Transpiration Transpiration is the loss of water from a plant by evaporation Water can only evaporate from the plant if the water potential is lower in the air surrounding the plant Most transpiration occurs via the leaves Most of this transpiration is via the stomata.

How Transpiration is Measured Water evaporates from the plant A Simple Potometer 1’’’’’’’’2’’’’’’’’3’’’’’’’’4’’’’’’’’5’’’’’’’’6’’’’’’’’7’’’’’’’’8’’’’’’’’9’’’’’’’’10’’’’’’’’11’’’’’’’’12’’’’’’’’13’’’’ Leafy shoot cut under water Air tight seals Capillary tube Plastic tubing Movement of meniscus is measured over time Graduated scale

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 volume of water taken up in given time Limitations measures water uptake 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

6 Environmental Factors Affecting Transpiration 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 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. 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. 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.

Intrinsic Factors Affecting the Rate of Transpiration. Leaf surface area Thickness of epidermis and cuticle Stomatal frequency Stomatal size Stomatal position

The Effect of Wind Speed on the Rate of Transpiration Stomatal transpiration rate / gcm-2s-1 In still air closing the stomata is less effective in controlling the transpiration rate moving air still air 10 20 Stomata diameter/µm

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 Lower  Boundary layer the  gradient is increased, so faster rate of water evaporation via the stomata

Movement of Water Through the Stomata Water moves from a higher (less negative) to a lower (more negative) water potential H2O Diffusion shells

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

Wilting If water lost by transpiration is greater than water uptake via the roots the plant cells become flaccid and the plant wilts. 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 When the guard cells are flaccid the stomata close

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/greenplantsasorganisms/0photosynthesisrev2.shtml The lower epidermis has stomata. The guard cells control the opening and closing of the stomata

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

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

A single stoma and guard cells Guard cells flaccid Guard cells turgid Stoma open Stoma 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

The guard cells control the opening and closing of the stomata Guard cells flaccid Guard cells turgid Thin outer wall Thick inner wall Stoma closed Stoma open

Regulating Stomatal Opening:-the potassium ion pump hypothesis Guard cells flaccid K+ K+ ions have the same concentration in guard cells and epidermal cells K+ K+ K+ K+ Light activates K+ pumps which actively transport K+ from the epidermal cells into the guard cells K+ K+ K+ K+ K+ K+ K+ Stoma closed

Regulating Stomatal Opening:-the potassium ion pump hypothesis H2O Increased concentration of K+ in guard cells K+ K+ Lowers the  in the guard cells K+ K+ K+ K+ K+ K+ K+ Water moves in by osmosis, down  gradient K+ K+ K+

Guard cells turgid Stoma open Increased concentration of K+ in guard cells H2O H2O K+ K+ Lowers the  in the guard cells K+ H2O H2O K+ K+ K+ K+ K+ K+ H2O H2O K+ Water moves in by osmosis, down  gradient K+ K+ Stoma open

Xerophytes Have Special Adaptations to Reduce the Rate of Transpiration Xerophytes live in hot, dry environments These cacti have reduced leaf area as the leaves are reduced to spines 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 Fleshy leaves to hold water Silver surface to reflect sun

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:- 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

Cross Section of Marram Grass Leaf 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 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 This reduces the  gradient so reduces water loss from the plant

Adaptation to Increase Water Uptake in Xerophytes Deep extensive root system to maximise water uptake Accumulation of solutes in the root system to reduce the , so making the  gradient from the soil to the root cells steeper Some very shallow roots to absorb dew which condenses on the soil at night

Graph to show stomatal opening over 24 hours Some plants close stomata during hottest time-saving water 100 Increased light intensity causes more stomata to open stomatal opening/% An adaptation to hot dry environments Stomata close as the sun sets Dawn-stomata begin to open 12 2 4 6 8 10 12 2 4 6 8 10 12

24h Cycle of Stomatal Opening and Closing 12.00 Why is this cycle an advantage to most plants? 09.00 15.00 18.00 06.00 21.00 3.00 24.00