospheres1 1 MPaMPa ≈≈ 145.5 145.5 psipsi30 30 MPaMPa = 4351 lb in= 4351 lb in--2 2

1 Atmosphere = 14.7 lbs in1 Atmosphere = 14.7 lbs in--2 2

1 Atmosphere = 760mm Hg1 Atmosphere = 760mm Hg1 Atmosphere = 0.1013 1 Atmosphere = 0.1013 MPaMPa1 Atmosphere = 1.013 Bar1 Atmosphere = 1.013 Bar1 Atmosphere = 1.013 X 101 Atmosphere = 1.013 X 1055 PaPa

Major Processes Related to Water Movement in PlantsMajor Processes Related to Water Movement in Plants

1.1. Short distance transport = Molecular DiffusionShort distance transport = Molecular Diffusion2.2. Long distance transport = Bulk FlowLong distance transport = Bulk Flow

Short Distance Transport Long Distance Transport

Processes of Water Movement in PlantsProcesses of Water Movement in PlantsShort Distance TransportShort Distance Transport

1.1. Molecular DiffusionMolecular Diffusion1.1. FickFick’’ss first lawfirst law

2.2. Importance: Identifies factors governing short distance transporImportance: Identifies factors governing short distance transport:t:1.1. Concentration gradientConcentration gradient2.2. DistanceDistance3.3. Medium through which something movesMedium through which something moves

Js = rate of transport

Ds = diffusion coefficient = how easily something moves in a medium

cs = difference in concentration between two points = concentration gradient

x = distance between two points/concentrations

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1.1. Average time for a particle to diffuseAverage time for a particle to diffuse

2.2. Example: Glucose travels Example: Glucose travels 50um in 2.5 seconds50um in 2.5 seconds1 meter in 32 years1 meter in 32 years

3.3. Point: Diffusion Point: Diffusion rapid over short distancesrapid over short distancesvery slow over long distancesvery slow over long distances

Processes of Water Movement in PlantsProcesses of Water Movement in PlantsShort Distance TransportShort Distance Transport

Influence of distanceInfluence of distance

L = distance traveled

D = diffusion coefficient

1.1. Molecules move in bulk driven by a pressure gradient.Molecules move in bulk driven by a pressure gradient.

2.2. Pressure driven bulk flow moves water long distancesPressure driven bulk flow moves water long distances

Processes of Water Movement in PlantsProcesses of Water Movement in PlantsLong Distance TransportLong Distance TransportBulk flow or Mass FlowBulk flow or Mass Flow

Volume flow rate = meters sec-1

r = radius of tubeη= viscosity of the liquid

ψp = pressure gradientx = distance between gradient points

1.1. Free energy per mole of waterFree energy per mole of water

2.2. Water potential Water potential ==

3.3. Units of chemical/water potentialUnits of chemical/water potentialEnergy: J molEnergy: J mol--11

Pressure: PascalPressure: Pascal

4. The Pascal: 1Pa = 0.000145 lb in4. The Pascal: 1Pa = 0.000145 lb in--22

1MPa 1MPa = 1,000,000 Pa= 1,000,000 Pa= 145 lb in= 145 lb in--22

= 10 bars= 10 bars= 9.87 atmospheres= 9.87 atmospheres

Chemical Potential of WaterChemical Potential of Water Pressure UnitsPressure Units

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Water Potential and Its ComponentsWater Potential and Its Components1.1. Water Potential EquationWater Potential Equation

ψψ = = ψψss + + ψψpp + + ψψgg

ψψ == water potential in water potential in MPaMPa

ψψss = solute or osmotic potential= solute or osmotic potential

ψψpp = pressure potential= pressure potential

ψψgg = gravity potential= gravity potential

Solute or Osmotic PotentialSolute or Osmotic Potential

1.1. Represents effect of solutes on water potentialRepresents effect of solutes on water potential2.2. Solutes always decrease water potentialSolutes always decrease water potential

…… always reduce free energy of wateralways reduce free energy of water3. 3. vanvan’’tt Hoff equation Hoff equation

ψψss = = --RTcRTcss

ψψss = solute potential= solute potentialRR = gas constant (8.32 J mol= gas constant (8.32 J mol--11 KK--11))TT = temperature (degrees K)= temperature (degrees K)ccss == solute concentration solute concentration

((osmolalosmolal = moles solute per liter water)= moles solute per liter water)

4. Independent of solute identity4. Independent of solute identity

Values of Osmotic PotentialValues of Osmotic Potential

1.1. Hydrostatic Pressure of solutionHydrostatic Pressure of solution2.2. TurgorTurgor pressure = Positive hydrostatic pressurepressure = Positive hydrostatic pressure3.3. Tension = Negative hydrostatic pressureTension = Negative hydrostatic pressure

