Types of transpiration, Theories of transpiration mechanism

1. TRANPIRATION
IN PLANTS

2. TRANSPIRATION
The loss of water in the form of vapour
from living tissue of aerial parts of the
plant is called TRANSPIRATION.
The loss of water due to transpiration
is quite high
 Almost 98-99% of water absorbed by
a plant is lost in transpiration.

3. MAGNITUDE OF TRANSPIRATION
 2 Lts/day in SUNFLOWER
 36-45 Lts/day in APPLE
 54 Gallons in CORN PLANT in
one growing season.
 30% of rainfall water is lost by
deciduous forest.

5. Depending upon the plant surface
transpiration is of 4 types:
 Stomatal transpiration
 Cuticular transpiration
 Bark transpiration
 Lenticular transpiration

6. 1) STOMATAL TRANSPIRATION
Most important type of transpiration.
Constitutes 50-97% of total transpiration.
It is the loss of water through special apertures which are present in
leaves called STOMATA. Few stomata are found on green stem, flower
and fruits.
Stomata expose the wet interior part of plant to the atmosphere.
Water vapour therefore, pass outwardly through stomata by diffusion.
Stomatal transpiration continues till the stomata are open.

8. STOMATAL APPARATUS
Tiny pores found in the epidermis of leaves and
other soft aerial parts.
Allow exchange of gases between plant and
atmosphere.
Number of stomata present per cm square
1,000-60,000.
In mesophytic plants ,stomata occur both on
adaxial and abaxial surfaces . While in grasses
and monocots ,their number is almost equal on
both the surfaces.

9. STRUCTURE OF STOMATA
Each stoma is surrounded by two small specialized cells
called guard cells .
Guard cells are connected to adjacent epidermal cells by
plasmodesmata .
They are rapidly influenced by turgor changes .
Microfibrils are oriented in a specific way to allow the
opening and closing of stomata .
Inner walls (concave side) of guard cells is thick and outer
wall (convex side) is thin ,hence they are kidney shaped .

10. 2) CUTICULAR TRANSPIRATION
It occurs through the cuticle or epidermal cells of leaves and
exposed parts of the plant.
In common land plants cuticular transpiration is only 3-10% of the
total transpiration.
In herbaceous shade loving plants where cuticle is very thin,
cuticle transpiration may be upto 50% of the total.
This transpiration continues throughout day and night.

12. 3) BARK TRANSPIRATION
Occurs through corky covering of the stem.
Its measured rate is more than lenticular transpiration due to
larger area.
Cork is generally impermeable. So, water loss occurs through
IMBIBITION.
Occurs continuously through day and night.

13. 4) LENTICULAR TRANSPIRATION
It is found only in woody stems and some fruits where lenticels
occurs.
Constitutes major part of water loss by deciduous trees during
leafless stage.
Occurs continuously day and night and there is no mechanism
to stop or reduce it.
Total water loss through them is only fraction of total i.e. 0.1%.

15. MECHANISM OF STOMATAL MOVEMENT
Three main theories :
I. HYPOTHESIS OF GUARD CELL PHOTOSYNTHESIS
II. CLASSICAL STARCH HYDROLYSIS THEORY
III. MALATE ION PUMP HYPOTHESIS (MODERN THEORY)
Stomata functions as turgor operated valves because their
opening and closing is governed by turgor changes of the
guard cells .
INCREASED TURGOR Pore is created
DECREASED TURGOR Pore is closed

17. HYPOTHESIS OF GUARD CELL
PHOTOSYNTHESIS
Guard cell contain chloroplast .During daytime chloroplast
perform photosynthesis and thus produces sugar .
Sugar INCREASES SOLUTE CONCENTRATION of guard cells as
compared adjacent epidermal cells .
Thus absorption of water by guard cells from epidermal cells .
Turgid guard cells bent outwardly and creates a pore in
between.

18. CLASSICAL STARCH HYDROLYSIS THEORY
 Proposed by SAYRE (1923)
Guard cells contains starch when stomata are closed
STARCH IS CHANGED TO SUGAR .
 MECHANISM:
In the morning carbon dioxide concentration is low (due to increase in
rate of photosynthesis ).
Reduced carbon dioxide leads to INCRESE IN PH (due to loss of
carbonic acid)

20. Hence glucose increases the OSMOTIC
PRESSURE of guard cells , withdraw water
from epidermal and subsidiary cells .
On absorption of water guard cells swell up
and pore is created.

21. THE ABOVE REACTION IS REVERSIBLE i.e. sugar
changes back into starch
This requires LOWERING OF PH.
During the evening time, CARBON DIOXIDE FIXATION
stops and RESPIRATION INCREASES THE
CONCENTRATION of carbon dioxide in guard cells.
Decreased Ph brings about PHOSPHORYLATION OF
GLUCOSE.

