1. Prof. Samih Tamimi Biology751
Gas Exchange Gateway
Prof. Samih Tamimi Biology751

2. 1. Definition
Transpiration is the evaporation of water from the aerial parts of plants.
Of all the water plant absorbs, over 95-99% is transpired to the air as water vapor.

3. 4. From where water is transpired?
Aerial parts of whole young plant
Lenticels (lenticular transpiration) 0.1%
Cutin (cuticular transpiration) 3%~10% Stomatum (stomatal transpiration) ~ 90%

4. Prof. Samih Tamimi Biology751
What is most likely leaving through the stomata of the leaf picture here?
Water (H2O)
What is this process called?
Stomatal Transpiration
Prof. Samih Tamimi Biology751 4

5. Why should symplasmic resistances be variable?
Because water molecules cross cell membranes through a Nobel prize winning molecule :
AQUAPORINS

6. Prof. Samih Tamimi Biology751
Stomatal transpiration
Cuticle
Prevents water loss
Mesophyll
Site of photosynthesis
Stomata
Guard cells
Openings allow gases and water to move in and out of leaf
Open and close the stomata
Prof. Samih Tamimi Biology751

7. Understanding water pathways in leaves…
Mesopyll cell wall
stomatal chambers

8. Stomata regulate gas exchange: CO2 in, O2 and water out
H2O
H2O

9. Water loss Water loss is regulated by :
Difference in water vapor concentration
The pathway resistance

10. Transpiration rate (a flux) is a function of the water vapor
diffusion gradient (the driving force) and stomatal aperture
(the conductance of the pathway).
Flux = driving force x conductance
PP04100.jpg

11. Prof. Samih Tamimi Biology751
Water Pathway through the Leaf
\figures\ch04\pp04100.jpg
Prof. Samih Tamimi Biology751

12. The driving force of transpiration is the “vapor pressure gradient
The driving force of transpiration is the “vapor pressure gradient.” This is the difference in vapor pressure between the internal spaces in the leaf and the atmosphere around the leaf
Diffusional resistance comprises leaf resistance and boundary layer resistance

13. 5. Characteristics of guard cells

14. Guard cell properties and their relationship with stomatal control
Thickness of CW varies in the ventral and dorsal part of the guard cells.
Contains chloroplast and can perform light reaction. (not dark reaction for the lack of key enzymes)
Structurally isolated from epidermal cells for the lack of plasmodesmata (water and ions transmit only through cellular pathway, thus helps to build up water gradient)
Little volume, little amount of water absorption or loss controls stomtal aperture.

15. Mechanical Features Required for Proper Stomatal Function
1) the cell walls of guard cells can allow large elastic deformations(30% volume increase).
2) thickening of the ventral wall surrounding the pore compared tothe dorsal wall (away from the pore) favors a bending mode of the cell.
3) radial re-inforcement of the cell wall of the guard cell by cellulose microfibrils favors elongation of the cell. .
Prof. Samih Tamimi Biology751

16. Prof. Samih Tamimi Biology751

18. The mechanism of guard cell movement

19. Ion and organic molecules Guard cell ys Decrease (ys = -RTCs)
yw decrease
Water moves into the guard cell
yp increase
Stoma open

21. 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

22. 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

23. 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+

24. Increased concentration of K+ in guard cells
Guard cells turgid
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

25. 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

26. 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

27. 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+

28. Increased concentration of K+ in guard cells
Guard cells turgid
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

29. (1). Light
Stomata of most plant open in the day and close at night, while CAM plants are just the opposite.
Stomata opening are sensitive to red light and blue light, and blue light is more effective, it stimulates opening by a blue-light receptor: zeaxanthin.

