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Minerals H2O

H2OCO2

CO2

O2

O2

Plant Transport:acquiring resources

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Ascent of xylem fluid:Transpiration pull generated by leaf

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Pulling Xylem Sap: The Cohesion-Tension Ho.§ According to the cohesion-tension hypothesis,

transpiration and water cohesion pull water from shoots to roots

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Transpirational Pull:§Water vapor in the airspaces of a leaf diffuses down its water potential gradient and exits the leaf via stomata§As water evaporates, the air-water interface retreats further into the mesophyll cell walls§The surface tension of water creates a negative pressure potential

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§ This negative pressure pulls water in the xylem into the leaf

§ The transpirational pull on xylem sap is transmitted from leaves to roots

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Xylemcells

Xylem sapMesophyll cellsStomaWater moleculeAtmosphere

Transpiration

Adhesion byhydrogen bonding

Cellwall

Cohesionby hydrogen

bondingCohesion andadhesion inthe xylem

Water moleculeRoot hairSoil particle

WaterWater uptake from soil

Wat

er p

oten

tial g

radi

ent

Outside air ψ= −100.0 MPa

Leaf ψ (air spaces)= −7.0 MPa

Leaf ψ (cell walls)= −1.0 MPa

Trunk xylem ψ= −0.8 MPa

Trunk xylem ψ= −0.6 MPa

Soil ψ= −0.3 MPa

Low water potential

High water potential

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§ Water molecules are attracted to each other through cohesion

§ Cohesion makes it possible to pull a column of xylem sap§ Thick secondary walls prevent vessel elements and

tracheids from collapsing under negative pressure§ Drought stress or freezing can cause a break in the chain

of water molecules through cavitation, the formation of a water vapor pocket

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Xylem Sap Ascent by Bulk Flow: A Review§ The movement of xylem sap against gravity is maintained

by the cohesion-tension mechanism§ Bulk flow is driven by a water potential difference at

opposite ends of xylem tissue, and by transpiration (does not require energy from the plant); “like photosynthesis it is solar powered”

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§ Bulk flow differs from diffusion:u It is driven by differences in pressure potential,

not solute potentialu It occurs in hollow dead cells, not across the

membranes of living cellsu It moves the entire solution, not just water or

solutesu It is much faster

Both the VW bus and the 911 GT3 are both forms of transportation, but the 911 is a far faster and more efficient mode.

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The rate of transpiration is regulated by stomata:§ Leaves generally have large surface areas and high

surface-to-volume ratios§ These characteristics increase photosynthesis, but also

increase water loss through stomata§ Guard cells help balance water conservation with gas

exchange for photosynthesis

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Stomata: Major Pathways for Water Loss:§ About 95% of the water a plant loses escapes

through stomata§ Each stoma is flanked by a pair of guard cells,

which control the diameter of the stoma by changing shape

§ Stomatal density is under genetic and environmental control

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Mechanisms of Stomatal Opening and Closing:§ Changes in turgor pressure open and close

stomata u When turgid, guard cells bow outward and the

pore between them opensu When flaccid, guard cells become less bowed

and the pore closes

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ChloroplastsEpidermal cell

NucleusGuard cell

Thickened innercell wall (rigid)

Stoma open Stoma closed

H2O

water moves into guard cells

H2O H2O H2O

H2O H2O

H2O

H2O

H2O H2O H2O H2O

Control of Stomates:

K+

K+

K+

K+

K+ K+

K+ K+

K+ K+K+K+

water moves out of guard cells

§ Uptake of K+ ions by guard cellsu proton pumpsu water enters by

osmosisu guard cells

become turgid§ Loss of K+ ions

by guard cellsu water leaves by

osmosisu guard cells

become flaccid

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Control of transpiration:§ Balancing stomate function ….

u always a compromise between photosynthesis & transpiration§ leaf may transpire more than its weight in water in

a day…this loss must be balanced with plant’s need for CO2 for photosynthesis

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Stimuli for Stomatal Opening:§ Generally, stomata open during the day and close

at night to minimize water loss§ Stomatal opening at dawn is triggered by

u Lightu CO2 depletionu An internal “clock” in guard cells

§ All eukaryotic organisms have internal clocks; circadian rhythms are 24-hour cycles

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§ Drought, high temperature, and wind can cause stomata to close during the daytime

§ The hormone abscisic acid (ABA) is produced in response to water deficiency and causes the closure of stomata

Stimuli for Stomatal Closing:

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Effects of Transpiration on Wilting and Leaf Temperature:§ Plants lose a large amount of water by transpiration§ If the lost water is not replaced by sufficient transport of

water, the plant will lose water and wilt§ Transpiration also results in evaporative cooling, which

can lower the temperature of a leaf and prevent denaturation of various enzymes involved in photosynthesis and other metabolic processes

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§ Loading of sucrose into phloem = translocationu flow through cells via plasmodesmatau proton pumpsu cotransport of sucrose into cells down proton gradient

Transport of sugars in phloem from source to sink:

Sucrose manufactured in mesophyll cells can travel via symplast (blue arrows) to sieve-tube elements. In some species, sucrose exists the symplast near sieve tubes and travels through the apoplast (red arrow). It is actively accumulated from the apoplast by sieve-tube elements and their companion cells.

The chemiosmotic mechanism is responsible for the active transport of sucrose into companion cells and sieve-tube elements. Proton pumps generate and H+ gradient, which drive sucrose accumulation with the help of a cotransport protein that couples sucrose transport to the diffusion of H+ back into the cell.

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Companion cells enhance solute movement between the apoplast and symplast

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Positive pressure flow in phloem:§ Mass flow hypothesisu “source to sink” flow

§ direction of transport in phloem is dependent on plant’s needs

u phloem loading§ active transport of sucrose

into phloem§ increased sucrose concentration

decreases H2O potentialu water flows in from xylem cells

§ increase in pressure due to increase in H2O causes flow

On a plant…What’s a source…What’s a sink?

can flow 1m/hr

1) Loading of sugar into the sieve-tube at the source reduces H2O potential inside the sieve-tube elements. This causes the tube to take up H2O by osmosis.

2) This uptake of H2O generates a positive pressure that forces the sap to flow along the tube.

3) The pressure is relieved by the unloading of sugar and the consequent loss of H2O at the sink. 4) In leaf-to-root translocation, xylem recycles H2O from sink to source.

Active transportof sugarhere

Passive transportof sugarhere

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In contrast to the unidirectional transport of xylem sap from roots to leaves, the direction that phloem sap travels is variable.

xylem phloem

One-way only

Water & minerals

No end walls Between cells

Stiff walls thickenedWith lignin

Water & food

Cells have end wallswith perforations

Two-way flow

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Experimentation:§ Testing pressure flow hypothesis

u using aphids to measure sap flow & sugar concentration along plant stem

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Maple sugaring:

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Don’t get mad…Get even!!