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TRANSPIRATION
Definitions
• Transpiration is the loss of water vapour by
evaporation from the aerial parts of a plant.
• The transpiration stream is the continuous flow of
water from the roots to the leaves of a plant
driven by evaporation.
Transpiration
• Transpiration involves
evaporation followed
by diffusion of water
vapour down a water
vapour potential
gradient out of the
leaves through the
stomata.
Transpiration
• Stomata are open
during daylight hours to
allow carbon dioxide to
enter the leaf.
• Water moves from the
xylem across the
mesophyll cells by the
symplast route (by
osmosis from cell to cell)
and by the apoplast
route(along the bundles
of cellulose fibres
forming the cell wall).
Cohesion -Tension
• The tension set up in the
leaf draws water up the
xylem vessels in the stem
of the plant.
• Water molecules form
hydrogen bonds causing
cohesion. This produces
an unbroken column up
the xylem.
Cohesion-Tension
• A stronger rate of
evaporation leads to more
tension in the water column
and a faster rate of water
movement up the trunk.
• Capillary action plays a part
in the movement of water up
a stem. The water molecules
adhere to the walls of the
xylem vessels which are very
narrow.
• Root pressure also
contributes due to the build
up of water in the xylem in
the roots
Measuring the rate of water uptake by
plants
• The potometer
measures the rate of
water uptake by a cut
shoot.
• This can be used as
an estimate of the rate
of transpiration.
• Why is the uptake of
water only an estimate
of transpiration rate?
Setting up the potometer
• https://www.youtube.com/watch?v=oUr1P9RZnEU
The potometer- details
• An air bubble is
introduced at the
bottom of the capillary
tubing.
• When it reaches the
scale, the distance
moved along the scale
in one minute can be
measured.
The potometer - details
• The rate of water taken
up can be calculated.
• First work out the
cross section area of
the capillary tube using
𝜋𝑟 2 ,
• Then multiply by the
distance travelled by
the bubble to get the
rate in 𝑚𝑚3 𝑚𝑖𝑛−1
The potometer- precautions
• The equipment must be
water tight so that there
are no leaks.
• The potometer is set up
under water. The leaves
must be allowed to dry
before taking any
readings.
• No air must be allowed
to build up under the cut
shoot and break the
water column.