Plant water relations

11

Principles of plant physiology Principles of plant physiology

Chapter TwoChapter TwoPlant-Water RelationsPlant-Water Relations

Beira Hailu Meressa, JUCAVM Beira Hailu Meressa, JUCAVM 20082008

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Water is absolutely essential for all living Water is absolutely essential for all living

organisms.organisms.

Why is water importantWhy is water important? ?

–Living cells are composed of 70-95 % waterLiving cells are composed of 70-95 % water

–Life absolutely depends on the properties of Life absolutely depends on the properties of

water.water.

–Life probably evolved in water. Life probably evolved in water.

–water covers ¾ of the earthwater covers ¾ of the earth

–When organisms go dormant, they loose When organisms go dormant, they loose

most of their watermost of their water

–Limiting resource for crop productivity Limiting resource for crop productivity

AgricultureAgriculture

– Water naturally exists in all three physicalWater naturally exists in all three physical


33

Tom Robbins, “Tom Robbins, “Even Cowgirls Get the Even Cowgirls Get the

Blues”Blues” • Water-the ace of elementsWater-the ace of elements

– Water dives from the clouds without Water dives from the clouds without parachute, wings parachute, wings

or safety net or safety net

– Water runs over the steepest precipice Water runs over the steepest precipice and blinks not a and blinks not a

lash lash

– Water is Water is buried and rises againburied and rises again; water ; water walks on firewalks on fire and and

fire gets the blisters fire gets the blisters

– Stylishly composed in any situation - solid, gas or liquid Stylishly composed in any situation - solid, gas or liquid

– speaking in penetrating dialects understood by all speaking in penetrating dialects understood by all

things - animal, vegetable or mineral things - animal, vegetable or mineral

– water travels intrepidly through three dimensions:water travels intrepidly through three dimensions:

• sustaining, sustaining,

• destroying, and destroying, and

• creatingcreating Beira Hailu Meressa, JUCAVM Beira Hailu Meressa, JUCAVM

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1.1 Properties of water important to 1.1 Properties of water important to plantplant

Life is absolutely dependent on the properties of water

Pure water: Colourless and has no smell and taste The hidden qualities of water make it a most interesting:

Physicochemical properties of water

I. Water is polar Water's unique properties due to its simple

composition and structure

hydrogen atoms are "attached" to one side of the hydrogen atoms are "attached" to one side of the

oxygen atomoxygen atom

Positive and negative charge Positive and negative charge

The separation between negative and positive charges The separation between negative and positive charges

creates a polar molecule creates a polar molecule Beira Hailu Meressa, JUCAVM Beira Hailu Meressa, JUCAVM

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55Beira Hailu Meressa, JUCAVM Beira Hailu Meressa, JUCAVM

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II. Hydrogen bondsII. Hydrogen bonds A weak bond that forms between a hydrogen A weak bond that forms between a hydrogen

atom and electronegative atomatom and electronegative atom Water can bond both to itself and to other Water can bond both to itself and to other

moleculesmolecules responsible for the responsible for the thermal properties of water

A.A. Water's high specific heat capacity Water's high specific heat capacity the amount of heat energy that must be the amount of heat energy that must be added added

or lostor lost to change the temperature of one gram to change the temperature of one gram

of water by 1°Cof water by 1°C

water can absorb a lot of heat before water water can absorb a lot of heat before water

molecules can move faster or begin to get hot molecules can move faster or begin to get hot

much more energy is needed disrupt the much more energy is needed disrupt the

hydrogen bonds in liquid waterhydrogen bonds in liquid water

• water resists rapid temperature fluctuations water resists rapid temperature fluctuations

• adding stability to earth's environments adding stability to earth's environments Beira Hailu Meressa, JUCAVM Beira Hailu Meressa, JUCAVM

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http://www.psc.edu/MetaCenter/MetaScience/Articles/Jordan/Jordan.html#Jordan-bio

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B.B. high heat of vaporizationhigh heat of vaporization the energy needed to change the phase of a liquid

to a gas (44 kJ/mole)

it takes a great deal of energy to break a molecule

free of its liquid partners

– water resists evaporation

– causes a cooling effect on plants

C. High latent heat of fusionC. High latent heat of fusion takes lots of energy to convert from solid to a takes lots of energy to convert from solid to a

liquid stateliquid state

lot of energy must be released by water to freezelot of energy must be released by water to freeze

