Transpiration

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.

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%

What is most likely leaving through the stomata of the leaf picture here?

• Water (H2O)

What is this process called? Stomatal Transpiration

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

Stomatal transpiration

Stoma Open

Stoma Closed

Guard Cells

Stoma

Importance of transpiration

Guard Cells

CO2

O2 H2O

What goes in?

What goes out?

What process involves using CO2 and H2O releasing O2 as a waste product?

• Photosynthesis

What is the plant using this process to make?

• Carbohydrates-glucose

If the plant needs water for photosynthesis, why is water coming out of the stoma?

Stoma Open Stoma Closed

Function of Stomata

Guard Cells

• These stomata (leaf openings) naturally allow water to evaporate out.

Why would the plant close stomata with guard cells?

• Prevent excess water loss through transpiration. (conserve water)

So what is the point of having stomata?

• Allows gas exchange for photosynthesis

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 Cells

4. Guard cells: • cells that open and

close the stoma

5. Stomata: openings in leaf’s surface; when open: • GAS EXCHANGE:

Allows CO2 in & O2 out

of leaf • TRANSPIRATION:

Stomata

5. Characteristics of guard cells

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.

6. Mechanism of stomatal 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.

7. Factors influencing stomatal aperture

• Light

• Temp.

• CO2

• Water content

• Plant hormone

(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.

(2) Temperature

• Stomatal aperture increase with Temp, within 20- 30 (the optimal).℃

(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.

(4) Water content

• Stomta open when the leaf contain enough water. When there is a water shortage, they close.

(6) Plant hormones

• CTK promotes opening

• ABA inhibits

Factors that influence transpiration

Transpiration from the leaf depends on two major factors:

1. Difference in water vapor gradient

2. Diffusional resistance

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 stomatal resistance and boundary layer resistance

Transpiration rate=Driving force/resistance

water vapor inside the leaf - water vapor of the air

= stomatal resistance + boundary layer resistance

Environmental factors that affect the rate of transpiration

1. Light

Plants transpire more rapidly in the light than in the dark. This is largely because light stimulates the opening of the stomata , Light also speeds up transpiration by warming the leaf .

2. Temperature

Plants transpire more rapidly at higher temperatures because water evaporates more rapidly as the temperature rises.

3. Humidity When the surrounding air is dry, diffusion of water out of the leaf goes on more rapidly.

• 4. Wind When a breeze is present, the humid air is carried away and replaced by drier air.

• 5. Soil waterA plant cannot continue to transpire rapidly if its water loss is not made up by replacement from the soil. When absorption of water by the roots fails to keep up with the rate of transpiration, loss of turgor occurs, and the stomata close. This immediately reduces the rate of transpiration. If the loss of turgor extends to the rest of the leaf and stem, the plant wilts.

Cells turgid/Stoma open

Changes in guard cell shape and stomatal opening and closing(surface view)

Radially orientedcellulose microfibrils

Vacuole

Cell wall

Guard cell

Cells flaccid/Stoma closed