Transpiration is the evaporation of water from plant surfaces, mainly leaves, through stomata. It occurs through three main processes: stomatal transpiration via openings in the leaves called stomata, cuticular transpiration through the plant cuticle, and lenticular transpiration through openings called lenticels. Stomata are regulated by guard cells which open and close to control transpiration rates in response to various environmental factors like light, temperature, humidity and water availability. Higher temperatures, light levels, low humidity and adequate water supply generally increase transpiration, while the reverse decreases it.

1. Transpiration
KAMALJIT SIDHU
ASSOCIATE PROFESSOR
DEPARTMENT OF BOTANY
KHALSA COLLEGE FOR WOMEN,CIVIL LINES
LUDHIANA

2. Transpiration

4. Transpiration
• The evaporation or the loss of water in the vapour
form from the exposed parts of the plant
• Amount of water lost by transpiration is between
0.5-2.5 gm/dm2/hr
• It can be measured in terms of dry matter-
Transpiration ratio
• It is minimum in CAM plants like Pineapple
• In C4 Plants -100-200units of dry matter
• In mesophytes-300-500 units of dry matter
• 98-99% of water absorbed is transpired
• Only 0.2% is used in photosynthesis
• Remaining used for hydration of the protoplast and
growth

5. Types of Transpiration

6. Stomatal Transpiration
• Stomatal transpiration- Takes place
through stomata
• 2 stages of stomatal transpiration-
Diffusion of water from the moist cell
walls into intercellular spaces and
passage of water vapours from the
intercellular spaces through the stomata
• 75% of the evapouration occurs through
the inner surface of the epidermal cells-
Peristomatal transpiration
• It stops only when the stomata are
closed completely

7. Cuticular Transpiration
• Cuticular Transpiration-occurs through cuticle
that lies on the outside of epidermal cells of the
leaves and other exposed aerial parts of the
plants
• Hedera helix and Tilia europea –stomata absent
on the upper surface of the leaves
• No transpiration from the upper surface in
Hedera and 29% in Tilia
• These variations are due to the thickness of the
walls of the epidermal cells and the cuticle
• Transpiration is inversely proportional to the
thickness of the walls
• Cuticle imbibes water from the epidermal cells
• Constitutes 3-10% of the total transpiration
• In herbs-50% is Cuticular transpiration

8. Lenticular Transpiration
• Lenticular Transpiration-Found only in
woody stems and some fruits where
lenticels occur
• Deciduous trees loose major portion of
water when they are leafless through
lenticels
• Occurs continuously throughout the
day and night
• Number of lenticels is less so water loss
is only 0.1%

9. Bark Transpiration
• Bark Transpiration-Through
the bark or cork occurring on
outside of woody stems
• Cork is impermeable
• Looses water slowly
• Looses 5 times more water
than all the lenticels together

10. Stomatal Apparatus
• Stomata are tiny pores found in the epidermis of the
leaves and soft aerial parts
• Allow the exchange of gases between the leaf cells
and the atmosphere
• Expose the wet interior parts of the plant to the dry
air outside which cause transpiration
• The stomatal pore is elliptical in outline
• Pore size varies in different plants
• Each stomata is surrounded by two specialized
epidermal guard cells
• Guard cells are kidney shaped
• They are joined at the ends
• Guard cell wall is thick on the concave or the inner
side and thin on the convex or the outer side
• In Graminae , Palmaceae and Cypreaceae the guard
cells are dumb bell shaped

12. Opening and Closing of
Stomata• Opening and closing of stomata-Controlled by changes in the turgor
pressure
• Turgor changes due to gain or loss of solutes from the adjacent or
subsidiary cells
• When turgid the volume increases by 40-100%
• Guard cells being small in size are more influenced by the turgor
changes
• Internally, the guard cells contain a number of mitochondria and
small chloroplast
• Chloroplasts have peripheral reticulum like C4 plants , having both
PS1 and PS11
• C-assimilation is absent due to absence of enzyme-ribulose
biphosphate carboxylase and NADP linked triose phosphate
dehydrogenase
• Hence, guard cells are heterotrophic, import sugars from the
adjacent cells,ATP synthesis occurs,grana well developed with large
starch accumulations when the guard cells are closed
• Stomata closed during the night,guard cells contain starch,mesophyll
cells are depleted of it