Thus water resists freezing Thus water resists freezing Beira Hailu Meressa, JUCAVM Beira Hailu Meressa, JUCAVM 20082008

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D. Capillary actionD. Capillary action

i.i. CohesionCohesion hydrogen bonds between water molecules make hydrogen bonds between water molecules make

liquid water self-stickyliquid water self-sticky

The hydrogens of one water molecule are The hydrogens of one water molecule are

attracted to the oxygen from other water attracted to the oxygen from other water

moleculesmolecules makes water bead up more on a surface makes water bead up more on a surface


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ii. Adhesionii. Adhesion

the attraction of water molecules to non-water the attraction of water molecules to non-water

hydrophilic substanceshydrophilic substances

This property of water gives it the ability to This property of water gives it the ability to

climb the wall of any container it is in climb the wall of any container it is in

The top of the water column assumes a The top of the water column assumes a uu-shape-shape


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iii. Surface tensioniii. Surface tension Water has a very high surface tension Water has a very high surface tension

the cohesion of water molecules to each the cohesion of water molecules to each

other and to the water molecules below at other and to the water molecules below at

the surface of a body of water the surface of a body of water

It is a measure of how difficult it is to It is a measure of how difficult it is to

break the surface of a liquid break the surface of a liquid

Thus water acts as thought it has a skin Thus water acts as thought it has a skin

because of cohesion because of cohesion

Plants take advantage of capillary action Plants take advantage of capillary action

to pull water from the root into themselves to pull water from the root into themselves



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III. Universal solventIII. Universal solvent

dissolves more different kinds of dissolves more different kinds of

substances than any other solventsubstances than any other solvent

Hydrogen bonds make water an Hydrogen bonds make water an

excellent solvent excellent solvent

wherever water goes, either through wherever water goes, either through

the ground or through plant body, it the ground or through plant body, it

takes along valuable chemicals, takes along valuable chemicals,

minerals, and nutrients. minerals, and nutrients.


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Water is a versatile solvent owing to its polarityWater is a versatile solvent owing to its polarity

Ionic compounds dissolve in water. Ionic compounds dissolve in water.

• Charged regions of polar water molecules Charged regions of polar water molecules

have an electrical attraction to charged ions have an electrical attraction to charged ions

• Water surrounds individual ions, separating Water surrounds individual ions, separating

and shielding them from one another and shielding them from one another

Polar compounds in general, are water-soluble. Polar compounds in general, are water-soluble.

Charged regions of polar water molecules have an Charged regions of polar water molecules have an

affinity for opposite charged regions of other polar affinity for opposite charged regions of other polar

molecules molecules


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IV. Water is a liquid at physiological IV. Water is a liquid at physiological

temperaturetemperature high boiling and melting point when compared high boiling and melting point when compared

to other similar sized moleculesto other similar sized molecules

Life exists between 0 and 100Life exists between 0 and 100ooC C

<0<0ooC too low to permit significant chemistry for C too low to permit significant chemistry for

metabolismmetabolism

> > 100 oC tends to disrupt bonds100 oC tends to disrupt bonds


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VV. Transparent to Light. Transparent to Light

Chloroplasts inside a cell are Chloroplasts inside a cell are

surrounded by watersurrounded by water

Photosynthesis if water were Photosynthesis if water were

opaque ????opaque ????

the penetration of light in water the penetration of light in water

determines the distribution of determines the distribution of

aquatic plantsaquatic plants Beira Hailu Meressa, JUCAVM Beira Hailu Meressa, JUCAVM

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VI. Chemically inertVI. Chemically inert::

water does not react unless enzymaticcally water does not react unless enzymaticcally

designed to react designed to react

VII. Affect the shape, stability and VII. Affect the shape, stability and

properties of biological molecules.properties of biological molecules.


