Transpiration B.Sc. III Sem V (botany)

1. Mr. M.D. Kolte
Assistant Professor
Department of Botany
Shri Shivaji Science & Arts College,
Chikhli
Transpiration

2. Introduction
 Plants absorb large quantities of water as
well as mineral salts from the soil with the
help of their roots. This water is
conducted in the upward direction upto
the tips of stem, branches and leaves by
a process called ascent of sap. We are
going to discuss about Transpiration
process, transpiration types and factor
affect transpiration.
 Major part of water is lost by way
of transpiration from the leaves which
have large surface area. Transpiration
process is very useful for plants for two
reasons. One to get rid of excess water
absorbed by roots (about 98%) and two
for cooling the plant in hot weather.

3.  WHAT IS
TRANSPIRATION
PROCESS ?
 The loss of water in the
form of vapours from the
aerial exposed parts of a
plant is
called transpiration
process.
TRANSPIRATION
TYPES
 Depending upon the
aerial part of the plant
through

4. 1. Stomatal transpiration :
 Loss of water vapours
through microscopic
pores (stomata
surrounded by
specialized
guard cells is
called stomatal trans
piration types.
Stomatal transpiratio
n accounts for upto
90% of total water
loss from the plants
through transpiration
process. Thus It js,

5. 2. Cuticular transpiration
:
 Some transpiration takes
place by the direct evaporation
of water from the outer walls of
the epidermal cells. This layer is
impermeable to water. But
some loss does take place
through this layer. Water loss
due to evaporation
through transpiration.
It constitutes upto 10% of total
water loss from the plants
through transpiration. It should
be noted that greater the
thickness of cuticle, lesser will
be the rate of transpiration. It

6. 3. Lenticular transpiration
:
 Water loss through the
lenticels in woody stems
and fruits is called
lenticular transpiration.
It constitutes about 1-2%
of total water loss
through transpiration
process. Lenticels
never close. They
remain open all the
times.
It was all
about transpiration
types.

7. STOMATAL APPARATUS (STOMATA)
 Stomata (sing. stoma) are microscopic pores surrounded by two
specialized kidney. shaped or dumb-bell shaped guard cells. They
are present in the epidermis of leaves and young green stems. They
are chiefly concerned with gaseous exchange during
photosynthesis, respiration and water loss through transpiration
process.
 Transpiration is an inevitable process through open stomata. The
guard cells surrounding a pore are highly specialized
epidermal cells with chloroplasts. They are placed side by side
(parallel) and are kidney shaped in dicot leaves. They are, however,
dumb-bell shaped in monocots.
 Their walls are differentially thickened. The opposing inner walls of
guard cells are thickened in the middle and are, thus, inelastic. The

8. Mechanism of stomatal
transpiration
 The mechanism of stomatal transpiration which
takes place during the day time can be studied in
three steps.
 i. Osmotic diffusion of water in the leaf from
xylem to intercellular space above the stomata
through the mesophyll cells.
 ii. Opening and closing of stomata (stomatal
movement)
 iii. Simple diffusion of water vapours from
intercellular spaces to other atmosphere through
stomata.

9. i. Osmotic diffusion:
 Water absorbed by the roots of a plant moves
upwards through the xylem vessels of roots, stems
and reaches up to the leaves
 Inside the leaf the mesophyll cells are in contact with
xylem, and on the other hand with intercellular
space above the stomata
 When mesophyll cells draw water from the xylem
they become turgid and their diffusion pressure
deficit (DPD) and osmotic pressure (OP) decreases.
 That they release water in the form of vapour in
intercellular spaces close to stomata by osmotic
diffusion.
 Now in turn, the O.P and D.P.D of mesophyll cells
become higher and hence, they draw water form

10. Opening and closing of the
stomata
 There are different mechanisms which are responsible
for the opening and closing of the stomata.
 The opening and closing depends upon the turgor
pressure in the guard cells.
 The swelling of guard cells due to absorption of
water causes opening of stomatal pores while
shrinking of guard cells closes the pores.
 Opening and closing of stomata occurs due to turgor
changes in guard cells. When guard cells are turgid,
stomatal pore is open while in flaccid conditions, the
stomatal aperture closes.
 There are other theories which explains the opening and
the closing of the stomatal pore.
 1. Starch – sugar Inter conversion theory
 2. Synthesis of sugars or organic acids in Guard cells
 3. ATP –Driven proton (H+ ) – K exchange pump

11. 1. Starch – Sugar Inter Conversion
Theory
 This classical theory is based on the effect of pH on starch
phosphorylase enzyme which reversibly catalyses the conversion
of starch + inorganic phosphate into glucose -1 phosphate.
 During the day, pH is guard cells in high. This favours hydrolysis
of starch (which is insoluble into glucose -1- phosphate (which is
soluble) so that osmotic pressure is increased in guard cells.
 Consequently water enters, into the guard cells by osmotic
diffusion from the surrounding epidermal and mesophyll cells.
Guard cells become turgid and the stomata open.
 During dark, reverse process occurs. Glucose 1- phosphate is
converted back into starch in the guard cells thereby decreasing
osmotic pressure. The guard cell release water, become flaccid
and stomata become closed.

