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Lecture #5 – Plant Transport. Image of waterfall. Key Concepts:. The importance of water Water potential: Ψ = P - s How water moves – gradients, mechanisms and pathways Transpiration – water movement from soil to plant to atmosphere The pressure flow model of phloem transport.

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Lecture #5 – Plant Transport

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Lecture #5 – Plant Transport

Image of waterfall


Key Concepts:

  • The importance of water

  • Water potential: Ψ = P - s

  • How water moves – gradients, mechanisms and pathways

  • Transpiration – water movement from soil to plant to atmosphere

  • The pressure flow model of phloem transport


WHY WATER???

  • Required for metabolism and cytoplasm

  • Nutrients are taken up and transported in water-based solution

  • Metabolic products are transported in water-based solution

  • Water movement through the plant affects gas exchange and leaf T

Diagram – movement of water through a tree


Water Potential (Ψ):

  • Controls the movement of water

  • A measure of potential energy

  • Water always moves from an area of HIGH water potential to an area of LOW water potential

  • Controlled by physical pressure, solute concentration, adhesion of water to cell structures and to soil particles, temperature, and gravity

Ψ = P - s


Diagram – water moves from high water potential to low water potential, sometimes toward a negative value; same next 3 slides


minus 4 is MORE NEGATIVE than minus 1


High

Low


Diagram – water potential is universal, including with waterfalls


Water Potential (Ψ):

  • Controls the movement of water

  • A measure of potential energy

  • Water always moves from an area of HIGH water potential to an area of LOW water potential

  • Controlled by physical pressure, solute concentration,adhesion of water to cell structures and to soil particles, temperature, and gravity

Ψ = P - s


P – Pressure Potential

  • By convention, set to zero in an open container of water (atmospheric pressure only)

  • In the plant cell, P can be positive, negative or zero

    • A cell with positive pressure is turgid

    • A cell with negative pressure is plasmolyzed

    • A cell with zero pressure is flaccid


Turgid

P > 0

Plasmolyzed

P < 0

Flaccid

P = 0


What are the little green things???

Micrograph – photosynthetic cells: turgid on left, plasmolyzed on right; same on next 3 slides


Turgid Plasmolyzed


Critical Thinking

  • How can you tell this tissue was artificially plasmolyzed?


Critical Thinking

  • How can you tell this tissue was artificially plasmolyzed?

  • Observe the cell on the far right – it is still turgid 


Crispy means plasmolyzed beyond the permanent wilting point 

Image – turgid plant on left, plasmolyzed on right


s

s

s

s – Solute Potential

  • s = zero for pure water

    • Pure H2O = nothing else, not a solution

  • Adding solutes ALWAYS decreases the potential energy of water

    • Some water molecules now carry a load – there is less free water


Remember, Ψ = P – s

Diagram – effect on water potential of adding salts to solutions separated by semi-permeable membrane


Ψ = P – s

Pressure can be +, -, or 0

Solutes always have a negative effect

Simplest way to calculate Ψ is by this equation


Flaccid cell in pure water – what happens???

…..what do you know???

….what do you need to know???


Ψ = ?

Flaccid cell in pure water – what happens???


Ψ = ?

P = ?.......s = ?

Flaccid cell in pure water – what happens???


Ψ = ?

P = 0.......s = about 0.7 MPa

Flaccid cell in pure water – what happens???


Ψ = -0.7 MPa

P = 0.......s = about 0.7 MPa

Flaccid cell in pure water – what happens???


Ψ = ?

Flaccid cell in pure water – what happens???

…..what do you know???

….what do you need to know???


Ψ = ?

P = ?.......s = ?

Flaccid cell in pure water – what happens???


Ψ = ?

P = 0.......s = 0

Flaccid cell in pure water – what happens???


Ψ = 0 MPa

P = 0.......s = 0

Flaccid cell in pure water – what happens???


Ψ = -0.7 MPa

?

Ψ = 0 MPa

Will water move into the cell or out of the cell???

Flaccid cell in pure water – what happens???


Ψ = -0.7 MPa

Ψ = 0 MPa

Water moves from high Ψ to low Ψ

Flaccid cell in pure water – what happens???


Ψ = -0.7 MPa

Ψ = 0 MPa

Then what happens???


Ψ = -0.7 MPa

Ψ = 0 MPa

P in cell goes up…..

Then what happens???


Ψ = 0 MPa

Ψ = 0 MPa

Dynamic equilibrium!

Then what happens???


Hands On

  • Prepare a section of plump celery and stain with T-blue

  • Examine and describe

  • Introduce a drop of salt water

  • Any change???