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Role of Water in plantsRole of Water in plants Solvent for the uptake and transport of molecules Solvent for the uptake and transport of molecules

Water is major component of cellsWater is major component of cells

Good medium for biochemical reactionsGood medium for biochemical reactions

React in many biochemical reaction React in many biochemical reaction

Provide structural support via turger pressure in Provide structural support via turger pressure in

leavesleaves

The medium for the transfer of plant gamete The medium for the transfer of plant gamete

Plant movements are the result of water moving in Plant movements are the result of water moving in

to and out of those plants to and out of those plants

Temperature stabilization Temperature stabilization

Plays a role in cell elongation and growthPlays a role in cell elongation and growth Beira Hailu Meressa, JUCAVM Beira Hailu Meressa, JUCAVM

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2.22.2 Concepts of water Concepts of water

potentialpotential diverse multivarious role of water in diverse multivarious role of water in

plant function plant function

Consider both Consider both

the state of water andthe state of water and

rate of movement of water in plantsrate of movement of water in plants

determined by values of water determined by values of water

potential or gradient in water potentialpotential or gradient in water potential


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Defn. Defn. is the chemical potential of water in is the chemical potential of water in

a a solutionsolution compared to compared to pure waterpure water at the at the

same temperature and pressuresame temperature and pressure

a measure of the free energy of watera measure of the free energy of water

the energy available to move the energy available to move water from one from one

place to another place to another

with out temperature change with out temperature change

Determines direction of water movement Determines direction of water movement


http://en.wikipedia.org/wiki/Water

2020

Values of water potential:Values of water potential:

Pure water has a defined water Pure water has a defined water

potential of zero. potential of zero.

However it is possible for the However it is possible for the

water potential to be water potential to be positivepositive or or

negativenegative Beira Hailu Meressa, JUCAVM Beira Hailu Meressa, JUCAVM

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Units of measurementUnits of measurement: :

measured in units of atmospheric measured in units of atmospheric

pressure:pressure:

Pascal (MPa)Pascal (MPa)

Pounds force/square inchPounds force/square inch

Bars Bars

dynes/square cmdynes/square cm


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Equation of water potentialEquation of water potential

In a simple system In a simple system

• Pressure potential (Ψp) Pressure potential (Ψp)

• Osmotic potential(Ψπ) or solute potential Osmotic potential(Ψπ) or solute potential

(Ψ(Ψss).).

Ψ = Ψp + Ψπ (Ψs)Ψ = Ψp + Ψπ (Ψs)

Complex Systems Complex Systems

Gravity potential (Ψg)Gravity potential (Ψg)

Matric potential (Ψm)Matric potential (Ψm)

Ψ = Ψπ + Ψp + Ψg + Ψm Ψ = Ψπ + Ψp + Ψg + Ψm Beira Hailu Meressa, JUCAVM Beira Hailu Meressa, JUCAVM

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A. Solute potential / osmotic potential/A. Solute potential / osmotic potential/

A measure of tendency for HA measure of tendency for H22O to cross O to cross

a selectively permeable membranea selectively permeable membrane

from low concentration to high from low concentration to high

concentration of soluteconcentration of solute

Pure water has a solute potential of Pure water has a solute potential of

zerozero Beira Hailu Meressa, JUCAVM Beira Hailu Meressa, JUCAVM

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Addition of solute makes the value of Addition of solute makes the value of

solute potential negative solute potential negative

decreasesdecreases the free energy of water the free energy of water

Negative contribution to water Negative contribution to water

potential potential

Beira Hailu Meressa, JUCAVM Beira Hailu Meressa, JUCAVM

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Van’t Hoff equationVan’t Hoff equation

Shows relationship of solute Shows relationship of solute

concentration (in molality) to solute concentration (in molality) to solute

potentialpotential

Ψπ = − miRTΨπ = − miRT

m, m, concentration of the soluteconcentration of the solute

i i , ionization constant of the solute (1 for , ionization constant of the solute (1 for

glucose, 2 for NaCl)glucose, 2 for NaCl)

RR , ideal gas constant , ideal gas constant

T T , temperature (K), temperature (K)Beira Hailu Meressa, JUCAVM Beira Hailu Meressa, JUCAVM 20082008

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For example, For example,

when a solute is dissolved in when a solute is dissolved in

waterwater

the water molecules are less the water molecules are less

likely to likely to

diffuse away via osmosis than diffuse away via osmosis than

when when there is no solutethere is no solute

SSSSSS

WWWW

SSSSSS

WWWW SSSSSS

WWWW

High free energy of water ?????