30. 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

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32. Prof. Samih Tamimi Biology751
But blue light activates
Prof. Samih Tamimi Biology751

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35. Guard cell protoplasts
Prof. Samih Tamimi Biology751

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38. Activation of H+ pump by blue light
Prof. Samih Tamimi Biology751

39. Prof. Samih Tamimi Biology751
Role of the proton-pumping ATPase in the regulation of stomatal movement
PP5e-Fig jpg
Prof. Samih Tamimi Biology751

40. Phosphorylation of threonin (Thr950) on C terminal is induced by blue radiation. For activation of H+-ATPase is necessary also binding of protein

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45. Prof. Samih Tamimi Biology751
Summary
Prof. Samih Tamimi Biology751

46. Ion Transport—stomatal opening
Proton extrusion makes membrane potential more negative
Water influx
Potassium uptake. Membrane hyperpolarization activate the Kin channel
Inside cell
Membrane
Prof. Samih Tamimi Biology751

47. Prof. Samih Tamimi Biology751
Stomatal swelling
E. Water influx increases pressure, but water is incompressible, so guard-cell volume increases. The increase results from stretching of the dorsal wall.
Prof. Samih Tamimi Biology751

48. Prof. Samih Tamimi Biology751
Stomatal closing
Membrane
Inside cell
Potassium efflux.
Mechanism: Membrane depolarization activates the Kout channel
Prof. Samih Tamimi Biology751

49. Air boundary layer
A thin film of still air on the surface of leaf and its resistance to water vapor diffusion is proportional to its thickness. The thickness of the boundary layer is determined primarily by wind speed.

50. When air surrounding the leaf is still, increases in stomatal aperture have little effect on transpiration rate. The thickness of the boundary layer is the primary deterrent to water vapor loss from the leaf.
When wind velocity is high, the stomatal resistance has the largest amount of control over water loss.

51. Boundary layer resistance
Thickness of the layer is determined by wind speed.
Still air – layer may be so thick that water is effectively stopped from leaving the leaf
Windy conditions – moving air reduces the thickness of the boundary layer at the leaf surface
The size and shape of leaves influence air flow – but the stomata itself play the most critical role leaf transpiration
Prof. Samih Tamimi Biology751

52. Osmotic potential decrease Water potential decrease
ADP + Pi
Osmotic potential decrease
Water potential decrease
Water enter into vacuole
Increase turgor
Stomata open

53. Mechanism for stomatal close
Uptake of Ca+2 into the cytosol
Depolarize the membranes
Anion channel opened and Cl- and malate released from the vacuole.
K+ channel opened and K+ released from vacuole and subsequently into subsidiary cells.

54. Osmotic potential increase Water potential increase
ADP + Pi
Ca+2
Ca+2
Osmotic potential increase
Water potential increase
Water comes out from vacuole
decrease turgor
Stomata close
Ca+2

55. 6. Mechanism of stomatal opening ----K+ absorption theory
H+-ATPase in PM is light activated
Its function is out-pumping H+
HCO3-+PEP
Mal PM
Mal－
　＋H+
light
Inward rectifier K+ channel is voltage dependent, PM hyperpolarization activates the channel and carry K+ inward H＋ H＋ V K＋ K＋ H＋ H＋
Cl- is transported through Cl- /H+ symport or Cl-/OH-antiport
Cl-
Cl-

56. 7. Factors influencing stomatal aperture
Light
Temp.
CO2
Water content
Plant hormone

57. Main channels of ion transport in guard cells plasmalemma , tonoplast
K+in, K+out, Ca2+in
from Schroeder et al. Annu. Rev. Plant Physiol. Plant mol. Biol

58. Blue radiation receptor can be zeaxanthin in guard cell chloroplasts
Blue radiation receptor can be zeaxanthin in guard cell chloroplasts. After its excitation, the signal is tranferred into cytoplasm where serin/threonin kinase is activated. This kinase activates ATPase and stimulate transport of H+  K+in. Zeaxanthin is formed by deepoxidation of violaxanthin (xanthophyl cycle). Zeaxanthin formation is dependent on electron transport rate and consumption of ATP and NADPH during CO2 fixation. Thus concentration of CO2 can affect response of stomata to irradiance (Zeiger, E. Trends Plant Sci. 5: 183, 2000)

60. Guard-Cell Function.