Water uptake, water transport and transpiration

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Water uptake, water transport and transpiration
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Topics
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1. General features
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
Translocation is not unidirectional
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2. Cohesion and adhesion in the transpiration
stream
Fig. 32.3
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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.
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/H
ydrogenBonds.html
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Descriptive movie clip
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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.
Typically yleaf -1 to - 4MPa
Water potentials of connected tissues defines
rate of water flows through a plant.
ysoil 0.01 to - 0.1 MPa
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4. 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 gt Tair then the leaf warms the air
Evaporative cooling depends upon latent heat of
evaporation
Only 1-3 of radiation is used in photosynthesis
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Wind speed influences transpiration
 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.
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/windsp
d.htm
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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.
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5. Measuring water potential
The pressure bomb!
Compressed air
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Field measurements of ?
Forest laboratory in south west Scotland
Measurement every hour for 7 days
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Diurnal pattern of shoot water potential
During daylight water loss from foliage exceeds
water gain from soil so shoot water potential
decreases. On sunny days ? reaches 2 Mpa
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Stomatal control clip
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6. Stomatal control of leaf water potential
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about osmosis?
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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
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Control of stomatal opening and closing
Guard cells actively take up K causing water to
enter by osmosis. The guard cells walls are
unevenly thickened causing the cells to bow as
they becomes turgid
Fig. 32.4
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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.
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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
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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 ?
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Cavitation and tree ecology
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
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Contraction of tissues
Measuring the extension rate of a conifer shoot
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Shoot extension
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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
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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