A B


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regions o

f hig

her

regions o

f hig

her

water p

otentia

l

water p

otentia

l

•less n

egative;

less n

egative;

•high fr

ee energy;

high fr

ee energy;

•more p

ositive;

more p

ositive;

•less so

lute

)

less so

lute

)

Regions of lo

wer

Regions of lo

wer

water p

otentia

l

water p

otentia

l

•more

negative,

more

negative,

•less fr

ee energy,

less fr

ee energy,

•less posit

ive,

less posit

ive,

•high solute

high solute

Water movement


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B. Pressure potential (turgor pressure) (Ψp)B. Pressure potential (turgor pressure) (Ψp)

• the hydrostatic pressure produced by a the hydrostatic pressure produced by a

solution in a space divided by a solution in a space divided by a

differentially permeable membranedifferentially permeable membrane

• due to a differential in the concentrations due to a differential in the concentrations

of solute of solute

• is increased as water enters a plant cell is increased as water enters a plant cell

• It is usually positiveIt is usually positive

• may be negative (tension) as in the xylemmay be negative (tension) as in the xylem


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• When a biological cell is in a hypotonic When a biological cell is in a hypotonic

environment (the cell interior contains environment (the cell interior contains

a lower concentration of water)a lower concentration of water)

• water flows across the cell membrane water flows across the cell membrane

into the cellinto the cell

• expand due to an increases in the expand due to an increases in the

total amount of water inside the cell total amount of water inside the cell

• exerts a pressure on the cell wall exerts a pressure on the cell wall


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Hypotonic



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C. Matric potential C. Matric potential (Ψm)(Ψm)

• due to the force of attraction of water for due to the force of attraction of water for

colloidal, charged surfaces colloidal, charged surfaces

• It is negative because it reduces the ability It is negative because it reduces the ability

of water to move of water to move

• In large volumes of water it is very small and In large volumes of water it is very small and

usually ignored usually ignored

• Water adheres electrostatically to solid Water adheres electrostatically to solid

hydrophilic surface hydrophilic surface

• Capillary rise is associated with matrix Capillary rise is associated with matrix

potential. potential. Beira Hailu Meressa, JUCAVM Beira Hailu Meressa, JUCAVM 20082008

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D.D. Gravitational potential (ΨgΨg)

• The potential energy associated with moving The potential energy associated with moving

water to heightwater to height

• when referring to the top of tall treeswhen referring to the top of tall trees

ΨgΨg= = ρgh ρgh

= 1000kg/M= 1000kg/M33 *9.8 m/S *9.8 m/S22 *h(m) *h(m)

= 0.01Mpa/m= 0.01Mpa/m

water in leaves at the top of a 100m tall tree water in leaves at the top of a 100m tall tree

suck water. suck water.


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Basic principles of water potentialBasic principles of water potential

• Water potential of pure water is zero, open to Water potential of pure water is zero, open to

the atmospherethe atmosphere

• Water potential in intact plant tissue is Water potential in intact plant tissue is

usually negative because of the large usually negative because of the large

quantities of dissolved solutes in cellsquantities of dissolved solutes in cells

• The addition of solute decreases water The addition of solute decreases water

potentialpotential

• The addition of pressure increases water The addition of pressure increases water

potentialpotential

• Pure water, under external pressure is Pure water, under external pressure is

positivepositiveBeira Hailu Meressa, JUCAVM Beira Hailu Meressa, JUCAVM

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• Plant cells will gain or lose water to Plant cells will gain or lose water to intercellular fluids depending upon intercellular fluids depending upon their water potentialtheir water potential– A flaccid cell placed in a A flaccid cell placed in a

hyperosmotic solution (low Ψw)hyperosmotic solution (low Ψw)

– lose water by osmosis lose water by osmosis

– the cell will plasmolyze the cell will plasmolyze

– protoplast moves away from cell protoplast moves away from cell wallwall


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• Gives a measure of water statusGives a measure of water status

– Leaves of well-watered plants have Leaves of well-watered plants have

water potential ranging from -0.2 and water potential ranging from -0.2 and

-0.6Mpa-0.6Mpa

– Leaves of plants in arid climates poses Leaves of plants in arid climates poses

water potential between -2 and -5water potential between -2 and -5


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Effect on physiological changesEffect on physiological changes


