Transpiration is the evaporation of water from plant leaves. Over 95-99% of water absorbed by plants is transpired through openings called stomata. Stomata are bordered by guard cells that open and close to regulate gas exchange and water loss. When open, stomata allow carbon dioxide and oxygen exchange for photosynthesis while also allowing water to transpire out as vapor. Environmental factors like light, temperature, humidity, and soil water availability influence transpiration rates by impacting stomatal openings and the vapor pressure gradient driving water movement.

1. Transpiration

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. What is most likely leaving through the
stomata of the leaf picture here?
• Water (H2O)
What is this process called?
 Stomatal Transpiration

5. 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

6. 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

7. 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

8. 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

9. 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

11. 5. Characteristics of guard cells

12. 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.

13. 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

14. 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.

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

16. (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.

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

19. (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.

20. (4) Water content
• Stomta open when the leaf contain
enough water. When there is a water
shortage, they close.

21. (6) Plant hormones
• CTK promotes opening
• ABA inhibits

22. Factors that influence transpiration
Transpiration from the leaf depends on
two major factors:
1. Difference in water vapor gradient
2. Diffusional resistance

23. 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

24. Transpiration rate=Driving force/resistance
water vapor inside the leaf - water vapor of the air
=
stomatal resistance + boundary layer resistance

25. 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 .

26. 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.

27. • 4. Wind
When a breeze is present, the humid air is
carried away and replaced by drier air.
• 5. Soil water
A 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.

28. Cells turgid/Stoma open
Changes in guard cell shape and stomatal opening and closing
(surface view)
Radially oriented
cellulose microfibrils
Vacuole
Cell
wall
Guard cell
Cells flaccid/Stoma closed