22. As a result osmotic concentration of guard cells is
falls.
Hence LOSS of TURGIDITY takes place
GUARD CELLS SHRINK
PORE CLOSES

24. OBJECTIONS:
1. GLUCOSE NOT FOUND in guard cells during stomatal
opening.
2. STARCH SUGAR (chemically slow) while opening
and closing of stomata are quite RAPID.
3. Wide changes in guard cells not due to carbon dioxide
concentration.
4. BLUE LIGHT is more effective for opening of stomata (not
explained by STARCH HYDROLYSIS THEORY).
5. There is no visible changes in the starch content of the
guard cells during opening and closing movement.

25. Proposed by LEVITT, 1974.
Opening of stomata is accompanied by increase in K+ ion
concentration.
In open state concentration of K+ ion= 400-800mM
In closed state =100mM
Stomatal opening is stimulated by sunlight, cytokinin cAMP and
other factors

26. Blue fraction of LIGHT sensitizes
PHOTOTROPIN
transfers the signals
PROTEIN PHOSPHATASE I (PPI)
activates
H+ ATPase
PROTONS PUMPS OUT FROM THE CYTOSOL
(interior of plasma membrane become more negative)
Voltage regulated K+ channel opens
K+ INTAKE BY GUARD CELLS
Cl- ions also intakes through PROTON CHLORIDE ANTIPORTER

27. Some proteins are picked by chloroplast and guard cell
CHLOROPLAST and MITOCHONDRIA.
Decrease in CO2 concentration & Increase in pH
Causes HYDROLYSIS OF STARCH to form ORGANIC
ACIDS.
STARCH
HEXOSE DIPHOSHPHATE
PHOSPHOGLYCERIC ACID
PHOSPHOPHENOL PYRUVATE

28. PEP + CO2 OXALOACETIC ACID (unstable)
MALIC ACID
H+ MALATE ION
H+ pass out of the guard cells & K+ ions enters the cell.
Cl ion taken from outside to maintain ionic balance.
In onion, Cl alone counterbalances K+.
The ions (K+, Cl , malate) pass into the small vacuoles and
gets stored & produce an osmotic potential. So, guard cell
absorbs water from nearby epidermal cells.

30. Due to elastic nature of guard cell walls, they swell up and
increase in volume by 40%-100% . A pore is created in
between the two guard cells
Sucrose is also involved in developing osmotic potential.
Stomatal closer occurs towards evening when light begin to
fall.
In absence of light,
H+ATPase of Plasmalemma stops its activities
H+ passes out of the guard cell mitochondria and chloroplast.

31. Decrease in pH of the guard cells.
Malate ion present in the guard cells combines with H+ to
form MALIC ACID.
Undissociated malic acid promotes leakage of ions.
Anion channel opens
Cl ions comes out of the guard cells
Plasma membrane depolarised.
Voltage gated K+ channel open.
Loss of K ions leads to decrease in osmotic concentration of
guard cells as compare to epidermal cells. Pore between the
guard cell closes.

32. To form vapours, exposed parts of the plant need a source of
heat energy.
The dry air of the atmosphere has high DPD (or low water
potential) - 13.4 atm at 99% R.H.
-140 atm at 90% R.H.
-680 atm at 60% R.H.
2055 atm at 20% R.H.
Such high DPD or low water potential has overcome various
resistances which water molecule have to meet from liquid phase
to vapour phase.

33. Intercellular spaces of transpiring organs (Saturated
with water vapours)
When stomata are open, water vapour are drawn from the
stomata to the outside air DUE TO HIGH DPD.
This INCREASES the DPD of sub-stomatal air, it draws more
water vapour from intercellular spaces.
The latter get water vapours from the wet walls of mesophyll
cells.
Stomatal transpiration continues till stomata are open.

35. FACTORS AFFECTING TRANSPIRATION
EXTERNAL OR ENVIRNMENTAL FACTORS
I. RELATIVE HUMIDITY AND VAPOUR DEFICIT
II. ATMOSPHERIC TEMPERATURE
III. SOIL TEMPERATURE
IV. LIGHT
V. AIR CURRENTS
VI. ATMOSPHERIC PRESSURE
VII. SUPPLY OF WATER

36. INTERNAL OR PLANT FACTORS
1. LEAF AREA
2. STOMATA
3. LEAF STRUCTURE
4. LEAF ORIENTATION
5. LEAF SIZE AND SHAPE
6. WATER CONTENT OF LEAVES
7. ROOT/SHOOT RATIO