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2.3 Water movement in cells and 2.3 Water movement in cells and tissuestissues

Mechanisms of movementMechanisms of movement 1.1. Bulk (or Mass) FlowBulk (or Mass) Flow

• mass movement of molecules in response mass movement of molecules in response

to a to a pressure gradientpressure gradient

• The molecules move from high to low The molecules move from high to low

pressure pressure

• functions in long-distance transport functions in long-distance transport

• is usually along the vertical axis of the is usually along the vertical axis of the

plantplant

– Vascular tissues Vascular tissues Beira Hailu Meressa, JUCAVM Beira Hailu Meressa, JUCAVM

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2. Diffusion2. Diffusion

• the net random movement of individual the net random movement of individual

molecules driven by random thermal motionmolecules driven by random thermal motion

• is rapid over short distancesis rapid over short distances

a region of a region of high high concentrationconcentration// high chemical high chemical potentialpotentialarea of high free energy low free energy

region of low region of low concentratioconcentratio

nn// low low chemical chemical potentialpotential


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•Molecules move until they reach dynamic

equilibrium

•At equilibrium the net movement stops

•the molecules continue to move randomlyBeira Hailu Meressa, JUCAVM Beira Hailu Meressa, JUCAVM

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Factors influencing the rate of Factors influencing the rate of

diffusiondiffusion i. Concentration Gradient i. Concentration Gradient

• Solutes move from an area of high Solutes move from an area of high

concentration to one of lower concentration to one of lower

concentrationconcentration

• Fick’s Law : relates the rate of diffusion Fick’s Law : relates the rate of diffusion

to the concentration gradient (C1–C2) to the concentration gradient (C1–C2)

and resistance (r)and resistance (r)

Js = (C1-C2)/r Js= flux densityJs = (C1-C2)/r Js= flux densityBeira Hailu Meressa, JUCAVM Beira Hailu Meressa, JUCAVM

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• The rate of diffusion is directly proportional The rate of diffusion is directly proportional

to the concentration gradientto the concentration gradient

• The greater the difference in concentration The greater the difference in concentration

between two areas, the greater the rate of between two areas, the greater the rate of

diffusiondiffusion

• if the gradient is zero, there will be no net if the gradient is zero, there will be no net

diffusiondiffusion

• The greater the resistance to diffusion, the The greater the resistance to diffusion, the

lower the rate of diffusionlower the rate of diffusion Beira Hailu Meressa, JUCAVM Beira Hailu Meressa, JUCAVM 20082008

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ii. Molecular Speedii. Molecular Speed

• atoms and molecules are always in motion at atoms and molecules are always in motion at

temperatures above absolute zero temperatures above absolute zero

• Molecular speed is directly proportional to Molecular speed is directly proportional to

temperature temperature

• At room temperature, the average velocity of At room temperature, the average velocity of

a molecule is =2 km/sec a molecule is =2 km/sec

• indirectly related to molecular weight indirectly related to molecular weight


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iii. Temperature iii. Temperature

• increases the rate of molecular increases the rate of molecular

movement movement

• therefore, increases the rate of therefore, increases the rate of

diffusiondiffusion

iv. Pressureiv. Pressure

increases speed of moleculesincreases speed of molecules

increase the rate of diffusionincrease the rate of diffusionBeira Hailu Meressa, JUCAVM Beira Hailu Meressa, JUCAVM 20082008

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V. Solute effect on the chemical potential V. Solute effect on the chemical potential of the solventof the solvent

• Solute particles decrease the free Solute particles decrease the free

energy of a solvent energy of a solvent

• factor is the number of particles factor is the number of particles

• Mole fraction of solventMole fraction of solvent = Number of solvent = Number of solvent

molecules/total number of solvent molecules + molecules/total number of solvent molecules +

solute molecules solute molecules

Water moves from an area of higher Water moves from an area of higher

mole fractionmole fraction Beira Hailu Meressa, JUCAVM Beira Hailu Meressa, JUCAVM

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3.Osmosis3.Osmosis • This is a specialized case of diffusion This is a specialized case of diffusion

• the diffusion of a solvent across a biological the diffusion of a solvent across a biological

selectively permeable membraneselectively permeable membrane

• Movement is driven by the sum of a Movement is driven by the sum of a

concentration gradient and pressure gradientconcentration gradient and pressure gradient

• Osmosis to occur :Osmosis to occur :

– two solutions of the same solventtwo solutions of the same solvent

– separated with selectively permeable membrane separated with selectively permeable membrane

– pressure and concentration gradientpressure and concentration gradient



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4. Dialysis:4. Dialysis:

• This is another specialized case This is another specialized case of diffusion of diffusion

• it is the diffusion of solute it is the diffusion of solute across a semi-permeable across a semi-permeable membrane. membrane.

