Transpiration

Intro• Water is absorbed by the plant roots from the soil.• It is translocated through the tracheary elements of xylem to the

leaves.• Only small fraction of the total water absorbed from the soil is

permanently retained by the plant.• It is estimated that 5% of the water absorbed is used in metabolic

reactions by an average plant and remaining 95% is released into the atmosphere as water vapour.

• This process of loss of water in the form of vapours from the aerial parts of the living plant is called TRANSPIRATION.

• Rate of transpiration is different for different plant e.g. sunflower transpires about 2 litres of water in a day. An apple tree transpires about 36-45 litres and a large Elm tree can transpire up to 1 tonne water by weight in a day.

Demonstration of Transpiration

• Transpiration can be readily demonstrated by placing a well

watered plant under a bell jar.• The pot and the soil are covered with a polythene sheet, or

aluminum foil so that no water can escape except through the plant.

• Soon the moisture collects on the inner surface of the bell jar.

• This moisture cannot be from the outer air of from the soil or pot since the bell jar is sealed with vaseline and the pot is covered by polythene.

• It is because the air inside the bell jar becomes saturated due to water vapours released by the plant and these get condensed and deposited on the inner surface of the jar.

• A bell jar without plant will not show drops of moisture under the same conditions.

Kinds of Transpiration• 1. Cuticular Transpiration: Cuticle is present

on the epidermis of leaves and herbaceous stem. The loss of water through the cuticle is known as Cuticular Transpiration.

• 2. Lenticular Transpiration: Lenticles are present in the bark of the stem. It consists of loosely arranged complementary cells. The loss of water through them is known as Lenticular Transpiration.

Cuticular transpiration

• 3. Stomatal Transpiration: It is the

transpiration that occurs through the stomata. The epidermis of leaves and green stems have numerous stomata. These are responsible for about 80-90 % of the total water transpiration. This is also known as FOLIAR TRANSPIRATION because stomata are mainly present on leaves.

• Cuticular and lenticular transpiration together contribute only 3-10% of total transpirated water a significantly smaller amount when compened to stomatal transpiration.

Structure of Stomata

Str. of Stomata• Stomata are minute pores of elliptical shape

surrounded by 2 specialized epidermal cells called guard cells.

• They are kidney shaped.• The wall of the guard cells surrounding the pore is

thick and inelastic but rest of the walls are thin.• Each cell has a cytoplasmic lining, a central vacuole

containing cell sap. • The cytoplasm contains a nucleus and number of

chloroplasts.

Str. of Transverse section of Stomata

Mechanism of Stomatal Opening and Closing

• The opening and closing of stomata depends upon the turgidity of their guard cells.

• When guard cells are turgid, pores are open but when flaccid, the pores are closed.

• When turgidity increases, the outer thin walls of guard cells stretch outward causing outward stretching of their inner wall.

• The inner wall being inelastic becomes concave and as a result the space surrounding the pore widens and the pore opens.

• During the day, plants photosynthesize, and the guard cells of stoma are fully turgid and thus stomata is fully open.

Mechanism of Transpiration• Roots absorb water and it is translocated by

xylem of the stem and distributed to leaves and all other aerial parts of the plant.

• The leaves consist of mesophyll cells.• This water is supplied to the mesophyll cells of

the leaves through xylem bundles which form the network of veins of the leaves.

• When leaves absorb radiant energy water is converted into water vapour and collect within intercellular spaces.

• The intercellular space form a connected system

extending up to sub stomatal cavity.

• When stomata remain open water vapours are diffused out of the leaf because its concentration is high in sub-stomatal cavity than the adjoining atmosphere.

• The vapours are formed in the first place from the thin film of water on outer surface of mesophyll cells of the leaves.

Stomata Upper v/s Lower Surface• A Dicotyledonous leaf

always has many more stomata on its lower surface i.e. the ventral surface so the transpiration is always more on the lower surface.

Facts• Generally, stomata are open during the day

and closed at night. But in succulents like cacti, opuntia, stomata close during the day and open at night.

• Fresh weight of plant or leaf would be maximum in the morning and minimum in the afternoon.

Demonstration of transpiration by cobalt chloride paper

• 2 filter papers are dipped in 5% cobalt chloride solution and dried.

