For B.Sc. III students

1. TRANSPIRATION
(for B.Sc. III. Students)
The loss of water in the vapour from exposed aerial parts
of plant is called transpiration.
Dr Jasmine Brar
Email : drjasmine1807@gmail.com

2. Evaporation Transpiration
Water vapours are formed at
free surface of water
Internal tissues of plant
It occurs at surface of non-
living objects
Exposed surface of plants
It is a physical force
controlled by environmental
conditions like RH & air
currents
It is both physical &
physiological influenced by
rate of water absorption,
osmotic pressure of cells,
thickness of cuticle & stomata
It stops when air is fully
saturated
It continues for sometimes
even after saturation due to
higher temp. of plant organs
exposed to sunlight. Hence,
higher saturation point for
their internal air

3. Evaporation Transpiration
Velocity of air currents directly
affects it
Velocity of air currents
indirectly affects it depending
upon intensity of light, closure
of stomata, bending of leaves
Light does not directly affect it Light affect it as stomata open
in sunlight & close at night
It is faster slow
Not influenced by structure of
evaporating surface
Controlled by anatomy of
transpiring organ
Not influenced by CO2,
hormones, pH
It is influenced by CO2,
hormones, pH
It continues as long as water is
available on surface
Dependent on absorption of
water from soil

4. Transpiration Ratio
Transpiration is measured in terms of dry matter
manufactured by the plant.
Water requirement is minimum in case of CAM plants like
Pine-apple where the value is 50 units.
C4 show a value of 100-200 units, Alfafa 900 units of water
For manufacture of single unit of dry matter.
Only 0.2 % water is used in transpiration

5. Fate of absorbed water
(Miller, 1938)

6. Stomatal & Cuticular Transpiration

7. Lenticular & Bark
Transpiration

8. Closed
Incompletely
Open
Stoma in V.S.

9. Closed Open
V.S. stoma passing
through middle of
guard cells

10. Arrangement of microfibrils in guard cells
Reniform
Dumb-Bell Shaped

11.  Stomatal frequency refers to the number
of stomata per unit area of the leaf. The number and
distribution of the stomata plays an important role in
determining the rate of gas exchange and water loss
from a leaf.
 Salisbury (1928) proposed the term ‘stomatal index’.
Stomatal index relates the number of stomata to number
of epidermal cells.
Stomatal frequency

12. Stomatal Frequency

13. Diffusion of water vapours

14. External Interference & Resistance
Mutual interference in their diffusion shells by
neighbouring stomata & resistance in the movement of
water vapours by the stationary air layer adhering the
transpiring surface

15. Transpiration as necessary evil
Transpiration is a necessary evil because it is an inevitable
process but harmful to plant. Its harmful effects include:
wilting, serious desiccation, and often death of a plant, if
there is shortage of water. Mild water stress results in
reduced growth rate, and reduction in yield.
However, transpiration is also beneficial to the plants in
several ways.
a. Mineral Absorption, b.Turgidity, c. Energy Exchange, d. Effect on
Growth and Development, e. Absorption of Water, f. Exchange of Gases