Transpiration
• The process involving the loss of water from the aerial parts of
the Plants (especially from leaves) in the form of Water Vapour
is called Transpiration.
• Water is a necessity in all forms of life and Plants, it is the
roots that perform the function of acquiring water from the
soil. However, the Plants for growth and metabolism use a
very small percentage of that water. The remaining amount of
water, which is almost 95-99%, is lost via transpiration and
guttation.
• The driving force that acts in favour of Transpiration is the
difference in Water Vapour between the leaf air spaces and the
external air.
• The higher is this difference in vapour pressure, the more is
the rate of Transpiration.

• Cuticular Transpiration: Cuticle is the waxy layer that covers the epidermis of
leaves and herbaceous stems. It is a polymer made of cutin, which is its chief
constituent, and wax. The polymer is composed of long-chain epoxy fatty acids,
attached via ester linkages. Even though the primary function of the cuticle remains
prevention of Transpiration, some Transpiration does take place through it, which is
about 5-10% of the total Transpiration that takes place in a Plant.
• Lenticular Transpiration: The openings in barks and stems that allow the gaseous
exchange between the inner living cells of the Plants and the atmosphere are termed
as lenticels. The loss of water in the form of Water Vapour from lenticels is called
lenticular Transpiration. Only 1-5% of the total Transpiration takes place through
lenticels.
• Stomatal Transpiration: Stomatal Transpiration accounts for approximately 90%
of the total Transpiration from Plants, which is the highest among the three types.
Stomata are specialized structures located on the epidermis of Plants for the
regulation of gaseous exchange between the Plant and its surroundings. The
opening and closing of stomata are regulated by turgor pressure.
• Various factors have been known to determine the rate of Transpiration, some of
them are light, temperature, humidity, and even the surface of the leaf from which
Transpiration is occurring. Even though leaves are the most common sites of
Transpiration, this phenomena of water loss can also occur from stems and flowers,
as well. This biological process is witnessed in all higher Plants and trees.
Transpiration can be divided into three types depending upon its location:

Introduction of Transpiration Pull
• Transpiration Pull is a physiological process that can be defined as a
force that works against the direction of gravity in Plants due to the
constant process of Transpiration in the Plant body.
• This force helps in the movement of water as well as the minerals
dissolved in it to the upper parts of the Plants.
• This movement of water takes place through the Xylem, a dead
tissue that is found throughout the length of Plants.
• Transpiration Pull can alternatively be described as a suction force
that Pulls up the groundwater in an upwards direction.
• The Plant, for Photosynthesis, utilizes a very small percentage of
that water and the remaining is transpired into the atmosphere via
Water Vapours.

Cohesion-tension theory
• In the process of Transpiration, the water molecules
from the soil combine, owing to their cohesive force, to
form a column in the Xylem. An adhesive force also
comes into play that acts between the water molecules
and the Xylem vessel. The pressure that is created by
the Transpiration Pull generates a force on the
combined water molecules and aids in their movement
in an upward direction into the leaves, stems and other
green parts of the Plant that is capable of performing
Photosynthesis. This theory explaining this
physiological process is termed as the Cohesion-
tension theory.

Mechanism of Transpiration Pull
• The mechanism underlying this biological phenomenon
is based on the upward movement of water, which
starts from the tip of the root, in the soil and ends in
the aerial parts of the Plant body.
• This movement of the water and the minerals
dissolved in it through the Xylem tissue is called the
ascent of sap.
• During the process of Transpiration in form of Water
Vapour into the atmosphere, a negative hydrostatic
pressure is also created in the mesophyll cells of leaves
to favour the draw of water from the roots to the veins
of the leaves.

FACTORS AFFECTING THE RATE OF
TRANSPIRATION
• There can be several factors affecting the rate of Transpiration. These
factors can be external; for example, environmental conditions or can also
be controlled by the Plants (internal) by adjusting the size of the stomatal
apertures.
• For environmental influences, the rate of Transpiration can be altered by
the evaporative demand of the atmosphere surrounding the site of
Transpiration, like boundary layer conductance, temperature, humidity,
wind, and incident sunlight.
• The level of soil, water and temperature of the soil can also affect
stomatal opening and closing, and hence on the Transpiration rates.
• The percentage of water loss from Transpiration also depends on the size
of the Plant or its leafiness.

Questions?
1. Are Transpiration and Transpiration Pull the same thing?
Ans: Transpiration and Transpiration Pull are related
phenomena. However, they do not denote the same thing.
Transpiration is the process of loss of water from the
stomata of leaves in the form of Water Vapours. This loss of
water is essential to cool down the Plant when in hot
weather. It is also thought to be a slight disadvantage
caused by the opening of stomata for the diffusion of
CO2 into the leaf cell. It is important to note that
Transpiration along with guttation is responsible for 95-
97% of the total water loss from the absorbed water.
Transpiration Pull is secondary to Transpiration as it arises
due to the water loss in leaves and consecutive negative
pressure in Xylem vessels.

• What is root pressure?
Ans: The transverse osmotic pressure generated within
the cells of the root system causes absorption of water
(moisture) from the soil and forward movement of
water molecules (along with dissolved minerals, now
called the “sap”), up in the Xylem is called root
pressure. It is a result of osmotic pressure built in the
root cells due to the accumulation of ions in absence of
Transpiration Pull (especially at night, as the stomata
remain closed and no Transpiration occurs).