  • Examine the stalk of celery that was in salt water vs. one that was in fresh water

  • Explain your observations in your lab notes.


Water Movement

  • Osmosis – the diffusion of water one molecule at a time across a semi-permeable membrane

    • Controlled by both P and s

  • Bulk Flow – the movement of water in bulk – as a liquid

    • Controlled primarily by P


Osmosis

Diagram – osmosis across a semi-permeable membrane; next slide also

Critical Thinking: Where does water move by osmosis in plants???


Osmosis

Critical Thinking: Where does water move by osmosis in plants???

Cell membrane is semi-permeable


Water Movement

  • Osmosis – the diffusion of water one molecule at a time across a semi-permeable membrane

    • Controlled by both P and s

  • Bulk Flow – the movement of water in bulk – as a liquid

    • Controlled primarily by P


Water Movement

  • Osmosis – the diffusion of water one molecule at a time across a semi-permeable membrane

    • Controlled by both P and s

  • Bulk Flow – the movement of water in bulk – as a liquid

    • Controlled primarily by P – no membrane, no solute gradient!


Critical Thinking

  • Where does water move by bulk flow in plants???


Critical Thinking

  • Where does water move by bulk flow in plants???

  • Primarily in the xylem, also in phloem and in the cell walls


Routes of water transportsoil  root  stem  leaf  atmosphere

Cell Wall

Cell Membrane

Cytoplasm

Diagram – apoplast, symplast and transmembrane pathways; same on next slide


Routes of water transportsoil  root  stem  leaf  atmosphere

Cell Wall

Cell Membrane

Cytoplasm


Diagram – Casparian strip; same on next 2 slides


The Casparian Strip is a band of suberin in the transverse and radial (but not the tangential) walls of the endodermis cells

Water CANNOT PASS THROUGH the Casparian Strip

Water must GO AROUND the Casparian Strip – through the tangential face of the endodermis


The Casparian Strip is a band of suberin in the transverse and radial (but not the tangential) walls of the endodermis cells

Water CANNOT PASS THROUGH the Casparian Strip

Water must GO AROUND the Casparian Strip – through the tangential face of the endodermis


Critical Thinking

  • Apoplast water is forced into the symplast at the Casparian Strip

  • What does this mean for the water???

  • What is the function of the Casparian Strip???


Critical Thinking

  • Apoplast water is forced into the symplast at the Casparian Strip

  • What does this mean for the water???

  • It has to cross a cell membrane (easy for water!)

  • What is the function of the Casparian Strip???


Critical Thinking

  • Apoplast water is forced into the symplast at the Casparian Strip

  • What does this mean for the water???

  • It has to cross a cell membrane (easy for water!)

  • What is the function of the Casparian Strip???

  • Solute uptake is regulated at the membrane!!!


Membrane Transport(review in text if necessary)

Diagram – review of membrane transport proteins


Water is on the move


Transpiration

Diagram – transpiration

  • Movement of water from soil  plant  atmosphere

  • Controlled by HUGE water potential gradient

  • Gradient controlled by P

    • Very little s contribution

Ψ = P - s


Stomates are the Valves:as long as the stomata are open, water will move through the plant

Micrograph – stomata


Transpiration

Diagram – transpiration

  • Movement of water from soil  plant  atmosphere

  • Controlled by HUGE water potential gradient

  • Gradient controlled by P

    • Very little s contribution

Ψ = P - s


Solar Heating Drives the Process

  • Air is dry because of solar heating

    • The air molecules bounce around more which causes air masses to expand

    • Warm air has tremendous capacity to hold water vapor

  • Warm, dry air dramatically reduces the Ψ of the atmosphere

  • Daytime gradient is commonly 30+ MPa


Critical Thinking

  • Why do we have life on this planet and not the others in our solar system???


Critical Thinking

  • Why do we have life on this planet and not the others in our solar system???

  • Liquid water!

  • Why do we have liquid water???


Critical Thinking

  • Why do we have life on this planet and not the others in our solar system???

  • Liquid water!

  • Why do we have liquid water???

  • 3rd rock from the sun!

    • The Goldilocks Zone – not too hot, not too cold

    • Plus, we have enough gravity to hold our atmosphere in place

    • It’s our atmosphere that holds the warmth


Life is Random

Model – our solar system


Solar Heating Drives the Process

  • Air is dry because of solar heating

    • The air molecules bounce around more which causes air masses to expand

    • Warm air has tremendous capacity to hold water vapor

  • Warm, dry air dramatically reduces the Ψ of the atmosphere

  • Daytime gradient is commonly 30+ MPa


  • 200

  • - 30

  • 0

asymptotic

Atmospheric water potential (MPa)

0

80

100

Relative Humidity (%)


  • 200

  • - 30

  • 0

asymptotic

Atmospheric water potential (MPa)

0

80

100

Relative Humidity (%)

Critical Thinking

  • Under what conditions does atmospheric water potential approach zero???


