Transcript Document
Transpiration
• Transpiration is the loss of water from a plant by
evaporation
• Water can only evaporate from the plant if the
water potential lower
in the
air
surrounding the plant is • Most transpiration occurs via the leaves • Most of this transpiration is
via the stomata
.
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How Transpiration is Measured A Simple Potometer Leafy shoot cut under water Air tight seals Plastic tubing Water evaporates from the plant Capillary tube
1’’’’’’’’2’’’’’’’’3’’’’’’’’4’’’’’’’’5’’’’’’’’6’’’’’’’’7’’’’’’’’8’’’’’’’’9’’’’’’’’10’’’’’’’’11’’’’’’’’12’’’’’’’’13’’’’
Graduated scale Movement of meniscus is measured over time
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How Transpiration is Measured
The rate of water loss from the shoot can be measured under different environmental conditions Water is pulled up through the plant Limitations
•measures water uptake
volume of water taken up in given time
1’’’’’’’’2’’’’’’’’3’’’’’’’’4’’’’’’’’5’’’’’’’’6’’’’’’’’7’’’’’’’’8’’’’’’’’9’’’’’’’’10’’’’’’’’11’’’’’’’’12’’’’’’’’13’’’’ •cutting plant shoot may damage plant •plant has no roots so no resistance to water being pulled up 3
6 Environmental Factors Affecting Transpiration
1. Relative humidity
:- air inside leaf is saturated (RH=100%). The lower the relative humidity outside the leaf the faster the rate of transpiration as the gradient is steeper
2. Air Movement
:- increase air movement increases the rate of transpiration as it moves the saturated air from around the leaf so the gradient is steeper.
3. Temperature
– :- increase in temperature increases the rate of transpiration as higher temperature Provides the latent heat of vaporisation – Increases the kinetic energy so faster diffusion – Warms the air so lowers the of the air, so gradient is steeper 4
4.
Atmospheric pressure
:- decrease in atmospheric pressure increases the rate of transpiration.
5.
Water supply
:- transpiration rate is lower if there is little water available as transpiration depends on the mesophyll cell walls being wet (dry cell walls have a lower ). When cells are flaccid the stomata close.
6.
Light intensity
:- greater light intensity increases the rate of transpiration because it causes the stomata to open, so increasing evaporation through the stomata. 5
Intrinsic Factors Affecting the Rate of Transpiration.
1. Leaf surface area 2. Thickness of epidermis and cuticle 3. Stomatal frequency 4. Stomatal size 5. Stomatal position
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The Effect of Wind Speed on the Rate of Transpiration
Stomatal transpiration rate / gcm -2 s -1 moving air In still air closing the stomata is less effective in controlling the transpiration rate
10 20
Stomata diameter/µm still air
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Moving Air Removes the Boundary Layer of Water Vapour From the Leaf Still air Moving air
Saturated air accumulates around leaf Water vapour is removed from the leaf surface
cross section through a leaf
Boundary layer Lower
the
gradient
is increased, so faster rate of water evaporation via the stomata 8
Movement of Water Through the Stomata Diffusion shells
H
2
O
Water moves from a higher (less negative) to a lower (more negative) water potential
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Increase in stomatal frequency increases the rate of transpiration
Boundary layer stoma If the distance between the stomata is less than 10 X the pore diameter the diffusion shells overlap So increasing the number of stomata per unit area will have no further effect on transpiration 10
Wilting
Subject to copyright clearance a suitable image showing plants wilting could be inserted here.
e.g. one similar to that found at:
http://pdc.unl.edu/sugarbeet/RhizochtoniaRootAndCrownRot/s uddenwilt.htm
If water lost by transpiration is greater than water uptake via the roots the
plant cells
become
flaccid
and the plant wilts. When the guard cells are flaccid the stomata close 11
Leaf section
The upper epidermis has no stomata Subject to copyright clearance a suitable image showing a leaf section could be inserted here.
e.g. one similar to that found at:
http://www.bbc.co.uk/schools/gcsebitesize/biology/greenplants asorganisms/0photosynthesisrev2.shtml
The lower epidermis has stomata.
The guard cells control the opening and closing of the stomata
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Lower Epidermis of Tradescantia
Subject to copyright clearance a suitable image showing the surface of the lower epidermis could be inserted here.
e.g. one similar to that found at:
www.lima.ohio-state.edu/academics/biology/images/lower.jpg
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Surface view of leaf epidermis showing the guard cells which are flaccid and the stoma closed.
