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TranspirationTranspiration
• Transpiration is the loss of water from a plant by Transpiration is the loss of water from a plant by evaporationevaporation
• Water can only evaporate from the plant if the Water can only evaporate from the plant if the water water potentialpotential is is lowerlower in the in the airair surrounding the plant surrounding the plant
• Most transpiration occurs via the leavesMost transpiration occurs via the leaves
• MostMost of this transpiration is of this transpiration is via the stomatavia the stomata..
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How Transpiration is MeasuredHow Transpiration is Measured
A Simple A Simple PotometerPotometer
1’’’’’’’’2’’’’’’’’3’’’’’’’’4’’’’’’’’5’’’’’’’’6’’’’’’’’7’’’’’’’’8’’’’’’’’9’’’’’’’’10’’’’’’’’11’’’’’’’’12’’’’’’’’13’’’’
Air tight seals
Plastic tubing
Graduated scale
Capillary tube
Leafy shoot cut under water
Water evaporates Water evaporates from the plantfrom the plant
Movement of meniscus is measured over time
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How Transpiration is Measured
1’’’’’’’’2’’’’’’’’3’’’’’’’’4’’’’’’’’5’’’’’’’’6’’’’’’’’7’’’’’’’’8’’’’’’’’9’’’’’’’’10’’’’’’’’11’’’’’’’’12’’’’’’’’13’’’’
The rate of water loss from the shoot can be measured under different environmental conditions volume of water volume of water
taken up in given taken up in given timetime
Limitations
•measures water uptake•cutting plant shoot may damage plant
•plant has no roots so no resistance to water being pulled up
Water is pulled Water is pulled up through the up through the plantplant
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6 Environmental Factors Affecting 6 Environmental Factors Affecting TranspirationTranspiration
1.1. Relative humidityRelative 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
2.2. Air MovementAir Movement:- increase air movement increases the rate of transpiration as it moves the saturated air from around the leaf so the gradient is steeper.
3.3. TemperatureTemperature:- 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
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4. Atmospheric pressureAtmospheric pressure:- decrease in atmospheric pressure increases the rate of transpiration.
5. Water supplyWater 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 intensityLight intensity :- greater light intensity increases the rate of transpiration because it causes the stomata to open, so increasing evaporation through the stomata.
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Intrinsic Factors Affecting the Rate of Intrinsic Factors Affecting the Rate of Transpiration. 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
Stomata diameter/µm
10 20
Stomatal transpiration rate
/ gcm-2s-1
In still air closing the stomata is less effective in controlling the transpiration rate
moving moving airair
still airstill air
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Moving Air Removes the Boundary Layer of Water Vapour From the Leaf
Boundary layer
Saturated air accumulates around leaf
Still airStill air Moving airMoving air
Lower Lower
Water vapour is removed from the leaf surface
the gradient gradient is increased, so faster rate of water evaporation via the stomata
cross section through a leaf
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Movement of Water Through the Stomata
H2O
Diffusion shells
Water moves from a higher (less negative) to a lower (more negative) water potential
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Increase in stomatal frequency increases Increase in stomatal frequency increases the rate of transpirationthe rate of transpiration
Boundary layer
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
stoma
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WiltingWilting
If water lost by transpiration is greater than water uptake via the roots the plant cellsplant cells become flaccidflaccid and the plant wilts.
When the guard cells are flaccid the stomata close
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/suddenwilt.htm
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Leaf section
The upper epidermis has no stomata
The lower epidermis has stomata.
