Lecture #5 – Plant Transport

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 through plant also essential component of hydrological cycle.

Ψ = P - s 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

Ψ = P - s 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 – 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 s

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???

Flaccid cell in pure water – what happens??? 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??? …..what do you know??? Ψ = ? ….what do you need to know???

Flaccid cell in pure water – what happens??? Ψ = ? P = ?.......s = ?

Flaccid cell in pure water – what happens??? P = 0.......s = 0 Ψ = ?

Flaccid cell in pure water – what happens??? Ψ = 0 MPa P = 0.......s = 0

Flaccid cell in pure water – what happens??? Ψ = 0 MPa Ψ = -0.7 MPa ? Will water move into the cell or out of the cell???

Flaccid cell in pure water – what happens??? Ψ = 0 MPa Water moves from high Ψ to low Ψ Ψ = -0.7 MPa

Then what happens??? Ψ = 0 MPa Ψ = -0.7 MPa

Then what happens??? Ψ = 0 MPa Ψ = -0.7 MPa P in cell goes up…..

Then what happens??? Ψ = 0 MPa Dynamic equilibrium!

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

Critical Thinking: Where does water move by osmosis in plants??? 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 transport soil  root  stem  leaf  atmosphere Cell Wall Cell Membrane Cytoplasm Diagram – apoplast, symplast and transmembrane pathways; same on next slide Water moves readily through all these cell components

Routes of water transport soil  root  stem  leaf  atmosphere Cell Wall Cell Membrane Cytoplasm

Diagram – Casparian strip; same on next 2 slides

Water CANNOT PASS THROUGH the Casparian Strip 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

Water CANNOT PASS THROUGH the Casparian Strip 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 Movement of water from soil  plant  atmosphere 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 Movement of water from soil  plant  atmosphere 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 Just because we can’t see air does not mean it’s not there

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 Solar heating drives the process

Atmospheric water potential (MPa) Relative Humidity (%) 100 80 200 - 30 asymptotic

Atmospheric water potential (MPa) Critical Thinking Under what conditions does atmospheric water potential approach zero??? Atmospheric water potential (MPa) Relative Humidity (%) 100 80 200 - 30 asymptotic

Atmospheric water potential (MPa) Critical Thinking Under what conditions does atmospheric water potential approach zero??? Only in the pouring rain Atmospheric water potential (MPa) Relative Humidity (%) 100 80 200 - 30 asymptotic

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

Atmospheric water potential (MPa) Relative Humidity (%) 100 80 200 - 30 asymptotic

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

Atmospheric water potential (MPa) Relative Humidity (%) 100 80 200 - 30 asymptotic

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 You can help change this!!! Image – deforestation snaps water cycle and also results in erosion guatemala Guatemala Panama

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!!! guatemala Guatemala Panama

Social Justice I’m not angry with you …… My anger is NOT directed at you – you all are our best hope…..

But I do expect you to DO something!!! Social Justice But I do expect you to DO something!!!

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 K+ accumulation is triggered by increased light, low carbon dioxide, circadian rhythms High [K+] does what to Ψ??? 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

This is the gradient that counts 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 O2 to first capture CO2 Both adaptations allow photosynthesis to proceed with stomata largely closed during the day Images and diagrams – metabolic adaptations to dry environments

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 Diagram – pressure flow model of phloem flow; this diagram is repeated throughout this section Xylem transport is uni-directional, driven by solar heating Phloem flow is multi-directional, driven by active transport – source to sink

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??? Why doesn’t the sugar just flow back into the leaf cell???

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 Transport Questions???

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