Students, this ppt give the details about the mechanism involved in the stomatal transpiration.

1. Mechanism of stomatal
Transpiration
Dr. K. GANESH KUMARI
ASSISTANT PROFESSOR
DEPARTMENT OF BOTANY
SRIMAD ANDAVAN ARTS AND SCIENCE COLLEGE (AUTONOMOUS)
TRICHY - 5

2. Introduction
• Loss of water vapour through
stomata of leaves is called stomatal
transpiration.
• It accounts for about 90% water loss
from the surface of a plant.
• Stomata are microscopic pores
found in the upper and lower
epidermis of leaves.
• The pore is surrounded by two
guard cells which in turn are
surrounded by subsidiary cells.

3. Cont…
• The opening and closing of stomata
are due to the activity of guard cells.
• When the stomatal pore is open (in
day time), transpiration occurs.
• When it is closed (in night)
transpiration does not occur.
• The mechanism of stomatal
transpiration involves three steps.
 Diffusion of water from lead xylem
to intercellular spaces.
 Opening and closing of stomata.
 Diffusion of water vapour from
intercellular spaces to atmosphere.

4. Diffusion of water from leaf
xylem to intercellular spaces
• Leaf xylem, in the veins, supplies
water to all the leaf tissues.
• In the leaves, vascular bundles are
surrounded by mesophyll tissue
which in most cases is composed of
palisade parenchyma and spongy
parenchyma.
• There are small intercellular spaces
between the cells of palisade tissue
but in the spongy parenchyma
intercellular spaces are larger in
size.

5. Cont…
• Since the mesophyll cells are
photosynthetic, they accumulate
sugars.
• So their osmotic pressure is very
high.
• The mesophyll cells draw water
from the xylem tubes through
the cell walls by osmosis.
• As a result, the mesophyll cells
become turgid and saturated
with water.

6. Cont…
• As the cell wall of mesophyll cells
contains cellulose microfibrils and
other hydrophilic components, water
adheres to the microfibrils and soaks
the cell wall.
• Consequently a thin film of water
develops on the surface of mesophyll
cells.
• Water from this thin film, exposed to
the intercellular spaces, gets
evaporated into water vapour

7. Cont…
• Then water from the cell wall of interior
mesophyll cells is brought to the
exposed cell walls and evaporated.
• Because of the continuous vapourization
of water from moist cell walls, the
intercellular spaces become surround
with water vapour.
• The heat demand for this vapourization
is met by solar radiation.
• Heat from the sunlight, thermal
radiations and warm air current are used
by the leaves to vaporize water
• Nearly 500cal of energy is utilized for the
vaporization of 1g of water.

8. Opening and closing of stomata
• After vapourization of water in the
intercellular spaces, the stomata must
open for the diffusion of the vapour into
the outer environment.
• The stomata open when the guard cells
swell, turgid and increase in length.
• The stomata close when the guard cells
become flaccid.
• Stomata of most plants remain open
during the day time and closed in the
night.
• This daily periodicity of stomatal
opening and closing is called stomatal
march.

9. Cont…
• The guard cells intake ions by active
absorption from other cells.
• Metabolic products are also accumulated in
the guard cells.
• Consequently, the osmotic pressure (OP)
and diffusion pressure deficit (DPD) of
guard cells become higher.
• Water from surrounding cells moves into
the guard cells.
• Hence they become turgid. As the turgidity
increases, the guard cells increase in width
and curve at the middle due to differential
stretching of walls.
• So the stomatal pores opens.

10. Cont…
• The turgidity of guard cells decreases
when osmotically active substances are
transported to the surrounding cells.
• As a consequences, the guard cells
become flaccid and straight because of
low OP and DPD.
• So, the inner membranes of the sister
guard cells come close to each other
and the stomatal pore is closed.
• The opening and closing of stomata are
controlled by several factors such as
light intensity and quality, temperature,
relative humidity and intracellular CO2
concentrations.

11. Theories to explain the mechanism of closing and
opening of stomata
• Von Mohl (1856) suggested that guard cells become turgid and
open due to synthesis of osmotically active substances by
photosynthesis.
• Llyod (1906) stated that conversion of starch into glucose and
vice versa are responsible for the turgidity of guard cells to open
and close the stomata.
• Linsbauer (1916) proposed that permeability change in guard
cells during day and night as the reason for turgidity change in
the guard cells.

12. Cont…
• Mouravieff (1959) noticed that blue light induces turgidity in
guard cells and red light reduces the turgidity.
• According the walker (1966), dark fixation of CO2 in guard cells is
associated with turgidity and flaccidity of guard cells.
• According to Zelitch (1969) increase in potassium concentration
in guard cells is responsible for turgidity.

13. Diffusion of water vapour
from intercellular spaces to
the atmosphere
• The water vapour goes out to the
atmosphere through the stomatal
pore by simple diffusion.
• The diffusion of water vapour is
determined by the concentration
gradient of water vapour between
the substomatal cavity and outer
atmosphere.
• Diffusion of water vapour is more
rapid through small pores in unit
area than free water space of equal
size.

14. Cont…
• The average time needed for a water molecule to escape the leaf
is approximately 0.042 seconds.
• It is given by the equation – T = L2 / D2
• Where
• L is the distance of diffusion (less than 1mm = 10-3m) and
• D is the diffusion coefficient of water in air (2.4 x 10-5 m2 s-1).

15. Thank you