  • 200

  • - 30

  • 0

asymptotic

Atmospheric water potential (MPa)

0

80

100

Relative Humidity (%)

Critical Thinking

  • Under what conditions does atmospheric water potential approach zero???

  • Only in the pouring rain


Gradient is HUGE

  • Pressure plumbing ~ 0.25 MPa

  • Fully inflated car tire ~ 0.2 MPa

  • Only in the pouring rain does atmospheric Ψ approach zero

  • Soil Ψ is ~ zero under most conditions

  • Remember – gradient is NEGATIVE

  • Water is pulled into plant under TENSION


Gradient is HUGE

  • Pressure plumbing ~ 0.25 MPa

  • Fully inflated car tire ~ 0.2 MPa

  • Only in the pouring rain does atmospheric Ψ approach zero

  • Soil Ψ is ~ zero under most conditions

  • Remember – gradient is NEGATIVE

  • Water is pulled into plant under TENSION


  • 200

  • - 30

  • 0

asymptotic

Atmospheric water potential (MPa)

0

80

100

Relative Humidity (%)


Gradient is HUGE

  • Pressure plumbing ~ 0.25 MPa

  • Fully inflated car tire ~ 0.2 MPa

  • Only in the pouring rain does atmospheric Ψ approach zero

  • Soil Ψ is ~ zero under most conditions

  • Remember – gradient is NEGATIVE

  • Water is pulled into plant under TENSION


The tension gradient is extreme, especially during the day

Sunday, 1 October 2006

8 am – RH = 86%

Noon – RH = 53%

4 pm – RH = 36%

8 pm – RH = 62%

5am, 23 September – 94% in light rain

Diagram – transpiration gradient from soil to atmosphere


  • 200

  • - 30

  • 0

asymptotic

Atmospheric water potential (MPa)

0

80

100

Relative Humidity (%)


Critical Thinking

  • Tension is a strong force!

  • Why doesn’t the water stream break???

  • Adhesion and cohesion

  • Why doesn’t the xylem collapse???

  • Lignin!


Critical Thinking

  • Tension is a strong force!

  • Why doesn’t the water stream break???

  • Adhesion and cohesion

  • Why doesn’t the xylem collapse???


Critical Thinking

  • Tension is a strong force!

  • Why doesn’t the water stream break???

  • Adhesion and cohesion

  • Why doesn’t the xylem collapse???

  • Lignin!!!


Diagram – transpiration gradient plus pathways


Table – water use by various crops

One hectare (2 football fields) of corn transpires about 6 million liters of water per growing season – the equivalent of 2’ of water over the entire hectare…


Transpiration is a powerful force!

  • A single broadleaf tree can move 4000 liters of water per day!!! (about 1000 gallons)

  • If humans had to drink that much water we would drink about 10 gallons per day!

  • Transpiration accounts for 90% of evapotranspiration over most terrestrial surfaces

  • Plants are the most important component of the hydrological cycle over land!!!


Tropical deforestation is leading to ecological and social disaster

Poverty, famine and forced migration

250 million victims of ecological destruction – that’s about how many people live in the US!

….and just a tiny fraction of the world’s impoverished people

Image – deforestation snaps water cycle and also results in erosion

You can help change this!!!

Panama

Guatemala


Tropical deforestation is leading to ecological and social disaster

Poverty, famine and forced migration

250 million victims of ecological destruction – that’s about how many people live in the US!

….and just a tiny fraction of the world’s impoverished people

You MUST help change this!!!

Panama

Guatemala


Social Justice

I’m not angry with you

……


Social Justice

But I do expect you to DO something!!!


Hands On

  • Examine variegated plant

    • Water with dye solution

    • What do you expect???

  • Set up experiments with white carnations

    • Vary conditions of light, temperature and air flow

    • Re-cut stems and place in dye solution – why?

  • Be sure to develop hypotheses

  • Discuss findings with team and be prepared to share conclusions with the class


Hands On

  • Work with team to develop hypotheses about how different species might vary in water transport – rely on locally available plant species, and vary species only (not environmental conditions)

  • As a class, develop several hypotheses

  • Collect plant samples

  • Set up potometers, record data

  • Summarize results and discussions in lab notes


Transpiration is a Natural Process

  • It is a physical process that occurs as long as the gradient exists and the pathway is open

  • Under adequate soil moisture conditions the enormous water loss is not a problem for the plant


Critical Thinking

  • What happens when soil moisture becomes limited???