13. Opening and Closing of
Stomata• Vacuoles in guard cells malate , citrate ,K and cl ions –
stomata open
• Guard cells curve outwardly when they are turgid-pore
develops
• Guard cells are surrounded by the variable number of
modified epidermal cells-subsidiary cells
• In xerophytes subsidiary cells lie outside the guard cells ,
guard cells become sunken
• Epistomatal cavity-Cavity formed outside the level of
guard cells
• Substomatal cavity-Cavity inner to the guard cells
• Stomatal complex-Subsidiary cells+ Guard cells + pore

14. Guard cells and Epidermal cells-
Differences

15. Stomatal Movement

16. Stomatal Movement

17. Theories of Stomatal
Movement•Hypothesis of guard cell photosynthesis-Put forth by
Schwendner
•Guard cells have chloroplasts,perform photosynthesis during
the day
•Produce sugar which increase their solute concentration
•Water enters the guard cells by endosmosis
•Guard cells swell up , bend outwards and create a pore
•Fails to explain the stomatal opening during the night in CAM
plants
•Appreciable sugar content has not been found in the guard

18. Theories of Stomatal Movement
• Classical Starch Hydrolysis Theory-Put forward by Sayre, modified by Steward

19. Mechanism of Stomatal movement

20. Theories of Stomatal Movement
• Objections to Starch Hydrolysis Theory
• Change in CO2 concentration in the leaf interior cannot cause a wide
variation of pH-4.5 to 7.0
• Rise in OP during the stomatal opening cannot be only due to
formation of glucose from starch
• Fails to explain the extra-effectiveness of blue light
• Starch-Sugar inter conversion too slow to account for rapid stomatal
movement
• Glucose is not detected in the guard cells at the time of stomatal
opening
• No visible change in starch content
• Hydrolysis cannot account for rise in OP of the guard cells

22. Theories of Stomatal Movement
• Malate or potassium ion pump theory-Put forth by Levitt , main features are
• Opening of stomata is accompanied by increase in K+ ion concentration in guard cells
• These ions come from the surrounding subsidiary or epidermal cells
• Stomatal opening is stimulated by sunlight , cytokinin cAMP
• Blue fraction of sunlight sensitizes the receptor phototropin which transfers signals to
protein phosphatse(PPI)
• This activates plamalemma based H+ - ATPase
• This pumps out the protons from the cytosol of the cell
• The interior of plasmalemma becomes more negative as compared to the exterior
• So K+ ion uptake is passive
• Another secondary mechanism operational is for bringing in the chloride ions from
outside-antiporter

23. Theories of Stomatal Movement
• Some protons picked up by mitochondria and chloroplast of guard cells
• Internal CO2 conc decreases, pH rises causes hydrolysis of starch to phosphoenol
pyruvate(PEP)
• PEP combines with CO2 to produce oxalo acetic acid , which is converted into
malic acid
• Malic acid dissociates into malate ions and H+ ions
• H+ ions pass out of the guard cells and K+ ions enter
• More K+ ions enter than which could be counter balanced so chloride ion is taken
in
• These ions pass into the vacuoles and get stored
• Produce suitable OP for the entry of water
• Guard cells become turgid and stomata open

24. Theories of Stomatal Movement

25. Theories of Stomatal Movement
• Stomatal closure towards the evening , when the light is less
• In absence of light H+ -ATP ase of plasmalemma stops its activity of hydrogen ions
• They diffuse out of the guard cells , chloroplast and mitochondria
• It decreases the pH of the guard cells
• Malate ions present in the guard cells combines with hydrogen ions to form malic
acid
• Excess of the malic acid inhibits its own biosynthesis
• Un dissociated malic acid promoted the leakage of ions
• Anion channels open
• This causes ex osmosis
• Turgidity of the cells lost , the pore closes

