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
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
11.
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
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
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
21.
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
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
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
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