• These papers are blue in colour. • Now take 2 dry pieces of filter paper of equal

size and fix each piece on either sides of a dorsi-ventral leaf of a potted plant.

• Keep the plant in light for some time.• It is observed that the filter paper fixed on the

lower side of the leaf turns pink first.• This shows that transpiration is more from the

lower side.

Poto-meter• It is an apparatus used to measure the rate of

transpiration.• Ganong’s Potometer consists of a long graduated tube

which is bent at right angle towards the lower side at one end.

• There is a small terminal opening at this end.• At the other end is a wide mouthed tube into which a

twig can be inserted.• Near this end, long tube is connected with other wide

mouthed tube with a stop-cock at its base.• This is called water reservoir.

Ganong’s Potometer

To measure the rate of transpiration using Ganong’s Potometer

• The potometer is filled with water. • A twig is fixed in the wide-mouthed tube. • The apparatus is made air tight.• Now an air bubble is introduced in the graduated arm & the tip of

the end is kept in a beaker filled with water.• Note the initial position of the air bubble and time.• Leave the whole apparatus for some time.• Now note the final position of the air bubble in the graduated arm

& time.• Distance travelled by the air bubble in a given time will give the

rough estimate of the rate of transpiration.• This expt. is based on the assumption that the amount of water

absorbed is almost equal to that of water transpired.

Simple Potometer

Simple potometer

To measure the rate of transpiration using Darwin’s Potometer

• Darwin’s potometer works on the same principle as

Ganong’s Potometer but here the capillary tube is vertical.

• It consists of a glass tube with a side tube as shown fitted with rubber corks.

• The upper end of side tube has the plant inserted while the lower end of straight tube has a capillary tube with a fitted scale.

• The procedure is essentially the same. The twig is cut under water in a slant.

• An air bubble is introduced and its distance travelled noticed to get a rough estimate of the rate of transpiration.

Things kept in mind while using Potometer

• It actually measures the rate of absorption of water. Most of this is transpired but some is used up in metabolic processes. So this gives a rough estimate not exact of transpiration.

• The air bubble should be introduced carefully in capillary tube.

• The twig should be cut under water so that no air goes into xylem vessels which might stop absorption.

Significance of Transpiration1. It cases the conc. of leaf cell sap to rise. Cell in this

condition will absorb water from their neighbours and even totally from the xylem vessels of leaf, then from stem and finally from root. So this transpiration stream is responsible for upward translocation of water.

2. Mineral salts absorbed with water are also distributed to various parts of the plant.

3. It increases the rate of water absorption because it concentrates the sap in the plants.

4. The rapid evaporation of water from the leaf surface and consequent absorption of latent heat from the leaf tissue keep the temperature of a leaf below harmful levels in the direct rays of the sun.

Transpiration

Advantages• It evaporates excess of

water.• This process helps in

ascent of sap.• It controls the body

temperature of plant.• By this process, water and

minerals reach to the various parts of the plants.

Disadvantages• Due to excessive

transpiration plants start to wilt.

• Transpiration creates water loss which may make the land a desert.

Transpiration and Water Cycle• Transpiration also effects our climate. • Such large quantities of water are

transpired into the atmosphere that they actually play a role in the water cycle and cause rainfall. That is why deforestation can lead to droughts.

• We feel cool under a tree during summer season.

Factors affecting Transpiration

External Factors• 1. Light• 2. Temperature• 3. Humidity of the Air• 4. Wind• 5. Soil Water

Internal factors• 1. Leaf Area• 2. Thick Cuticle• 3. Number of Stomata• 4. Sunken Stomata• 5. Presence of Hair• 6. Leaf Modifications

External Factors1. Light:

- It is an important factor.- It affects the rate of transpiration in 2 ways. It controls the opening and closing of stomata

and it increases the temperature.- So with the increase in light intensity the rate

of transpiration increases because the stomata get opened and the temperature increases.

2. Temperature:

- The increase in temperature results in increasing the rate of evaporation of water from cell surface & reduces the humidity of the atmosphere.

- So the rate of transpiration will increase.3. Humidity in the Air:

- When the atmosphere is very humid, the rate of transpiration will be low because the difference in water vapour conc. in the sub-stomatal cavity & atmosphere is less.

- On the other hand, when air is dry this difference will be more & transpiration rate will be higher.