4.4. ψψpp = 0 in a standard state= 0 in a standard state

Pressure PotentialPressure Potential

Flaccid Turgid Tension

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1.1. Gravity causes water to move downwardGravity causes water to move downward2. ψgg = = ρρww g h g h

ρρww = density of water (1.0 kg/m= density of water (1.0 kg/m3 3 at 4C))h h = height (meters)= height (meters)gg = acceleration due to gravity (9.8 ms= acceleration due to gravity (9.8 ms--22))

ρρww g = 0.01 M Pa mg = 0.01 M Pa m--11

For a 100 meter tree: For a 100 meter tree: ψgg = 0.01 = 0.01 MPaMPa mm--11 X100 = 1.0 X100 = 1.0 MPaMPa1MPa = = 145 lb in1MPa = = 145 lb in--2 2

5. Here5. Here’’s the point: s the point: In cells: In cells: ψgg is negligible (no change in height)is negligible (no change in height)In tall trees (100 meters) = 1.0 In tall trees (100 meters) = 1.0 MPaMPa change in change in ψww

Gravity PotentialGravity Potential1.1. MatricMatric = surface which binds water= surface which binds water2.2. Can be strong or negligibleCan be strong or negligible

1.1. NeglibibleNeglibible hydrated tissueshydrated tissues2.2. Strong Strong Dry surfaces (e.g. seeds)Dry surfaces (e.g. seeds)

Examples: Examples: Dry seed Dry seed ψ ψ ≈≈ ––50 to 50 to --350 350 MPaMPa

Hydrated seed Hydrated seed ψ ψ ≈≈ 0 0 MPaMPa

3. Here3. Here’’s the point: s the point: In hydrated tissue: In hydrated tissue: ψmm is negligibleis negligibleIn dry tissue: In dry tissue: ψmm is very importantis very important

4. Application 4. Application Soil Soil matricmatric potential is a critical variable in crop yield , potential is a critical variable in crop yield , runoff, runoff, evapotranspirationevapotranspiration and irrigation scheduling.and irrigation scheduling.

ψψmm = = MatricMatric PotentialPotential

Numerical Examples of Water Potential & Its ComponentsNumerical Examples of Water Potential & Its ComponentsNumerical Examples of Water Potential and Its ComponentsNumerical Examples of Water Potential and Its Components

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Numerical Examples of Water Potential and Its ComponentsNumerical Examples of Water Potential and Its ComponentsHofflerHoffler diagram: Water Potential and Change in Cell Volumediagram: Water Potential and Change in Cell Volume

Major ideas…

As cell water potential decreases first 5%

1. In a turgid cell, Water potential drop results in sharp turgor drop with little change in cell volume.

2. As cell volume decreases below 90%, osmotic potential decreases faster than turgor (Turgor = 0) with further decline in water potential

Sensitivity of Various Physiological Processes to DehydrationSensitivity of Various Physiological Processes to Dehydration

END END Chapter 3Chapter 3

Water and Plant CellsWater and Plant Cells

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Chapter 4Chapter 4Water Balance of PlantsWater Balance of Plants

Mechanisms & driving forces Mechanisms & driving forces operating on water transport in operating on water transport in

plantsplants

Physical Characteristics of Soil

Field Capacity WetClay soil 40%Sand 3%

Relative Size of soil particles

ψsoil = ψs + ψp

ψp = about -2MPa in dry clay soil.Permanent Wilting Point = ψsoil > ψplant

Components of soil water potential Soil Hydraulic ConductivitySoil hydraulic conductivity ≈ how easily water moves through soil

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Role of Water Evaporation in Water MovementT = surface tension of water

= 7.28 X 10-8 MPa m

r = radius of curvaturerT

P2−

=Ψ

Water Uptake by the Root

Water Movement: Soil to Root

Root Hairs increase surface area.

Concave menisci form at soil-water interface..creating very negative matric potentials.

More water removedMore acute menisci

More negative ψsoil .

Water Absorption: Root

Epidermis Cortex

Aquaporins regulate water movement through endodermis.

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Role of Aquaporins in water transport Root Pressure and Guttation

How does water move up a tree?

