1. General features of water flow through plants
2. Adhesion and cohesion in the water column
3. Water potential and how water moves through the plant
4. The energy budget of foliage
5. Measuring water potential
6. Stomatal control of leaf water potential
7. Consequences to the plant of changes in water potential

+
1. General features
Translocation is not unidirectional
1. General features
Water flows through the plant with rate largely determined by physics
Plant structure and control systems minimize loss and
‘shortage’ in tissues
Functions: cooling
CO
2
/O
2 exchange nutrient flow translocation physiological processes

2. Cohesion and adhesion in the transpiration stream
Fig. 32.3

Hydrogen Bonds and Cohesion
Water molecules have a weak negative charge at the oxygen atom and weak positive charge at the hydrogen atoms.
The positive and negative regions are attracted to the oppositely-charged regions of nearby molecules. The force of attraction, dotted line, is called a hydrogen bond. Each water molecule is hydrogen bonded to four others.
O
H
H
H
O
H
O
H
O
H
H
H
O
H
H
The hydrogen bond has ~ 5% of the strength of a covalent bond.
However, when many hydrogen bonds form, the resulting union can be sufficiently strong as to be quite stable.
Adhesion is the tendency of molecules of different kinds to stick together. Water sticks to the cellulose molecules in the walls of the xylem, counteracting the force of gravity. http://www.ultranet.com/~jkimball/BiologyPages/H/HydrogenBonds.html

Descriptive movie clip

3. Water potential and 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 of it. Think of squeezing a sponge or cloth.
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.
At 22 o C (72F) and 50% Relative Humidity y air
= 100 MPa negative
Typically y leaf
=
1 to 4MPa y soil
= 0.01 to 0.1 MPa
Water potentials of connected tissues defines rate of water flows through a plant.

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

2.0
1.5
1.0
0.5
3.0
2.5
Stomatal aperture,
 m
Wind speed influences transpiration
The boundary layer around a leaf is thick in still air, and constitutes a major resistance to the flux of H
2
O 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 http://forest.wisc.edu/forestry415/lecture6/windspd.htm

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.

The pressure bomb!
Compressed air


Forest laboratory in south west
Scotland
Measurement every hour for
7 days

Diurnal pattern of shoot water potential
Midnight
Midday
Transpiration
Mg/sec/tree
500
400
300
200
100

Shoot water potential
MPa
-1
-2
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

Stomatal control clip

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

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 Fig. 32.4

Factors that can affect stomatal aperture
How could you analyze them experimentally?
Low leaf water potential
High temperature
Low CO
2 concentration in the air spaces of the leaf causes a plant to open its stomata
Circadian rhythm: a cycle of opening during daylight and closing during the dark.

Leaf hairs increase boundary layer resistance
Trichomes or hairs cells grow out of the surface of the epidermis. These may be uni-or multicellular depending on species. Both uni-and multicellular hairs may be branched. Some leaves have glandular hairs with an enlarged cell or group of cells at the end of a stalk.
Curatella americana from Cerrado (Brazil). Leaf surface showing stomata. Note the silicified hairs in the shape of a star

7.
Consequences of changes in water potential
Cavitation and tree ecology:
The water column in wood can break when the tension on it exceeds the forces of adhesion and cohesion
Contraction of tissues:
At low
 tissues that have not developed strong thickening can contract
Cessation of physiological processes:
Different physiological processes may cease at different


Under very low water potentials the water column in xylem can break and cells become filled with air. This is called cavitation
Air is in solution in water, but as water potential decreases it may come out of solution unless the forces of cohesion and adhesion are strong enough to overcome that.
Cavitation be caused by a sustained period of dry conditions, i.e., a summer drought.
It can also occur when xylem water freezes. Air has very low solubility in ice, so bubbles form; when the water thaws, the bubbles will coalesce and cause cavitation.
Conifers have smaller conducting cells than angiosperm trees.
Conifers only have tracheids while angiosperm trees have vessels.
So, conifers may grow less quickly, but they may also cavitate less and this may help them survive in more stressed environments

Measuring the extension rate of a conifer shoot

Shoot extension
Mm/h
Shoot extension
2
0
-2
8
MJm 2
6
4
2
Total radiation
24 June 25 June
Midnight
26 June 27 June

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

Sections you need to have read
32.1 32.2 32.3 32.4
Botany 371/372 Plant physiology laboratory