High salt

Low salt buffer


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Transport of water in Transport of water in PlantsPlants

Levels of transport in plants:Levels of transport in plants:

Movement of water and solutes into and Movement of water and solutes into and

out of individual cells out of individual cells

Localized transport of material from cell Localized transport of material from cell

to cell at the level of tissues and organs to cell at the level of tissues and organs

Long-distance transport of sap Long-distance transport of sap

throughout the vascular tissues at the throughout the vascular tissues at the

whole-plant level whole-plant level Beira Hailu Meressa, JUCAVM Beira Hailu Meressa, JUCAVM 20082008

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Water diffuse from the Water diffuse from the soilsoil to the to the

plant rootplant root and then to the and then to the airair : :

Water potential gradient is established Water potential gradient is established

between the root cell sap and the soil between the root cell sap and the soil

solutionsolution

Water potential may be established by:Water potential may be established by:

• Increasing the concentration of Increasing the concentration of

solutessolutes..

– Water potential of soil solution is Water potential of soil solution is

highest than airhighest than airBeira Hailu Meressa, JUCAVM Beira Hailu Meressa, JUCAVM 20082008

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•Physical state of water Physical state of water

– highest whenhighest when water is a liquid water is a liquid

and and

– lowest when water is a gas in lowest when water is a gas in

airair. .

intimate contact between root intimate contact between root

hair and soil particleshair and soil particles



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Water Transport in the RootWater Transport in the Root

Water is taken in to the root Water is taken in to the root hairhair

move into and within the plant move into and within the plant root in a variety of routs root in a variety of routs


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A.A. Apoplastic path wayApoplastic path way

movement of water through intercellular movement of water through intercellular

spaces and cell wall spaces and cell wall

continuum formed between the continuous continuum formed between the continuous

matrix of cell wallsmatrix of cell walls

Water and solute move without entering a Water and solute move without entering a

cellcell

involves the non-living vascular tissue involves the non-living vascular tissue

due to capillary action due to capillary action Beira Hailu Meressa, JUCAVM Beira Hailu Meressa, JUCAVM 20082008

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blocked by the casparian strip at blocked by the casparian strip at

endodermis endodermis

Impermeable band of suberin Impermeable band of suberin

inside walls of endodermal cells inside walls of endodermal cells

regulates the quantity and type regulates the quantity and type

of minerals and ions reach the of minerals and ions reach the

xylem xylem


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B. Syplastic path wayB. Syplastic path way

the continuum of cytoplasm the continuum of cytoplasm

within a plant tissuewithin a plant tissue

formed by the plasmodesmata formed by the plasmodesmata

which pass through pores in the which pass through pores in the

cell walls. cell walls.

responsible in order water and responsible in order water and

minerals to reach the xylem. minerals to reach the xylem.

This path way involves the living This path way involves the living

part of the cellpart of the cell

water moves by osmosiswater moves by osmosisBeira Hailu Meressa, JUCAVM Beira Hailu Meressa, JUCAVM

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C. Transmembrane path wayC. Transmembrane path way

• Water sequentially moves from one Water sequentially moves from one

cell to the next cell cell to the next cell

• by repeatedly crossing plasma by repeatedly crossing plasma

membranes and cell walls.membranes and cell walls.