Critical Thinking

  • What happens when soil moisture becomes limited???

  • Water stress causes stomata to close

  • What then???


Critical Thinking

  • What happens when soil moisture becomes limited???

  • Water stress causes stomata to close

  • What then???

  • Gas exchange ceases – no CO2 = no photosynthesis


What happens when soil moisture becomes limited???

  • Water stress causes stomata to close

  • Closed stomata halt gas exchange

    • P/T conflict  P/T compromise

  • Stomata are generally open during the day, closed at night

    • Abscissic acid promotes stomata closure daily, and under water stress conditions

    • Other structural adaptations limit water loss when stomata are open

    • Other metabolic pathways (C4, CAM) limit water loss


Normally, stomata open during the day and close at night in response to changes in K+ concentration in stomata guard cells

  • High [K+] does what to Ψ???

  • K+accumulation is triggered by increased light, low carbon dioxide, circadian rhythms

Micrograph – turgid guard cells; same next 4 slides


Normally, stomata open during the day and close at night in response to changes in K+ concentration in stomata guard cells

K+accumulation is triggered by increased light, low carbon dioxide, circadian rhythms

High [K+] lowers water potential in guard cells

What does water do???


Normally, stomata open during the day and close at night in response to changes in K+ concentration in stomata guard cells

K+accumulation is triggered by increased light, low carbon dioxide, circadian rhythms

High [K+] lowers water potential in guard cells

Water enters, cells swell and buckle


Normally, stomata open during the day and close at night in response to changes in K+ concentration in stomata guard cells

K+accumulation is triggered by increased light, low carbon dioxide, circadian rhythms

High [K+] lowers water potential in guard cells

Water enters, cells swell and buckle

Pore opens


Normally, stomata open during the day and close at night in response to changes in K+ concentration in stomata guard cells

K+accumulation is triggered by increased light, low carbon dioxide, circadian rhythms

High [K+] lowers water potential in guard cells

Water enters, cells swell and buckle

Pore opens

Reverse at night closes the pores


Diagram – open and closed stomata


Abscissic acid is the hormone that mediates this response

Diagram – hormone mediated stomatal opening and closing


Cellulose orientation determines shape of turgid cells

Diagram – spoke-like orientation of cellulose microfibrils


What happens when soil moisture becomes limited???

  • Water stress causes stomata to close

  • Closed stomata halt gas exchange

    • P/T conflict  P/T compromise

  • Stomata are generally open during the day, closed at night

    • Abscissic acid promotes stomata closure daily, and under water stress conditions

    • Other structural adaptations limit water loss when stomata are open

    • Other metabolic pathways (C4, CAM) limit water loss


Micrograph – location of stomatal gradient

This is the gradient that counts


Images – structural adaptations to dry environments


Spatial separation helps C4 plants be more efficient in hot climates

Temporal separation does the same for CAM plants

Both use an enzyme that can’t fix O2to first capture CO2

Both adaptations allow photosynthesis to proceed with stomata largely closed during the day

Images and diagrams – metabolic adaptations to dry environments


Hands On

  • Work with your team to make hypotheses about stomata number and placement on various types of leaves

  • Use nail polish to make impressions of stomata

    • Put a tab of paper under the polish

    • Make a dry mount of the impression

  • Count stomata in the field of view and estimate the number of stomata per mm2

  • Be prepared to discuss your findings


Phloem Transport

  • Most of phloem sap is water (70% +)

  • Solutes in phloem sap are mostly carbohydrates, mostly sucrose for most plant species

  • Other solutes (ATP, mineral nutrients, amino acids, hormones, secondary metabolites, etc) can also be translocated in the phloem

  • Phloem transport driven by water potential gradients, but the gradients develop due to active transport – both P and s are important


The Pressure Flow Model For Phloem Transport

  • Xylem transport is uni-directional, driven by solar heating

  • Phloem flow is multi-directional, driven by active transport – source to sink

Diagram – pressure flow model of phloem flow; this diagram is repeated throughout this section


The Pressure Flow Model For Phloem Transport

  • Sources can be leaves, stems or roots

  • Sinks can be leaves, stems, roots or reproductive parts (especially seeds and fruits)


The Pressure Flow Model For Phloem Transport

  • Sources and sinks vary depending on metabolic activity, which varies daily and seasonally

  • Most sources supply the nearest sinks, but some take priority


Active transport (uses ATP) builds high sugar concentration in sieve cells adjacent to source

Diagram – the transport proteins that actively transport sucrose into the phloem cells from the leaf cells


The Pressure Flow Model For Phloem Transport

  • High [solute] at source end does what to Ψ???