12.  According to Steward 91964), the conversion of
starch and inorganic phosphate into glucose-1-
phosphate does not cause any appreciable change in
the osmotic pressure because the inorganic
phosphate and glucose-1-phosphate are equally
active osmotically.
 In this scheme he has suggested that,
 Glucose-1-phosphate should be further converted into
glucose and inorganic phosphate for the opening of
stomata.
 Metabolic energy in the form of ATP would be

13. ATP –Driven proton (H+ ) – K exchange pump mechanism in
Guard cells:
 According to this mechanism, there is accumulation of K+
ions in the guard cells during day light period.
 The protons (H+ ) are ‘pumped out’ from the guard cells
into the adjacent epidermal cells and in exchange K+ ions
are mediated through ATP and thus are an active process.
 ATP is generated in non-cyclic photophosphorylation in
photosynthesis in the guard cells. The ATP required in ion
exchange process may also come through respiration.
 The accumulation of K ion is sufficient enough to
significantly decrease the water potential of guard cells
during day light.
 Consequently, water enters into them from the adjacent
epidermal and mesophyll cells thereby increasing their
turgor pressure and opening the stomatal pore.
 Reverse situation prevails during dark when stomata are
closed. There is no accumulation of ‘K’ in g cells in dark.

14. Simple Diffusion Of Water Vapours
 The last step in the mechanism of transpiration is
the simple diffusion of water vapours from the
intercellular spaces to the atmosphere through
open stomata.
 This is because the intercellular spaces are more
saturated with moisture is comparison to the
outer atmosphere in the vicinity of stomata.

15. Factors affect
TRANSPIRATION
 Rate of Transpiration is affected both by the
Internal and External Factors.
(A) Internal factors :
 (1) Leaf area : Larger the surface area of the
leaves, more is the transpiration.
(2) Leaf structure : Presence of thick cuticle,
coating of wax, sunken stomata, compact
mesophyll and reduction in the number of
stomata reduce the rate of transpiration.
(3) Water content of leaves: Due to insufficient
absorption of water by roots, if the water content
of the leaves decreases, the leaves wilt
and transpiration is reduced.

16. (B) External factors :

17. ADAPTATIONS IN PLANTS TO
REDUCE TRANSPIRATION
 Many plants specially xerophytes have evolved a
variety of permanent adaptations to cut
down transpiration. Some of these external
adaptations are : -
(a) Thick cuticle : The leaves may be covered by
very thick cuticle e.g., Banyan.
(b) Loss of leaves : In some plants leaves may be
dropped or may be absent as in most cacti.
(c) Narrow leaves : To reduce the surface area
for transpiration, leaves in some plants become
narrower, e.g., Nerium.
(d) Fewer stomata : In some plants, the number of
stomata may be reduced.
(e) Sunken stomata : In some plants, the stomata
are sunken (stomata depressed below the adjoining
epidermal cells) or covered by hair, e.g., Nerium.

18. Significance of Transpiration in
Plants
 Transpiration helps in the conduction of water and
minerals to different parts of the plants.
 Due to the continuous elimination of water from the
plant body, there is a balance of water maintained
within the plant.
 It maintains osmosis and keeps the cells rigid.
 A suction force is created by transpiration that helps in the
upward movement of water in the plants.
 Certain hydrophilic salts are accumulated on the surface of
the leaves, which keeps the leaves moist.
 It maintains the turgidity of the cells and helps in cell
division.
 Optimum transpiration helps in the proper growth of the
plants.
 The cooling effect of a tree is due to the evaporation of
water from its leaves.

19. Antitranspirants
 A number of substances are known which when applied to
the plants retard their transpiration. Such substances are
called as antitranspirants. Some examples of
antitranspirants are colourless plastics, silicone, oils, low
viscosity waxes, phenyl mercuric acetate, abscisic acid,
CO2, etc.
 Colourless plastic, silicone oils and low viscosity waxes
belong to one group as these are sprayed on the leaves,
form after film which is permeable to O2 and CO2 but not to
water.
 Fungicide phenyl mercuric acetate, when applied in low
concentration (10-4 m), it exercised a very little toxic effect
on leaves and resulted in partial closure of stomatal pores
for a period of two weeks.
 Similarly ABA a plant hormone also induces stomatal
closure.
 CO2 is an effective antitranspirants. A little rise in CO2

20. Guttation
 In some plants such as garden nasturtium, tomato,
colocasia etc, water drops ooze out from the uninjured
margins of the leaves where a main vein ends. This is
called as guttation and takes place usually early in the
morning when the rate of absorption and root pressure are
high while the transpiration is very low.
 The phenomenon of guttation is associated with the
presence of special types of stomata at the margins of the
leaves which are called as water stomata or hydathodes.
Each hydathode consists of a water pore which remains
permanently open.
 Below this there is a small cavity followed by a loose tissue
called as epithem. This epithem is in close association with
the ends of the vascular elements of veins. Under high root
pressure the water is given to the epithem by the xylem of
the veins.
 From epithem water is released into the cavity. When this