mechanism of stomatal regulation

1. PLANT PHYSIOLOGY
Stomatal Regulators
Maham Naveed
M.Sc Roll # 05
M.Sc Botany
12 Nov,2014

2. Cuticle
Prevents
water loss
Cuticle
Mesophyll
Stomata Guard cells
Site of
photosynthesis
Openings allow gases
and water to move in
and out of leaf
Open and close
the stomata
Leaf structure

3. 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.

4. Guard Cells
4. Guard cells:
• cells that open and
close the stoma
4. Stomata: openings in
leaf’s surface; when
open:
• GAS EXCHANGE:
Allows CO2 in & O2 out
of leaf
• TRANSPIRATION:
Stomata

5. Function of Guard Cells
How do the
guard cells react
to the availability
of water?
 Dry – guard
cells CLOSE
 lots of H2O –
guard cells
OPEN
http://www.ualr.edu/~botany/images.html

8. Guard cell properties and their
relationship with stomatal control
• Thickness of cell wall varies in the
ventral(0.5μm) and thicker 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.

9. Stomata frequency upper and lower epidermis

10. History
• The recent theory rejected the old classical
theory of mechanism proposed by
Scarth(1932).
• Its replacement by Fujino's theory (1959)
concept of active K+ transport.
• Solution accumulation especially of K+ , Cl-and
malate is principally responsible.

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

12. When the stomatum is opening, the [K+] rises to 0.5M, anions
rise to 0.2-0.5M, the osmotic potential drops 2MPa, thus bring
water in.

13. Changes in ionic concentrations, pH and membrane
potential for epidermal cells, subsidiary cells and guard cells.
O = open stomata; C = closed stomata

14. Driving force -> osmotic uptake of water -> increase
of hydrostatic pressure -> push the thin dorsal walls
outward into the neighboring epidermal cells. -> cause
cell to arch.
In dumbbell-shape guard cells, the bulbous ends of the
cells push against each other as they swell, driving the
central handles apart in parallel and widening the
pore.
In 1960, it became evident that K+ levels are high in
open guard cells and very low in closed guard cells.
Upon opening, large amounts of K+ move from the
subsidiary and epidermal cells into the guard cells.

15. • Ion and organic molecules
• Guard cell
" ys Decrease
" yw decrease
• Water moves into the guard cell
" yp increase
• Stoma open

16. • Osmotic potential decrease
• Water potential decrease
• Water enter into vacuole
• Increase turgor
• Stomata open

17. 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.

18. • Osmotic potential increase
• Water potential increase
• Water comes out from vacuole
• decrease turgor
• Stomata close

19. Role of ABA
• Seed development
• Dormancy
• Germination
• Stomatal closure
• Leaf senescence
• Environmental stress response
• During drought ABA triggers a signaling in guard cells that
result stomata closure and inhibiting stomata opening.

20. • In guard cells of Arabidopsis thaliana ABA induced
calcium oscillations occur in 10.3 min & therefore too
slow to encode a signal that’s activate anions
channels with a lag time of 2 min.
• Calcium cytology activates 2 different types of anions
channels.
• S Type (slow activating sustained)
• R Type (Rapid transient )

21. • Both mediate Anions release from guard cells causing
Depolarization.
• This change in membrane potential deactivates K+
inward and activate K+ outward channels,resulting in
K+ efflux from guard cells.
• ABA causes an alkalization of guard cells cytosol
which directly enhance K+ outward channels.
• Anions and K+ contribute loss of guard cell turgor
and leads stomata close.

22. Factors influencing stomata
aperture
• Light
• Temp
• CO2
• Water content

23. (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.

24. Blue light has direct effect on stomatal opening.
At low fluence rate, blue light causes stomatal
opening, but not red light.
At high fluence rate, stomatal opening under blue
light is consistently higher than under red light.
Blue light -> blue/UV-A cytochrome -> proton
extrusion -> photosynthesis -> ATP production ->
stomatal opening.

26. (2) Temperature
Stomatal aperture increase with Temp, within 20-
30℃ (the optimal).
Temp increase -> stimulate respiration and impair
photosynthesis -> CO2 conc. increase -> stomata
close
Midday closure
temp increase -> photosynthesis reduced
water deficit occur -> photosynthesis reduced

27. (3). CO2
Low CO2 conc. promotes stomatal opening,
while high CO2 conc. inhibits stomatal opening
through its acidification of the guard cell thus
inhibits PM hyperpolarization.

28. Effect of CO2 on stomatal movement
CO2 conc. decrease => stomata open => to uptake more
CO2
CO2 conc. increase => stomata close
The response of the stomata is to the intracellular conc. of CO2
in the guard cells.
When CO2 level decrease or photosynthesis is needed, guard
cells will take up water and swell to open the pores, in order to
take more CO2.
When CO2 level increase or the water stress override the
photosynthesis, the guard cells will close.

29. (4) Water content
Stomata open when the leaf contain enough
water. When there is a water shortage, they
close.
At night, no photosynthesis
Stomata close, preventing unnecessary loss of
water.
Sunny morning, photosynthesis is demanding,
supply of water is abundant.

30.  Hydro passive closure
Water loss from the guard cells exceeds the rate of
movement into the guard cells. => decrease in
turgidity of the guard cells => stomata closure
Hydro active closure
ABA (abscisic acid,) referred to as an antitranspirant.
Plants sense water deficit => ABA is released into the
apoplast and then transferred to the guard cells =>
stomata close => ABA conc. is increased => more
stomata close

31. References
• F.B Salisbury:ross “Plant Physiology” 3rd
Edition.
• Taiz & Zeiger “plant Physiology” 5th Edition.
• Cowan, I. R. and Farquhar, G. D. (1977).
Stomatal function in relation to leaf
metabolism and environment:471-505.
• http://www.wikipedia.stoamatal regulation.

32. Any Question?