26. Theories of Stomatal Movement
• Mid-day closure is directly mediated by ABA
• It induces the formation of phosphotidic acid
• This blocks the action of PPI
• This de polarises the plasma membrane
• Outward anion channels open ,chloride ions pass out
• Potassium ions pass out
• This efflux reduces OP
• Decrease in the size of pores
• Stomata close

27. Theories of Stomatal Movement

28. Factors Affecting Stomatal
Movement• Light-In some plants light induces the opening of stomata , darkness-closing,red and blue parts
of spectrum are favourable for photosynthesis are suitable for the stomatal movement as well,
blue light is exceptionally favourable.
• Temperature-Rate of stomatal opening has a temperature coefficient Q10 equal to 2,high
temperature can offset the effect of light
• Atmospheric humidity-higher the humidity ,stomata remain open for a longer time . At 70%
humidity the stomata open 50%humidity the stomata tend to close
• Water content of the leaves-It reaches a minimum of -7to 18 atm, guard cells loose turgidity
and close.
• Mechanical shock-Different types of shocks , as sudden high wind velocity closes the stomata
• Mineral status-Deficiency of N,P or K stomata become sluggish
• CO2 concentration-Low internal CO2 conc inside, stomata open
• Oxygen-Stomata close when oxygen is deficient
• Growth Hormones-Cytokinins are essential for stomatal opening , AbA for closure
• pH-Rise in pH, Stomata open

29. Factors Affecting Transpiration

30. External Factors Affecting Transpiration
• Relative humidity-Inversely proportional to the relative humidity
• Atmospheric temperature- this influences the rate in 4 ways-change in vapour
pressure deficit of the air ,change in VP of the air inside the leaf, Altering relatve
humidity of atmosphere , changing the rate of diffusion of vapours from the
leaves
• Soil temperature- the soil temperature less 5-8°,low transpiration , rises
beyond12°,after 20°C,rise is slowed down
• Light-Rate of transpiration high in light,falls in darkness
• Air currents-In moving air the rate of transpirations shows a rapid increase in the
beginning followed by a slow increase
• Atmospheric pressure-Low atmospheric pressure
• Supply of Water-The water uptake from the roots decreases, rate of transpiration
decreases

31. Internal Factors Affecting Transpiration
• Transpiring surface area-Transpiration directly proportional to transpiring surface area
• Stomata-Rate of transpiration is influenced by theirnumber,spacing,distribution,
• Pecularities , size of stomatal aperture and periodicity of their opening
• Leaf Structure-Determines the rate of transpiration in 3 ways-thickness of the cuticle, number ,
density and thickening of epidermal hairs, Ratio of internal exposed surface area of the leaf
• Leaf Orientation-solar radiations more heating when the flat surface lies perpendicular to the
incidence of light
• Leaf size and shape-Thin boundary prevents over heating
• Water content of the leaves-Optimum transpiration when moisture is sufficient
• Root shoot ratio-Low root/shoot ratio low rate of transpiration
• Mucilage and solutes-rate decreases by holding water tenaciously
• Diseases-rate of transpiration high in diseased plants

32. Advantages of Transpiration
• Ascent of sap
• Removal of excess water
• Cooling Effect
• Mechanical tissue
• Root system
• Quality of fruits
• Assimilation Products
• Water Supply to Leaf cells
• Resistance
• Drainage of soil water
• Absorption and distribution of salts

33. Disadvantages of Transpiration
• Transpiration is an Essential Evil
• Modifications
• Reduced photosynthesis
• Reduced growth
• Formation of AbA
• Reduced Yield
• Desiccation
• Breakdown of organic compounds
• Fall in metabolism
• Formation of proline
• Accumulation of Betaine

34. Antitranspirants

35. Types Of Antitranspirants