Transcript PowerPoint format

Water uptake, water
transport and transpiration
Topics
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
+
Water flows through the
plant with rate largely
determined by physics
1. General features
Plant structure and
control systems
minimize loss and
‘shortage’ in tissues
Translocation is
not unidirectional
Functions:
cooling
CO2/O2 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.
H
O
H
H
H
O
H
O
H
O
H
O
H
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 22oC (72F) and 50% Relative Humidity
yair =
100 MPa
negative
Typically yleaf
= -1 to - 4MPa
ysoil = 0.01 to - 0.1 MPa
Water potentials of
connected tissues
defines rate of
water flows
through a plant.
4. The energy budget of foliage
Radiation
input
Some radiation
is reflected and
some energy is
re-radiated
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
In addition to
radiation input leaf
temperature can
also be affected by
wind speed and
humidity because
these conditions
affect rate of cooling
Transpiration flux, g H2O/cm2 leaf surface/second X10-7
3.0
Wind speed influences
transpiration
2.5
The boundary layer
around a leaf 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.
2.0
1.5
1.0
0.5
Stomatal aperture, m
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
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.
5. Measuring water potential
The pressure bomb!
Compressed air
Field measurements of 
Forest laboratory in south west
Scotland
Measurement every hour for
7 days
Diurnal pattern of shoot water potential
Midnight
Midday
500
Transpiration
Mg/sec/tree
400
300
200
100

Shoot water
potential
MPa
0
-1
-2
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
6. Stomatal control of leaf water potential
… about osmosis?
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
Fig. 32.4
Factors that can affect stomatal aperture
How could you analyze them experimentally?
Low leaf water potential
High temperature
Low CO2 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 
Cavitation and tree ecology:
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
Contraction of tissues:
Measuring the extension rate of a conifer shoot
Midnight
Shoot extension
Shoot
extension
4
Mm/h
2
0
-2
8
MJm2
Total radiation
6
4
2
24 June
25 June
26 June
27 June
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
Sections you need to have read
32.1 32.2 32.3 32.4
Courses that deal with this topic
Botany 371/372 Plant physiology laboratory