Transpiration and Water Status

Ascent of sap: water flows to replace transpiration loss

General principle: J = concentration * driving force/resistance = kcw(-d /dx)/r

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Ascent of sap: water flow to replace transpiration loss

General principle: J = concentration * driving force/resistance = kcw(-d /dx)/r

Driving force: the gradient in water potential

Water in xylem moves through vessels, with perforation plates,and tracheids, all interconnected by pits

Resistance: pits,cavitation, embolisms

Kiwifruit vine: H2O-filled xylem (bright spots) shown in cross-section by magnetic resonance imaging

Cavitationand/or airembolisms produceempty vessels(arrows)

The Ψ gradient in the stem depends on demand(transpiration), cross section of xylem available(unblocked), and resistance to flow.

A pipe of radius 40 µm can transport water at 4 mm/swith a pressure gradient of -0.2 MPa per 10 m (seeEssays 4.3 at http://5e.plantphys.net).

Pits in xylem provide more resistance to water flow(expect ΔΨ of -0.2 to -0.3 MPa per 10 m height).

Resistance to water flow causes water potential to dropin different parts of a plant at different times.

Does this sequence show whether water is pulled or pushed through the plant?

Water movement into the root: the endodermis forces water to crossat least two membranes with high resistance

Water movement into the root: the endodermis forces water to crossat least two membranes with high resistance

Web topic 4.3 calculates that it takes2 x 104 Pa (20 kPa) to move wateracross two membranes at 4 mm/s.

In the second slide of this lecture,the difference in water potentialbetween soil and root xylem was 0.1MPa. (100 kPa).

Clearly, the membranesrepresent a significant, but not theonly resistance to water flow intothe root.

Water movement through soil

♦Resistance variable, depending on degree of saturation: low saturation, small fluid cross-section

♦Thus dry soils are bad water sources for two reasons: low Ψp and high resistance

Measurements of water status (Ψ)

Constant weight method measures Ψ by comparison

1. Cut and weigh sub-samples of a tissue2. Soak samples in solutions of different Ψs3. Reweigh

This can also bedone by equilibratingsamples in smallclosed chamberscontaining referencesolutions of differentΨs

Vapor pressure psychrometer measures Ψ by RH of air

1. Equilibrate sample in a small chamber: Ψsample = Ψair2. Measure the relative humidity of chamber air with wetand dry thermocouples• cold thermocouple is warmed by condensation• rate of condensation is a function of RH• rate of condensation is measured by the electric

current needed to keep temperature constant3. Freeze-thaw sample to measure Ψs (reduce Ψp to 0)

Scholander pressure bomb measures Ψp of xylem

Pressure bomb: Hypothetical values

On the plant After cut In the bombxylem !p + !s = ! -1+(-0.1)=-1.1 -0.8+(-0.1)=-0.9 -1+1+(-0.1)=-0.1

intercellularspace !p = ! -1.1 -0.9 -1.1+1=-0.1

leafcell !p + !s= ! +0.5+(-1.6)=-1.1 +0.7+(-1.6)=-0.9 +0.5+1+(-1.6)=-0.1

Summary! Sun comes up; stomata open; water vapor diffuses out! Water evaporates from cell walls and xylem; "p goes negative

! Water flows up xylem, reducing "p in root xylem

! Water flows from soil through root, reducing "p in soil

! Water flows through soil! At sunset (or stress), stomata close and water flow equalizes "

How does thisexperimentdemonstrate:

a) the effect ofresistance ontranspiration;b) the degree of resistanceto water flowthrough the plant;c)the effect ofsaturation onwater flow through soil;d) the water potential of leaves of theplant?