1. Diffusion

2. Capillarity

3. It’s pushed up

4. It’s pulled up

1.1. Average time for a particle to diffuseAverage time for a particle to diffuse

2.2. Example: Glucose travels Example: Glucose travels 50um in 2.5 seconds50um in 2.5 seconds1 meter in 32 years1 meter in 32 years

3.3. Point: Diffusion Point: Diffusion rapid over short distancesrapid over short distancesvery slow over long distancesvery slow over long distances

Processes of Water Movement in PlantsProcesses of Water Movement in PlantsDiffusion is a very slow processDiffusion is a very slow process

L = distance traveled

D = diffusion coefficient

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Could capillarity generate the force needed to draw water up a tree?

To calculate the height liquid can be raised by capillarity:h = 2Tcosø ÷ dgr

h = height of capillary (meters)T = surface tension (@20C, T for water = 0.0728Nm-1)cos ø = lifting component of meniscus angle

(approx = 1)d = density of liquid (998kgm-3)g = gravitational constant (9.80 ms-2)r = radius of capillary (in meters)

Capillarity as a Mechanism of Long Distance Water Transport

Capillary diam (µm) height of water column (m)0.005 30001 15.310 1.5340 0.38

Point: Height to which a column of water can be lifted by a capillary is inversely related to the diameter of the capillary.

Capillarity as a Mechanism of Long Distance Water Transport

Xylem and Water Transport Circular Bordered Pits

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• Requires less pressure than movement through cells

• From Pouiseuille’s equation:• Rate = 4mm s-1

• Tube radius = 40um

Water Movement Through Xylem

0.02Xylem2 X 108Layer of cells

Pressure gradient (MPa m-1)Tissue

• Pressure gradient = 0.02 MPa m-1

• Tree height = 100 meters (a Redwood tree)• Total Pressure Needed = 2.0 MPa

• Weight of Water … added Pressure to overcome– 100 meters X 0.01MPa m-1 = 1MPa

• Total Pressure Needed = 3MPa

Pressure Needed to Move Water Up a Tree

Fig 4.16Fig 4.16 SoilSoil--PlantPlant--Atmosphere ContinuumAtmosphere Continuum SoilSoil--PlantPlant--Atmosphere Atmosphere ContinuumContinuum

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Relative Humidity and Absolute Water Concentrations

( )% 100wv

wv sat

cRH xc

=

RH = Relative Humidity (%)

Cwv = water concentration (moles m-3 )

Cwv(sat) = saturation water concentration (moles m-3)

Ψw = water potential (kPa)R = gas constant

= 8.314 kPa L mol-1 K-1

T = temperature (oK)Vw = partial molal volume of water

= 0.018 Liters / mol RH = relative humidity (0 - 1.0)Note: MPa = kPa/1000

ln( )ww

RT RHV

Ψ =

Relative Humidity and Water Potential

Diurnal course of Relative Humidity over a deciduous forest1. Water Vapor diffuses from Leaf to Air = Transpiration2. Diffusion influenced by 2 factors

1. Water Vapor Concentration Gradient2. Diffusional Resistance

Water Movement from Leaf To Atmosphere

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Stomatal aperture and Boundary Layer Resistance at leaf surface Role of Water Evaporation in Water Up a Tree

Water Evaporation in a leaf generates Negative Pressure in the Xylem

Fig. 4.16 Soil-Plant-Atmosphere Continuum

Experimental Demonstration of Water Under Tension

1. Dye uptake in cut stems

2. Change in stem diameter (Fritts 1958 – Beech Trees)

3. Direct measurement – Pressure bomb

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Measuring Water Potential in Plants with a Pressure Bomb

1. Turgor increase in guard cell solute concentration … pores open2.Turgor decrease in guard cell solute concentration … pores close

Guard Cells Control Short Term Regulation of Water Loss

Physical Challenges of CohesionTracheary Elements are connected by Pits

1. Break strength of water is ≈ 30MPa << Pressure needed to raise water.

2. Water in a metastable state - 3MPa ≈ - 30 Atmospheres

3. Cavitation or embolismA. Controlled by

Redissolving trapped gasSecondary growth replaces xylem

Physical Challenges of Cohesion-Tension

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Physical Challenges of Cohesion-TensionPit pairs regulate embolism

Trachary element cavitation in trees infected with wilt diseases

Transpirational Water Loss

Plant Water transpiredPotato 25 gallonsWinter wheat 25 gallonsTomato 34 gallonsCorn 54 gallons

Silver Maple Tree 58 gallons hr-1

per kg corn grain 600 kg (about 158 gallons)per kg corn plant 225 kg (about 60 gallons)

Species vary in xylem hydraulic conductivity …drought tolerance

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Water Transport in MangrovesMangroves in Esmeraldas-Ecuador, Majagual Region. Florida Mangroves

Water Transport in Mangroves

ENDWater Balance of Plants