Transpiration is the process of water vapor loss from plants, constituting 98-99% of the water absorbed and significantly influencing atmospheric moisture. It occurs through three main types: stomatal (98%), cuticular (3-10%), and lenticular (0.1%), with stomatal transpiration being the most crucial for gas exchange and cooling. Factors affecting transpiration rates include internal aspects like stomatal density and leaf area, and external elements such as light intensity, humidity, temperature, and wind, highlighting its essential role in plant health and metabolic processes.

1. Transpiration
By: Dr. Sunita Sangwan
Assistant professor, Dept of Botany
Higher education Dept. Haryana

2. Introduction
• The loss of water in the form of vapour from living tissue of
aerial parts of the plant is called TRANSPIRATION.
• The loss of water due to transpiration is quite high.
• Almost 98-99% of water absorbed by a plant is lost in
transpiration.
• According to recent studies it has be revealed that about 10
% of the moisture found in the atmosphere is released by
plants through transpiration. The remaining 90% is mainly
supplied by evaporation from, ocean, seas, lakes, rivers,
streams etc.

3. • Transpiration is the process
by which the water that is
absorbed by plants, through
roots, is evaporated into the
atmosphere from the
internal tissues of living
plants through the aerial
parts, such as leaves, green
shoots etc., where it is
converted from a liquid form
into a gaseous form in the
presence of sunlight

4. Types of
transpiration
Stomatal
Transpiration
(98%)
Lenticular
Transpiration
(0.1%)
Cuticular
Transpiration
(3-10%)

5. 1) STOMATAL TRANSPIRATION
• Most important type of transpiration. Constitutes 50-
97% of total transpiration.
• It is the loss of water through special apertures which
are present in leaves called STOMATA. Few stomata are
found on green stem, flower and fruits.
• Stomata expose the wet interior part of plant to the
atmosphere.
• Water vapour therefore, pass outwardly through
stomata by diffusion.
• Stomatal transpiration continues till the stomata are
open.

7. 2) CUTICULAR TRANSPIRATION
• It occurs through the cuticle or epidermal cells
of leaves and exposed parts of the plant.
• In common land plants cuticular transpiration
is only 3-10% of the total transpiration.
• In herbaceous shade loving plants where
cuticle is very thin, cuticle transpiration may be
up to 50% of the total.
• This transpiration continues throughout day
and night.

9. 3) LENTICULAR TRANSPIRATION
• It is found only in woody stems and some
fruits where lenticels occurs.
• Constitutes major part of water loss by
deciduous trees during leafless stage.
• Occurs continuously day and night and
there is no mechanism to stop or reduce
it.
• Total water loss through them is only
fraction of total i.e. 0.1%.

11. Mechanism of transpiration
• To form vapours, exposed parts of the plant need a
source of heat energy.
• The dry air of the atmosphere has high DPD (or low
water potential)
• - 13.4 atm at 99% R.H.
• -140 atm at 90% R.H.
• -680 atm at 60% R.H.
• 2055 atm at 20% R.H.
• Such high DPD or low water potential has overcome
various resistances which water molecule have to meet
from liquid phase to vapour phase.

13. • Intercellular spaces of transpiring organs (Saturated
with water vapours)
• When stomata are open, water vapour are drawn from
the stomata to the outside air due to high DPD.
• This increases the DPD of sub-stomatal air, it draws
more water vapour from intercellular spaces.
• The latter get water vapours from the wet walls of
mesophyll cells.
• Water is passively transported into the roots and then
into the xylem.
• The forces of cohesion and adhesion cause the water
molecules to form a column in the xylem. Water
moves from the xylem into the leaves.

14. So mechanism of transpiration involves three basic
steps:
• Absorption at the roots.
• Capillary action in the xylem vessels.
• Evaporation at the leaf.

17. Internal factors/ plant parameters
affecting the transpiration rate
Stomata:
• Stoma is the openings in the leaf that allow gas exchange
where water vapor leaves the plant and carbon dioxide
enters. When stomata are open, transpiration rates
increase; when they are closed, transpiration rates
decrease.
• More stomata will provide more pores for transpiration.
Leaf area:
A plant with large leaf area will show more transpiration
than another plant with less leaf area. More leaves mean
a bigger surface area and stomata for transpiration.

18. Cuticle:
• The cuticle is the waxy layer present on all above-ground
tissue of a plant and is hydrophobic or water-repelling.
Therefore, water does not move through it very easily.
• Thicker the cuticle layers on the leaf surface, slower the
transpiration rate. Sun leaves have much thicker cuticles
than shade leaves.
Root/Shoot Ratio:
• A low root/shoot ratio decreases the rate of
transpiration while a high ratio increases the rate of
transpiration.
Mesophyll:
• Compact mesophyll reduces transpiration while a loose
mesophyll increases transpiration.

