The document summarizes the physiology of stomata, including their structure, number, location, and mechanisms of opening and closing. It discusses three main theories of stomatal movement: guard cell photosynthesis hypothesis, classical starch hydrolysis theory, and malate/K+ ion pump hypothesis. The malate/K+ ion pump hypothesis, proposed in 1974, is currently considered the most accurate. It states that stomatal opening is accompanied by an increase in potassium ion concentration in guard cells and involves the production of malic acid, dissociation into hydrogen and malate ions, and transport of potassium malate into vacuoles, causing osmotic water intake and increased turgor pressure that opens the stomata

1. Physiology of Stomata
By: Dr. Sunita Sangwan
Assistant professor, Dept of Botany
Higher education Dept. Haryana

2. 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.
Stomata
Number
Location

3. Structure Of Stomata

4. How do the guard cells react to the availability of
water?
DRY
Guard cells CLOSE
lots of H2O
guard cells OPEN
2

5. Mechanism of stomatal movement
Stomata functions as turgor operated valves because their
opening and closing is governed by turgor changes of the
guard cells .
Cell absorb water
Become turgid & increase
in TP
Stretching of the stomatal
cell wall outside
Stomata opens
Water removes from cell
Become flaccid & decrease in
TP
Reliving of the stomatal cell
wall to prior stage
Stomata closes

7. Theories of Stomatal Movement
1
• Hypothesis of guard cell photosynthesis
• Given by Von Mohl (1856)
2
• Classical starch hydrolysis theory
• Given by Lloyed (1905), Loftfield (1921), Sayre (1926), Scarth (1932) &
Steward (1964)
3
• Malate/K+ ion pump hypothesis (modern theory)
• Proposed by Levit (1974)

8. 1. Theory of Guard Cell
Photosynthesis
Light
Photosynthesis by guard cell
Formation of Sugar
Increase in osmotic pressure of cell sap
Endosmosis
Entrance of water from mesophyll cells
Increase in turgidity of guard cells
Stomata opens

9. Objection to Guard Cell Photosynthesis hypothesis
1. Chloroplast of guard cell perform insignificant
photosynthesis.
2. Starch are also present in the leaves of plant placed in
dark.
3. Rate of increase of concentration of starch is very high as
compare to very little amount of chloroplast in guard cell.
4. Guard cells of such plants whose guard cell does not
posses chloroplast show presence of starch grain in their
guard cell.

10. 2. Classical starch hydrolysis theory
The CO2 becomes accumulated in intercellular spaces
The pH of cell sap of guard cells is reduced to 5.
Soluble Sugars are converted into starch
OP of guard cell reduced
Exosmosis from guard cell occurs which resulting in flaccid stage of guard cell &
closing of stomata
Changes takes place during closing of
stomata

11. 2. Classical starch hydrolysis theory
The CO2 concentration in leaves reduced due to
photosynthesis
The pH of cell sap of guard cells is increased (pH 7).
Starch is converted into soluble sugars due to action of
enzyme phosphorylase
OP of guard cell increased
Endosmosis from mesophyll cell to guard cell occurs
which resulting in Turgid stage of guard cell & opening of
stomata
Changes takes place during Opening of
stomata

14. Objections to Classical starch hydrolysis theory
 Monocots plants guard cell do not contain starch even
then they behaves like other dicots plants.
 Closer of stomata in noon without any change in starch
quantity.
 Insufficient rate of inter-conversion of starch & sugar for
opening & closing of stomata.
 Insufficient CO2 for guard cell activation & change in pH
of cell sap.
 Some time starch is converted to malate in place of
sugars.

15. Malate /K+ ion hypothesis
 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

17. Opening of stomata
Malate /K+ ion hypothesis
Light Starch
Production of
Malic acid in
guard cell
Dissociation
in to hydrogen
& malate ions
Influx of K+ &
eflux of H+
ions
Formation of
Potassium
malate
Transport of
potassium
malate into
the vacuoles
Osmotic
entrance of
water into
guard cell
Increase of
turgor
pressure
Stomata Open

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