Stomatal movement is controlled by the guard cells surrounding each stoma. Guard cells take in water when the environment causes an increase in their osmotic potential, causing the cells to swell and open the stoma. When osmotic potential decreases, guard cells lose water and become flaccid, closing the stoma. Stomata open in response to light, potassium ion accumulation, and a higher pH/sugar concentration, and close in response to higher CO2 levels, low water availability, and the plant hormone abscisic acid. Multiple environmental factors and biochemical processes precisely regulate stomatal openings to control gas exchange and transpiration.

1. STOMATAL
MOVEMENT
RUCHI 4136429055
RUCHIGARG 4136429056

2. Theories:
• Starch-sugar hypothesis
• pH regulates stomatal movement
• Role of k⁺
• Extrusion of proton

3. • The epidermal surface of a leaf
bears a great number of pores
called stomata.
• The stomata are microscopic and
are bordered by two specialized
epidermal cells called guard cells,
which control the opening and
closing of stomata.
• The stomatal movement is
generally understood to be a
direct response to increase or
decreases in the osmotic
potential of the guard cells.
• As a result of this changes in
water potential cause water to
move in or out of the guard cells.
• If water moves in, the cells
expand, the stoma is open.
• If water moves outcalls go flaccid,
the stoma is closed.

4. Anatomy and cytology of stoma
• Although the changes in turgor provide the motive
force for the opening and closing of stomata, an
unusual feature of the ground cell wall causes the
stomata to open in the manner that they do.
• The cellulose microfibrils making up the walls of a
guard cell are arranged radially (from the pore outward
and relatively particular to the pore) rather than
longitudinally , or along its length.
• This orientation of the cellulose microfibrils is termed
radial micellation.
• In addition , the guard cell wall adjacent to the
stomatal pore is thicker than the outer wall.

5. Factors affecting stomatal movement
• The environmental factors having the greatest
influence on the opening and closing of stomata are
light; presence of potassium, chloride and hydrogen
malate; CO₂ concentration; water deficits and abcissic
acid; and temperature.
• light: generally, the stomata of a leaf are open when
exposed to light and closed in dark.
• The amount of light necessary to achieve maximal
stomatal opening is usually considerably less than is
needed for maximal photosynthesis activity.
• The stomata of some species may be induced to open
by bright moonlight. eg CAM plants.

6. • It was observed in tobacco leaf stomata that no opening
occurred when stomata were exposed to either far-red or
ultraviolet irradiation . Good stomatal opening was
obtained in the red and blue light but no opening in the
green light.
How does the light bring about the opening of
stomata?
• Early worker assumes that guard cells, when exposed to
light and warmth, increased their output of osmotocally
active substances through the process of
photosynthesis.(more –ve osmotic potential and a
subsequent increase in turgor cause the stomata to open).

7. • pH : sayre in his work with rumex patientia noted that
stomata are sensitive to changes in pH . Generally a
high pH favors opening and a low pH favors closing of
the stomata.
• A high pH is accompanied by a decrease in starch and
increase in osmotically active sugars and results in an
increase in turgor.
• This PH effect is explained by Yin and Tung , who
obtained evidence of the presence of a phosphorylase
in chloroplasts that favors the degradation of starch at
pH 7.0 and favors starch synthesis at pH 5.0.

8. • Potassium, chloride, hydrogen and organic acids: Evidence
indicate that the turgidity of guard cells of many species of plants is
regulated by K⁺, Cl⁻, H⁺ and organic acids.
• When plants are exposed to light the guard cells accumulate large
amounts of K⁺. Light stimulate the active movement of K⁺ ions from
surrounding cells into the vacuoles of the guard cells.
• Potassium accumulation is also accompanied by starch degradation,
organic acid(mostly malic acid) build up and an increase in pH.
• The K⁺ accumulation seems to be due to the operation of an active
exchange process in which the protons(H⁺) are “pumped” out of the
guard cell into the accesory cells.

9. • In some species, as the K⁺ ions migrate, they are
accompanied by Cl⁻; the anion apparently moves in
response to electric difference created by the K⁺ uptake
into the guard cells.
• A malic acid build up in the guard cells of illuminated
leaves as protons (H⁺)move from the guard cells into
the accesory cells.
• The malate ions within the guard cells are neutralized
by the influx of potassium.
• Thus in guard cells, osmotic and hence water potentials
become very negative due to the presence of K⁺ Cl⁻
potassium malate or dipotassium malate or both.

11. CO₂ concentration: stomata are quite sensitive to
variations in CO₂ concentration.
• An increase in CO₂ concentration above that
found in air will cause stomata to close even in
the light.
• Indeed, stomatal closure can be induced by
merely breathing on leaves.
• It appears that CO₂ concentration in the leaf
intercellular spaces rather in the external air
primarily controls stomatal movement.

12. Water deficits and abcissic acid :
• Whenever the rate of transpiration exceeds
the rate of absorption for any period of time, a
water deficit is created in the plant.
• Under conditions of water deficit, the
stomatal pores of many mesophytes will close,
thereby reducing transpiration significantly.

13. • Abcisic acid : ABA causes the opening of anion specific
channels in the plasma mambrane of guard cells. Acco. To
one model , ABA activates calcium channels in plasma
membrane. Calcium moves into the cytosol from the cell
wall and acivates protein kinase. These protein kinases
cause opening of anion channels in the plasma membrane
and anion primarily Cl⁻ and malate⁻² move out from the
cytosol
• K⁺ also exudes out.
• In response to this , water also moves out of the guard cells
leading to stomatal closure.
• When this response to ABA is to be reversed and ABA is
removed , gradually the K⁺ and Cl

15. From the discussion it becomes clear that a
number of endogenous and environmental
signals influence stomatal opening and all
these signals work through regulating water
content by the discussed mechanism.