19. Leaf Modifications:
• Formation of prickles, leaf spines, scaly leaves,
phyllodes, phylloclades, are all modifications found
in xerophytes to reduce transpiration

20. External factors affecting the
transpiration rate
Light intensity:
• It warms the leaves up quicker.
• It also cause the stomata to open,
• Thus more transpiration occurs during the day and the
rate is higher on a sunny day compared to a cold dull
day.

21. Relative Humidity:
• At low humidity there is a lower concentration of water
molecules in the air around the leaves. This
concentration gradient helps the transport of water
molecules from the leaves by diffusion.
• High humidity means the air around the leaves is
already saturated and has a higher concentration of
water molecules than inside the leaves.
• The drier the atmosphere, larger the driving force for
water movement out of the plant, increasing rates of
transpiration.

22. Temperature:
• An increase in the air temperature warms
the water inside the leaves more quickly
causing it to evaporate quicker.
• It also increases the capacity of the air to
absorb more water.
• Warmer air holds more water creating a
large driving force for water movement out
of the plant, increases rates of transpiration
and cooler air decreases.

23. Wind:
• Transpiration relies on diffusion. Windy conditions
cause the air molecules to be blown away from the
leaves, preventing the air around the leaves
becoming saturated with water molecules.
• During high wind- the transpiration becomes very
active because the water vapour around the leaves
is instantly removed.
• At very high wind velocity - the stomata are closed
because of rapid loss of water from the guard cells
and the rate of transpiration is declined.

24. Soil water:
• The source of water for transpiration in a plant comes
from the soil.
• Plants with adequate soil moisture will normally
transpire at high rates because the soil provides the
water to move through the plant.
• Plants cannot continue to transpire if the soil is very
dry.
• This condition causes the leaf to lose turgor or
firmness, and the stomata to close. If this loss of
turgor continues throughout the plant, the plant will
wilt.

25. Significance of Transpiration in
Plants
• Transpiration is of immense importance to plant
life and the same is briefly described below
1. Gaseous exchange:
• Transpiration is essential in the life of land plants. It
helps in the absorption of carbon dioxide (CO2)
from the atmosphere during photosynthesis as the
openings of stomata in day time facilitate gaseous
exchange.

26. 2. Cooling effect:
• The leaves absorb the radiant energy. Some of the light
energy is utilized in photosynthesis; rest is converted into
heat energy resulting in an increase in leaf temperature.
How ever, transpiration brings down their temperature
thus inducing cooling effect.
3. Effect on mineral transport:
• Mineral salts remain dissolved in the soil water and are
absorbed by the roots. Minerals that are absorbed and
accumulated in the xylem duct of the root move up and
are distributed in the plant by the transpiration stream.

27. 4. Effect on water movement:
• The absorbed water is transported from roots to
leaves through the xylem vessels which is greatly
influenced by transpiration pull. Water loss due to
transpiration results in the development of low water
potential in the leaf tissues. Thus water moves from
the xylem vessels to the leaf cells and helps in the
ascent of sap.
5. Development of mechanical tissues :
• Greater amount of transpiration helps in the
development of mechanical tissues in plants and
hence the plants become healthier and more compact.
The cell walls become thick and cutinized and the
plants are able to resist the attack of fungi and
bacteria.

28. 6. Maintenance of turgidity:
• Transpiration maintains an optimum degree of
turgor in cells. Under favorable conditions
plants absorb excess amount of water, which is
given off by transpiration to maintain the
optimum turgor for better growth.
7. Increase of taste of fruits:
• The solutes inside the cell become more
concentrated when transpiration is rapid. The
concentration of sugar solution in the cells of
fruits increases and fruits taste sweeter.

29. Transpiration as necessary evil
• Curtis has said that transpiration is a “necessary evil”
because of the following facts:
1. A large amount of absorbed water is lost during
transpiration which is harmful to plants.
2. Unnecessary wastage of energy takes place during the
process of water absorption which is lost due to
transpiration.
3. When the rate of transpiration is high in plants growing in
soil deficient in water, an internal water deficit develops in
plants which may affect metabolic process.
4. Many xerophytes plants undergo structural modifications
and adaptations to check transpiration.
5. Considering both the beneficial and harmful effects of
transpiration, it may be concluded that transpiration is
definitely advantageous in spite of its harmful features.

30. Transpiration vs Evaporation
Vs

31. Transpiration Evaporation
Transpiration is a physiological process
only occurs in plants.
Evaporation is physical process in any
free surface.
Water moves through the epidermis
with its cuticle or through the stomata
or lenticels.
Any liquid can evaporate. The living
epidermis and stomata are not
involved.
Various forces such as vapour
pressure, osmotic pressure etc. are
involved.
Not much forces are involved
Formation of vapour continues for
some time even after the saturation of
air.
Evaporation stops when air is fully
saturated.
It is largely dependent upon
absorption of water from soil.
It continues as long as water is
available on the surface.
It is comparatively slow process. It is faster process.
Water vapours are formed in the
internal tissue of the plant.
Water vapours are formed at the free
surface of the water.

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