1. Transpiration is the process by which plants release water vapor through their leaves. Most water absorbed by roots (99%) is released this way. 2. There are three main types of transpiration: stomatal (through leaf pores called stomata), cuticular (through the plant cuticle), and lenticular (through bark openings called lenticels). Stomatal transpiration accounts for about 90% of total transpiration. 3. Stomata open and close to control transpiration rates. Opening is triggered by guard cell turgor changes driven by starch to sugar conversions, while closing involves the reverse process and involves potassium ion transport.

1. Savitribai Phule Pune University
S. Y. B. Sc. Botany: Paper II
Dr. Abhijit Surendra Limaye
Assistant Professor, Department of Botany,
Nowrosjee Wadia College, Pune
BO: 232: Plant Physiology
Chapter 4: Transpiration

2. Transpiration
❖Transpiration is the physiological
process where water is lost in the form
of vapours from the aerial parts of
plants.
❖Water is essential for plant to carry
out almost all physiological processes
but plant utilizes very small amount of
water absorbed while as per modern
physiology 99% of water absorbed by
the roots is released in the form of
vapours.

3. Transpiration
Depending on the organ that performs transpiration, there are
three types namely,
1.Stomatal Transpiration: It is loss of water through small
specialized pores present on leaves known as ‘Stomata’.
Maximum amount of water (Around 90%) is lost through
stomata.
2.Cuticular Transpiration: Cuticle is an impermeable layer
present on leaves and stem and around 8 to 10% of
water is lost through cuticles. Cuticular transpiration is
very less in xerophytes as they possess thick cuticles.
3.Lenticular Transpiration: Lenticels are tiny openings
present on woody bark and water lost through lenticels is
negligible (about 0.1 % to 1%).

4. Stomatal Transpiration
● Stomata are minute openings present in the
epidermis of leaf.
● Each stoma consists of a stomatal pore which
is surrounded by specialized cells termed as
guard cells.
● Guard cells contain a nucleus, chloroplasts and
a layer of cytoplasm with central vacuole filled
with cell sap.
● The inner wall of guard cells is thick and non-
elastic in nature whereas outer wall is thin and
elastic.
● Guard cells are kidney shaped in dicots and
dumb-bell shaped in monocots.
● Stomata are surrounded by
subsidiary cells which play vital
role in opening and closing of
stomata

5. Opening and closing of stomata
● The opening and closing of stomata is popularly
known as stomatal movement.
● This opening and closing of stomata is controlled by
concentration of solutes in the guard cells.
● Guard cells have starch containing chloroplasts, during day time, pH of guard cells is
neutral or slightly alkaline which favours the conversion of starch into sugar in the
presence of inorganic phosphate with the help of enzyme phosphorylase.
● Sugar being soluble in water, increases the osmotic pressure of guard cells. As a
result of this, guard cells absorb water from the mesophyll cells. Due to diffusion of
water into guard cell, guard cells become turgid.
● As outer wall of guard cells is thin and elastic stomata gets open during day time.

6. Opening and closing of stomata……..
Continue
● During night time, CO2 is released in the guard cells
but that is not utilized for photosynthesis.
● Due to release of CO2, the pH of guard cells become
acidic (pH= 5.00).
● The acidity favours the conversion of sugar into starch.
● Starch is insoluble and osmotically inactive substance
and therefore, osmotic potential of guard cells is
decreased.
● As a result, water diffuses from the guard cells to
mesophyll cells and stomata gets closed.
pH = 7.5
pH = 5.00

7. Overall process

8. Steward’s Hypothesis
In 1964, Steward proposed a modified scheme for interconversion of starch and
sugar for stomatal movement. Followings are the salient features of Steward’s
hypothesis.
1. Conversion of glucose 1 phosphate is not sufficient to increase osmotic
pressure.
2. Further it should be converted to glucose for stomatal movement
3. Apart from phosphorylase, there are other enzymes like phosphoglucomutase,
phosphatase, hexokinase are used in overall reaction.
4. Rate of photosynthesis, concentration of CO2 and pH play key role in conversion
of starch to glucose.

9. Overall reaction

10. In 1974, Levit proposed K-pump hypothesis for opening and closing of stomata. It is
also known as ‘Theory of Proton Transport’ or ‘Active K+ transport mechanism’.
Following are the key features of his hypothesis
1. According to this theory, potassium ions (K+) play important role in opening and
closing of stomata.
2. During day conditions, K+ enter into guard cells from neighbouring epidermal
cells which acts as reservoir of potassium ions. This process is active (energy
driven or requires ATP).
3. Starch is converted into malic acid during day time which dissociates into malate
ions and H+ ions.
Active K- Transport

11. 4. Exchange takes place between K+ ions from epidermal cells and and H+ ions
form guard cells.
Malic acid Malate ions + 2H+
5. In a guard cells, potassium malate is formed which is osmotically active and thus
responsible to increase the osmotic pressure and stomata gets open.
6. During night time concentration of CO2 decreases and therefore inhibits the
uptake of K+ ions. K+ ions are transported back to the epidermal cells.
7. The osmotic potential inside the guard lowers that results in exosmosis (water is
given out). The guard cells become flaccid and the stomata gets closed.
The overall process of the entry and exit of K+ and H+ as well as formation of
potassium malate, etc is given in the table.

13. Factors affecting rate of transpiration
The amount of water that plants transpire varies greatly over the time. There are several
factors that affect the rate of transpiration of plant.
Factors affecting transpiration rate
External factors
Internal factors
1. Temperature
1. Number of leaves
2. Light intensity
2. Number of stomata
3. Humidity
3. Size of leaves

14. External Factors

16. Significance of transpiration
1. Transpiration helps in the conduction of water and minerals to different parts of
the plants.
2. there is a balance of water maintained within the plant due to continuous
elimination of water from the plant body
3. Certain hydrophilic salts are accumulated on the surface of the leaves, which
keeps the leaves moist.
4. It maintains the turgidity of the cells and helps in cell division.
5. A suction force is created by transpiration that helps in the upward movement
of water in the plants.
6. Optimum transpiration helps in the proper growth of the plants.
7. The cooling effect of a tree is due to the evaporation of water from its leaves.
Transpiration is termed as necessary evil

17. Acknowledgement
I sincerely acknowledge the following resources used during creation of this
presentation
1. www. wikipedia.com
2. A text book of Plant Physiology, Nirali Publication
3. Plant physiology by Taiz and Zeiger