Subject to copyright clearance a suitable image showing the surface of the lower epidermis could be inserted here.
e.g. one similar to that found at:
www.lima.ohio-state.edu/academics/biology/images 14
A single stoma and guard cells
Guard cells turgid Stoma open Guard cells flaccid Subject to copyright clearance a suitable image showing a single stoma and guard cells could be inserted here.
e.g. one similar to that found at:
http://www.plantsci.cam.ac.uk/Webb/Overview/Overview.html
Stoma closed
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The guard cells control the opening and closing of the stomata Guard cells flaccid Thin outer wall Guard cells turgid Stoma closed Thick inner wall Stoma open 16
Regulating Stomatal Opening:-the potassium ion pump hypothesis Guard cells flaccid
K + K + K + K + K + K + K + K + K + K + K + K +
K + ions have the same concentration in guard cells and epidermal cells Light activates K cells + pumps which actively transport K + from the epidermal cells into the guard Stoma closed 17
Regulating Stomatal Opening:-the potassium ion pump hypothesis
H 2 O H 2 O H 2 O Increased concentration of K + in guard cells K + H 2 O K + K + K + K + K + K + K + K + K + K + K + H 2 O Lowers the
cells in the guard Water moves in by osmosis, down
gradient
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Guard cells turgid
K + H 2 O K + K + H 2 O K + H 2 O K + K + H 2 O + K + H 2 + O K + H 2 O K + K +
Stoma open
Increased concentration of K + in guard cells Lowers the
cells in the guard Water moves in by osmosis, down
gradient
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Xerophytes Have Special Adaptations to Reduce the Rate of Transpiration Xerophytes live in hot, dry environments
Subject to copyright clearance a suitable image showing typical xerophyte features could be inserted here.
e.g. one similar to that found at:
http://arboretum.ag.arizona.edu/tour/lowband/cactusgarden.html
These cacti have
reduced leaf area
as the leaves are reduced to spines
Fleshy leaves
to hold water
Silver surface
to reflect sun 20
Adaptations to Reduce Water Loss in Xerophytes
• Thick waxy cuticle to reduce evaporation • Reduced leaf area e.g.needles
• Hairy leaves :- the hairs trap a layer of saturated air • Sunken stomata :- the pits above the stomata become saturated • Rolled leaves roll :- this reduces the area exposed to the air and keeps the stomata on the inside so increasing the water vapour inside the
Increasing the water vapour around the stomata reduces the water potential gradient so slows water loss
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Cross Section of Marram Grass Leaf
Subject to copyright clearance a suitable image showing a cross section of marram grass could be inserted here.
e.g. one similar to that found at:
www.microscopy-uk.org.uk/schools/images/marram.html
Leaf is rolled with sunken stomata on the inside Hairs trap water vapour Water evaporating via the stomata collects in the air trapped in the rolled leaf This reduces the
gradient so reduces water loss from the plant
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Adaptation to Increase Water Uptake in Xerophytes
• Deep extensive root system to maximise water uptake • Accumulation of solutes in the root making the system to reduce the , so gradient from the soil to the root cells steeper • Some very shallow roots soil at night to
absorb dew
which condenses on the 23
100 stomatal opening/%
Graph to show stomatal opening over 24 hours
Increased light intensity causes more stomata to open Some plants close stomata during hottest time-saving water An adaptation to hot dry environments Stomata close as the sun sets 0 12 2 Dawn-stomata begin to open 4 6 8 10 12 2 4 6 8 10 12
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24h Cycle of Stomatal Opening and Closing Why is this cycle an advantage to most plants?
12.00
09.00
15.00
06.00
18.00
3.00
24.00
21.00
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Questions 1. What is transpiration? Give three environmental factors which will increase transpiration rate.
2. Explain how potassium ions are moved into the guard cells in light, and how this affects the guard cells and stomata.
3. Give three adaptations a xerophyte may have to reduce transpiration and explain how they do this.
4. Plants close their stomata at night and some also close their stomata around mid day. Explain why this is advantageous to the plant
Click on the marks above to check your answer 26
Answer Q 1 • Transpiration is the loss of water from a plant by evaporation • Higher temperature, increased air movement, lower humidity
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Answer Q 2 • Potassium ions are pumped into the guard cells by active transport • against the concentration gradient • this lowers the water potential inside the guard cells • water is drawn in by osmosis • from the surrounding cells which have a higher water potential/down the water potential gradient • guard cells swell and become turgid • guard cells bend • causing the stomata to open any 6 from the above
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Answer Q3 Any three from: • Thick waxy cuticle on leaves reduces evaporation • Curled leaves reduce evaporation by trapping humid air inside the curl so reducing the water potential gradient • Reduced leaf area, e.g. spines, reduces the area from which evaporation can occur • Hairy leaves -trap a layer of humid air around the leaf,so reducing the water potential gradient • Sunken stomata – moist air trapped above stomata, so reducing the water potential gradient
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Answer Q4 • Stomata closed at night when there is no light for photosynthesis, so reducing water loss by evaporation/transpiration via the stomata • Closing stomata at mid day, which is the hottest part of the day, is an advantage in hot dry environments, as transpiration is reduced.
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Can you think of any synoptic links?
• Water potential • Osmosis • Active transport • Respiration (energy required for active transport) • Photosynthesis (light and CO 2 required for photosynthesis, CO 2 enters via stomata, water used in photosynthesis) 31