1. Water uptake, water transport and transpiration

2. Things to know from today’s lecture
How water molecules show cohesion and adhesion and why this is important in water transport
The pathways of water movement in a plant
What water potential is, how to measure it, how it varies in a plant during a day and what effect this variation may have
Basic aspects of leaf energy budget
How stomatal opening is controlled
Trichomes

3. Hydrogen Bonds and Cohesion
Water molecules have weak negative charges at the oxygen atom and positive charges at the hydrogen atoms. + H O
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.
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.

4. 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 y (psi), is measured in megapascals, Mpa, (SI, SystÈme Internationale) units.
For pure water at standard temperature and pressure (STP) y = 0 Mpa.
negative
At 22oC (72F) and 50% Relative Humidity yair = MPa
Water potentials of connected tissues defines rate of water flows through a plant.
Typically yleaf = -1 to - 4MPa
ysoil = 0.01 to MPa

5. Stoma xylem Water Uptake in Growth Regions hair cell Upper epidermis
Photosynthetic cells
(mesophylll)
Stoma
leaf vein
lower epidermis
1 Driving Force is Evaporation
xylem
phloem
Growing cells also remove small amounts of water from
xylem
Water Uptake in
Growth Regions
2 Cohesion in Xylem
vascular cylinder
hair cell
soil particle
Water molecule
cortex
endodermis
Fig. 30.9, p. 523
3 Water Uptake from Soil by Roots

6. Measuring water potential
The pressure bomb!
Compressed air

7. Field measurements of 
Forest laboratory in south west Scotland
Measurement every hour for 7 days

8. Diurnal pattern of shoot water potential
100
200
300
400
500 -2 -1
30 Jul
31 Jul
1 Aug
2 Aug
3 Aug
4 Aug
5 Aug
6 Aug
Midnight
Midday
Transpiration
Mg/sec/tree
Shoot water
potential
MPa 
During daylight water loss from foliage exceeds water gain from soil so shoot water potential decreases. On sunny days  reaches –2 Mpa

9. 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

10. The energy budget of foliage
Some radiation is reflected and some energy is re-radiated
Radiation
input
In addition to radiation input leaf temperature can also be affected by wind speed and humidity because these conditions affect rate of cooling
If Tleaf > Tair then the leaf warms the air
Only 1-3% of radiation is used in photosynthesis
Evaporative cooling depends upon latent heat of evaporation

11. Wind speed influences transpiration
Stomatal aperture, m
Transpiration flux, g H2O/cm2 leaf surface/second X10-7
0.5
1.0
1.5
2.0
2.5
3.0
Wind speed influences transpiration
 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.
A slight increase in wind speed will reduce the boundary layer, and increase transpiration.
Further increase in wind speed may reduce transpiration, especially for sunlit leaves, because wind speed will cool the leaf directly 

12. 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

13. 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

14. Trichomes increase boundary layer resistance
Trichome: hairlike projection from a plant epidermal cell.
Trichomes do have other functions
Coleus
Foxglove
Peltate trichomes
Olive
Curatella americana

15. 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.

16. Components of experiments
There is usually a THEORY behind each experiment
An experiment has:
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. Control
The 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
And must be
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