4. Wind:

- Transpiration gradually increases the water vapour conc. of the air so rate of transpiration gradually falls.

- However, when air is dispersed by wind then transpiration will increase again.

- Moving air currents continually bring fresh dry air in contact with leaf surfaces, so high rate of transpiration is maintained.

5. Soil Water:- When soil water is sufficient then root absorbs water

normally & transpiration rate is maintained.- Deficiency of water in the soil decreases the rate of

transpiration indirectly by decreasing the rate of water absorption.

Internal Factors1. Leaf Area:

- Some plants have smaller leaf lamina to reduce transpiring area.

- Dense foliage also has a similar effect because of shading effect which keeps the temperature near stomata lower & inhibits rapid air movement.

2. Thick Cuticle:- In many plants especially xerophytes, a thick

cuticle layer is present outside epidermis.- It reduces the rate of transpiration.

3. Number of Stomata:- Number of stomata in leaves affect the rate of

transpiration.- If the number is more, the rate will be high, and

if the number is less, the rate will be low.

4. Sunken Stomata:- Sometimes, leaves in stomata are sunk deep in

the epidermis. - This reduces the air movement near stomata,

thus decreasing rate of transpiration.

5. Presence of Hair:

- Hairs are present in Xerophytic leaves.- These structures form a microenvironment

around the leaf surface.- When this is saturated with water vapour, the

rate of transpiration decreases.Leaf Modifications:

- Sometimes leaves are modified into prickles spines etc.

- In some plants the leaves are scale like. - Such modifications also serve the purpose of

reducing transpiration.

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6 Environmental Factors Affecting Transpiration

1. Relative humidity:- air inside leaf is saturated (RH=100%). The lower the relative humidity outside the leaf the faster the rate of transpiration as the gradient is steeper

2. Air Movement:- increase air movement increases the rate of transpiration as it moves the saturated air from around the leaf so the gradient is steeper.

3. Temperature:- increase in temperature increases the rate of transpiration as higher temperature– Provides the latent heat of vaporisation– Increases the kinetic energy so faster diffusion– Warms the air so lowers the of the air, so gradient is steeper

4. Atmospheric pressure:- decrease in atmospheric pressure increases the rate of transpiration.

5. Water supply:- transpiration rate is lower if there is little water available as transpiration depends on the mesophyll cell walls being wet (dry cell walls have a lower ). When cells are flaccid the stomata close.

6. Light intensity :- greater light intensity increases the rate of transpiration because it causes the stomata to open, so increasing evaporation through the stomata.

Intrinsic Factors Affecting the Rate of Transpiration.

1. Leaf surface area

2. Thickness of epidermis and cuticle

3. Stomatal frequency

4. Stomatal size

5. Stomatal position

Difference between

Transpiration Evaporation1. This is vital and physiological activity.

1. This is a physical activity.

2. In this process, water evaporates from aerial parts of the plants.

2. Water is evaporated from any surface.

3. Transpiration takes place by leaves and stomata.

3. Evaporation occurs without stomata.

4. This is restricted to living plants only.

4. Evaporation occurs from living and non-living bodies.

Guttation

• Plant growing in moist, warm soil and under humid conditions possess a specialized structure called HYDATHODES on tips and margins of leaves.

• Water escapes in liquid form at night and accumulates in drops at the ends of veins.

• This process is known as GUTTATION. • Guttation generally occurs at night when

transpiration has stopped & the rate of absorption is high.

• Under these conditions, root pressure is

maximum which forces the water upwards through the xylem elements & eventually exudes in the form of drops from hydathodes at the leaf margins.

• Guttation may also occur during day time if the plants are growing in moist & warm soils or in humid conditions, which increases root pressure.

• Guttation is very commonly observed from the tips & edges of leaves of grasses, wheat, rye, barley, nasturtium, Colocasia & tomato early in the morning after a moist & warm night.

Bleeding• The exudation of liquid from the cut

or injured surface of plants, such as date palm and Palmyra palm is known as BLEEDING.

Difference between

Bleeding Guttation1. The process starts from cuts found in plants.

1. It occurs from edges of leaves.

2. More cell sap comes out in this process.

2. Less cell sap comes out in this process.

3. It happens due to pressure in sieve tubes.

3. It happens due to the pressure present in xylem.

4. Hydathodes do not take part in this process.

4. Hydathodes take part in this process.