Critical Thinking

  • Remember the water potential equation

    Ψ = P - s

  • What happens to Ψ as s increases???


Critical Thinking

  • Remember the water potential equation

    Ψ = P - s

  • What happens to Ψ as s increases???

  • Water potential is reduced

  • This is what happens at the source end of the phloem


The Pressure Flow Model For Phloem Transport

  • High [solute] at source end decreases Ψ

  • What does water do???


Critical Thinking

  • Remember the water potential equation

    Ψ = P - s

  • What does water do when Ψ decreases???


Critical Thinking

  • Remember the water potential equation

    Ψ = P - s

  • What does water do when Ψ decreases???

  • Water moves toward the area of lower water potential

  • This is what happens at the source end of the phloem

  • Where does the water come from???


Critical Thinking

  • Remember the water potential equation

    Ψ = P - s

  • What does water do when Ψ decreases???

  • Water moves toward the area of lower water potential

  • This is what happens at the source end of the phloem

  • Where does the water come from???

  • The adjacent xylem – remember structure and function are related!


The Pressure Flow Model For Phloem Transport

  • High [solute] at source end decreases Ψ

  • Water moves into the source end of the phloem

  • What does this do to P at the source end?


Critical Thinking

  • What will happen to water pressure in any plant cell as water moves in???


Critical Thinking

  • What will happen to water pressure in any plant cell as water moves in???

  • It increases

  • Why???


Critical Thinking

  • What will happen to water pressure in any plant cell as water moves in???

  • It increases

  • Why???

  • The cell wall limits expansion – it “pushes back”


The Pressure Flow Model For Phloem Transport

  • High [solute] at source end decreases Ψ

  • Water moves into the source end of the phloem

    • This increases the pressure


The Pressure Flow Model For Phloem Transport

  • Increased pressure at source end causes phloem sap to move to any area of lower Ψ= sinks


The Pressure Flow Model For Phloem Transport

  • At sink end, the sugars are removed by metabolism, by conversion to starch, or by active transport


The Pressure Flow Model For Phloem Transport

  • What then happens to the Ψ at the sink end of the phloem???


Critical Thinking

  • Remember the water potential equation

    Ψ = P - s

  • What happens to Ψ as s decreases???


Critical Thinking

  • Remember the water potential equation

    Ψ = P - s

  • What happens to Ψ as s decreases???

  • Water potential is increased

  • This is what happens at the sink end of the phloem


The Pressure Flow Model For Phloem Transport

  • Ψ goes up at the sink end of the phloem

  • What does water do???


Critical Thinking

  • Remember the water potential equation

    Ψ = P - s

  • What does water do when Ψ increases???


Critical Thinking

  • Remember the water potential equation

    Ψ = P - s

  • What does water do when Ψ increases???

  • Water moves away from the area of higher water potential

  • This is what happens at the sink end of the phloem

  • Where does the water go???


Critical Thinking

  • Remember the water potential equation

    Ψ = P - s

  • What does water do when Ψ increases???

  • Water moves away from the area of higher water potential

  • This is what happens at the sink end of the phloem

  • Where does the water go???

  • The adjacent xylem – remember structure and function are related!


The Pressure Flow Model For Phloem Transport

  • Ψ goes up at the sink end of the phloem

  • Water leaves the phloem at the sink end, thus reducing Ψ

  • Adjacent xylem provides and accepts the water


The Pressure Flow Model For Phloem Transport

  • Thus the phloem sap moves – from source to sink

    • Some xylem water is cycled into and out of the phloem in the process


The Pressure Flow Model For Phloem Transport

  • Active transport is always involved at the source end, but only sometimes at the sink end


Critical Thinking

  • What about the structure of the sieve cells facilitates the movement of phloem sap???

Micrograph – sieve cells; same next slide


Critical Thinking

  • What about the structure of the sieve cells facilitates the movement of phloem sap???

  • The open sieve plate

  • The lack of major organelles


The Pressure Flow Model For Phloem TransportQuestions???


Key Concepts: Questions???

  • The importance of water

  • Water potential: Ψ = P - s

  • How water moves – gradients, mechanisms and pathways

  • Transpiration – water movement from soil to plant to atmosphere

  • The pressure flow model of phloem transport


Hands On

  • For tomorrow – bring some soil from your yard and/or garden

  • Put it in a clear, water-tight container (glass jar is easiest)


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