Water uptake, water transport and transpiration

Things to know from today’s lecture

How water molecules show cohesion and adhesion and why this is important in water transport

What water potential is, how to measure it, how it varies in a plant during a day and what effect this variation may have

The pathways of water movement in a plant

Basic aspects of leaf energy budget

How stomatal opening is controlled

Trichomes

Positive and negative regions are attracted. The force of attraction, dotted line, is called a hydrogen bond. Each water molecule is hydrogen bonded to four other water molecules – the force of Cohesion.

The hydrogen bond has ~ 5% of the strength of a covalent bond. However, when many hydrogen bonds form, the resulting Cohesion is sufficiently strong as to be quite stable.

Hydrogen Bonds and Cohesion

H H

H

H H

HH

H

HH

O

O

O

O

O

http://www.ultranet.com/~jkimball/BiologyPages/H/HydrogenBonds.html

Adhesion is the tendency of molecules of different kinds to stick together – by a similar process. Water sticks to cellulose molecules in the walls of the xylem, counteracting the force of gravity.

Water molecules have weak negative charges at the oxygen atom and positive charges at the hydrogen atoms. +

How water moves through the plant Water potential indicates how strongly water is held in a substance. It is measured by the amount of energy required to force water out.

Water potential , referred to as (psi), is measured in megapascals, Mpa, (SI, SystÈme Internationale) units.

For pure water at standard temperature and pressure (STP)

= 0 Mpa.

negative

At 22oC (72F) and 50% Relative Humidity air = 100 MPa

soil= 0.01 to - 0.1 MPa

Typically leaf-1 to -

4MPa Water potentials of connected tissues defines rate of water flows through a plant.

Fig. 30.9, p. 523

1 Driving Force is Evaporation

xylem phloem

Upper epidermis

2 Cohesion in Xylem

Growing cells also remove small amounts of water from xylem

Water Uptake inGrowth Regions

3 Water Uptake from Soil by Roots

vascular cylinder hair cell soil particleWater molecule

cortex

endodermis

leaf vein

Photosynthetic cells(mesophylll)

lower epidermis

Stoma

Measuring water potentialThe pressure bomb!

Compressed air

Field measurements of

Forest laboratory in south west Scotland

Measurement every hour for 7 days

100

200

300

400

500

-2

-1

0

30 Jul 31 Jul 1 Aug 2 Aug 3 Aug 4 Aug 5 Aug 6 Aug

During daylight water loss from foliage exceeds water gain from soil so shoot water potential decreases. On sunny days reaches –2 Mpa

Diurnal pattern of shoot water potential

TranspirationMg/sec/tree

Midnight

Midday

Shoot waterpotential

MPa

Cessation of physiological processes:

Cell growth and wall synthesis are very sensitive and may stop at -0.5 MPa

Photosynthesis, respiration and sugar accumulation are less sensitive. They may be affected between -1 and -2 MPa

The energy budget of foliage

In addition to radiation input leaf temperature can also be affected by wind speed and humidity because these conditions affect rate of cooling

Only 1-3% of radiation is used in photosynthesis

Evaporative cooling depends upon latent heat of evaporation

Some radiation is reflected and some energy is re-radiated

Radiationinput

If Tleaf > Tair

then the leaf

warms the air

 The boundary layer around a leaf extends out from the leaf surface. In it air movement is less than in the surrounding air. It is thick in still air, and constitutes a

major resistance to the flux of

H2O from the leaf.

Stomatal aperture, m

Tra

nsp

irat

ion

flu

x, g

H2O

/cm

2 lea

f su

rfac

e/se

con

d X

10-7

0.5

1.0

1.5

2.0

2.5

3.0Wind speed influences transpiration

Further increase in wind speed may reduce transpiration, especially for sunlit leaves, because wind speed will cool the leaf directly 

http://forest.wisc.edu/forestry415/lecture6/windspd.htm

A slight increase in wind speed will reduce the boundary layer, and increase transpiration.

Review of osmosis

Diffusion of water across a selectively permeable membrane from a hypotonic to a hypertonic solution

Hyper - above

Hypo - below

Water crosses the membrane until the solute concentrations are equal on both sides

Control of stomatal opening and closing

Guard cells actively take up K causing water to enter by osmosis. The guard cell’s walls are unevenly thickened causing the cells to bow as they becomes turgid

Trichomes increase boundary layer resistance

Trichome: hairlike

projection from a

plant epidermal cell.

Curatella americana

Coleus Foxglove

Olive

Peltate trichomes

Trichomes do have other functions

Laboratory measurement of transpiration

A laboratory potometer

1. Fill the potometer by submerging it – make sure there are no air bubbles in the system.

2. Recut the branch stem under water and, keeping the cut end and the potometer under water, put the cut end into the plastic tubing.

Components of experiments

An experiment has:

And must be

1. Hypothesis A statement predicting alternative responses: “If this is done that will happen otherwise it will not.”

2. Treatment A specific, designed, manipulation

3. Measurement sufficiently accurate to detect response to the treatment

4. ControlThe same measurement is made but the treatment is not applied. This provides the essential contrast.

5. Replication Enables the degree of response to be defined and helps to protect against obtaining results by chance

6. Repeated Required to establish the degree of certainty that can be attributed to a result, e.g., repetition with the same and different species

There is usually a THEORY There is usually a THEORY behind each experimentbehind each experiment