The guard cells control the opening and closing of the 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/greenplantsasorganisms/0photosynthesisrev2.shtml
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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
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A single stoma and guard cells
Guard cells Guard cells turgidturgid
Stoma Stoma openopen
Guard cells Guard cells flaccidflaccid
Stoma closedStoma closed
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
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Thin outer wall
The guard cells control the opening and closing of the stomata
Guard cells flaccid
Stoma closed Stoma open
Guard cells turgid
Thick inner wall
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Regulating Stomatal Opening:-the potassium ion pump hypothesis
Guard cells flaccid
Stoma closed
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+ pumps which actively transport K+ from the epidermal cells into the guard cells
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Regulating Stomatal Opening:-the potassium ion pump hypothesis
K+K+
K+
K+
K+K+
K+
K+
K+
K+K+
K+
Increased concentration Increased concentration of Kof K++ in guard cells in guard cells
Lowers the Lowers the in the guard in the guard cellscells
Water moves in by Water moves in by osmosis, down osmosis, down gradient gradient
HH22OO
HH22OO
HH22OO
HH22OO
HH22OO
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Stoma open
Guard cells turgid
K+
K+
K+
K+
K+
K+
K+
K+
K+
K+
K+
K+
Increased concentration Increased concentration of Kof K++ in guard cells in guard cells
Lowers the Lowers the in the guard in the guard cellscells
Water moves in by Water moves in by osmosis, down osmosis, down gradient gradient
HH22OO
HH22OO
HH22OOHH22OO
HH22OO
HH22OO
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Xerophytes Have Special Adaptations to Reduce the Rate of Transpiration
These cacti have reduced reduced leafleaf areaarea as the leaves are reduced to spines
Xerophytes live in hot, dry environments
Fleshy leavesFleshy leaves to hold water
Silver surfaceSilver surface to reflect sun
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
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Adaptations to Reduce Water Loss in Adaptations to Reduce Water Loss in XerophytesXerophytes
• Thick waxyThick waxy cuticlecuticle to reduce evaporation
• Reduced leaf areaReduced leaf area e.g.needles
• Hairy leavesHairy leaves:- the hairs trap a layer of saturated air
• Sunken stomataSunken stomata:- the pits above the stomata become saturated
• Rolled leavesRolled leaves:- this reduces the area exposed to the air and keeps the stomata on the inside so increasing the water vapour inside the roll
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
Leaf is rolled with sunkensunken stomatastomata on the inside
Water evaporating via the stomata collects in the air trapped in the rolled leafrolled leafThis reduces the gradientgradient so reduces water loss from the plant
Hairs Hairs trap water vapour
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
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Adaptation to Increase Water Uptake in Adaptation to Increase Water Uptake in Xerophytes Xerophytes
• Deep extensive rootDeep extensive root system to maximise water uptake
• Accumulation of solutes in the rootAccumulation of solutes in the root system to reduce the , so making the gradient from the soil to the root cells steeper
• Some very shallow rootsSome very shallow roots to absorb dewabsorb dew which condenses on the soil at night
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Graph to show stomatal opening over 24 hours
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2 4 6 8 10
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2 4 6 8 10 12
stomatal opening/%
0
100
Dawn-stomata begin to open
Increased light intensity causes more stomata to open
Stomata close as the sun sets
Some plants close stomata during hottest time-saving waterAn adaptation to
hot dry environments
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18.00
12.00
15.00
21.00
24.00
3.00
06.00
09.00
24h Cycle of Stomatal Opening and Closing
Why is this cycle an advantage to most plants?
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Questions
1. What is transpiration? Give three environmental factors which will increase transpiration rate.(2marks)
2. Explain how potassium ions are moved into the guard cells in light, and how this affects the guard cells and stomata.(6marks)
3. Give three adaptations a xerophyte may have to reduce transpiration and explain how they do this. (4marks)
4. Plants close their stomata at night and some also close their stomata around mid day. Explain why this is advantageous to the plant (2marks)
Click on the marks above to check your answer
finish
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Answer Q 1
• Transpiration is the loss of water from a plant by evaporation• Higher temperature, increased air movement, lower humidity
Back to question
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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
Back to question
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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
Back to question
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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.
Back to question
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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 CO2 required for photosynthesis, CO2 enters via stomata, water used in photosynthesis)