• NB. Water and solute molecules can NB. Water and solute molecules can

move move

– by any one of these routes orby any one of these routes or

– a combination through switching from one a combination through switching from one

to anotherto another..Beira Hailu Meressa, JUCAVM Beira Hailu Meressa, JUCAVM

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6161Apoplast

Symplast

Transmembrane



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The xylem pathwayThe xylem pathway

1. Root Pressure1. Root Pressure

• This a push forceThis a push force

• Generated as solute accumulates in the xylemGenerated as solute accumulates in the xylem

Due to the root's active absorption of dissolved Due to the root's active absorption of dissolved

nutrients nutrients

the water in the soil tends to be lower in the water in the soil tends to be lower in

solutes than the water inside the root's cells solutes than the water inside the root's cells

solute potential gradient developssolute potential gradient developsBeira Hailu Meressa, JUCAVM Beira Hailu Meressa, JUCAVM

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water flows into the roots through osmosis water flows into the roots through osmosis

and and osmotic pressureosmotic pressure increases increases

called root pressure called root pressure

a mechanism used by vascular plants to a mechanism used by vascular plants to

transport water through the xylem to the transport water through the xylem to the

plant's higher reachesplant's higher reaches

only provide modest push water up the stemonly provide modest push water up the stem

is not enough to account for the movement of is not enough to account for the movement of

water to leaves at the top of treeswater to leaves at the top of trees


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(A)

(B)

Mineral ions


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2. TACT Mechanism2. TACT Mechanism

The transpirational pull on the xylem The transpirational pull on the xylem

sap is transmitted to the soil solutionsap is transmitted to the soil solution

Four forces combine to transport water Four forces combine to transport water

solutions from the roots through the solutions from the roots through the

xylem elements, and into the leavesxylem elements, and into the leaves

These TACT forces are: These TACT forces are:

• Transpiration Transpiration

• Adhesion Adhesion

• Cohesion Cohesion

• Tension Tension Beira Hailu Meressa, JUCAVM Beira Hailu Meressa, JUCAVM

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i. Transpirationi. Transpiration

involves the pulling of water up involves the pulling of water up

through the xylem of a plant through the xylem of a plant

Utilize the energy of evaporation Utilize the energy of evaporation

and the tensile strength of water. and the tensile strength of water.


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ii. Adhesionii. Adhesion

is the attractive force between water is the attractive force between water

molecules and other substances.molecules and other substances.

both water and cellulose are polar both water and cellulose are polar

moleculesmolecules

there is a strong attraction for water to there is a strong attraction for water to

the hydrophilic walls of xylem cells the hydrophilic walls of xylem cells

The small diameter of vessels and The small diameter of vessels and

tracheids is important to the adhesion tracheids is important to the adhesion

effecteffect Beira Hailu Meressa, JUCAVM Beira Hailu Meressa, JUCAVM 20082008

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iii. Cohesioniii. Cohesion

is the attractive force between molecules of is the attractive force between molecules of

the same substance. the same substance.

high cohesive force due to the 4 hydrogen high cohesive force due to the 4 hydrogen

bonds bonds

water's cohesive force within xylem give it a water's cohesive force within xylem give it a

tensile strength equivalent to that of a steel tensile strength equivalent to that of a steel

wire of similar diameter.wire of similar diameter.

Cohesion of water allows for the pulling of Cohesion of water allows for the pulling of

water from the top of the plant without water from the top of the plant without

breaking the "chain".breaking the "chain".Beira Hailu Meressa, JUCAVM Beira Hailu Meressa, JUCAVM

20082008

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iv. Tensioniv. Tension

a stress placed on an object by a pulling a stress placed on an object by a pulling

force. force.

is created by the surface tension which is created by the surface tension which

develops in the leaf's air spaces. develops in the leaf's air spaces.

The upward pull of sap causes tension (negative The upward pull of sap causes tension (negative

pressure) in xylempressure) in xylem

decreases water potential and decreases water potential and

allows passive flow of water from soil into stele allows passive flow of water from soil into stele Beira Hailu Meressa, JUCAVM Beira Hailu Meressa, JUCAVM

20082008


20082008

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Soil factors affecting water Soil factors affecting water absorptionabsorption

Reading AssignmentReading Assignment

Until Next class Until Next class


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1.1. What is the importance of the water What is the importance of the water

potential concept in plant physiology? potential concept in plant physiology?

What are the components of water What are the components of water

potential? potential?

2.2. List three unique properties of water that List three unique properties of water that

make it such a good for cellular make it such a good for cellular

functioning functioning

3.3. Can plant cells have negative turgor Can plant cells have negative turgor

pressure values? Explain pressure values? Explain

4.4. Describe the casparian strip and its Describe the casparian strip and its

function. function.
