PLANT PHYSIOLOGYStomatal Regulators

Maham Naveed

M.Sc Roll # 05

M.Sc Botany

12 Nov,2014

Cuticle

Cuticle

Mesophyll

Stomata Guard cells

Prevents water loss

Site of photosynthesis

Openings allow gases and water to move in and out of leaf

Open and close the stomata

Leaf structure

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.

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

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

Function of Guard Cells

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.

Stomata frequency upper and lower epidermis

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.

Mechanism of stomata opening K+ absorption theory

H＋

light

K ＋

H＋

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

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.

Changes in ionic concentrations, pH and membrane potential for epidermal cells, subsidiary cells and guard cells.

O = open stomata; C = closed stomata

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.

• Ion and organic molecules

• Guard cell

∀ ψ s Decrease

∀ ψ w decrease

• Water moves into the guard cell

∀ ψ p increase

• Stoma open

• Osmotic potential decrease• Water potential decrease

• Water enter into vacuole

• Increase turgor

• Stomata open

Mechanism for stomatal close

Uptake of Ca+2 into the cytosolDepolarize the membranesAnion channel opened and Cl- and malate

released from the vacuole.K+ channel opened and K+ released from

vacuole and subsequently into subsidiary cells.

• Osmotic potential increase• Water potential increase

• Water comes out from vacuole

• decrease turgor

• Stomata close

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.

• 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 )

• 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.

Factors influencing stomata aperture

• Light

• Temp

• CO2

• Water content

(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.

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.

(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

(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.

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.

(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.

Hydro passive closureWater 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 closureABA (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

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.

Any Question?