mechanism of stomatal regulation

1. MECHANISM OF STOMATAL
REGULATION
Presented by: TAHIRA ALI RAI

2. Introduction:
 Plant breathe through tiny openings in leaves are called stomata.
 The stomata are very minute holes.
 Stomata open and close to allow the intake of carbon dioxide and the
release of oxygen.
 found on the epidermis of the leaves.
 Each stoma is surrounded by two kidney-shaped epidermal cells,
known as guard cells.
 The stomata found in all the aerial parts of the plant.
 They are never found on roots.
 The epidermal cells surrounding the guard cells of the stoma are
known as accessory or subsidiary cells.

3. Stomata on leave surface

4.  The guard cells contain chloroplasts.
 These cells contain much amount of protoplasm
 The number of stomata may range from thousands to lacs
per square centimeter on the surface of the leaf.
 The stomata may be found on both the surfaces of the leaf,
but their number is always greater on the lower surface.
 The upper surface of the leaves of several xerophytes also
lacks the stomata.

5.  The free floating leaves of the water plants bear stomata
only on their upper surface.
Frequency of stomata on the upper and lower surface of leave

6. Basics Mechanism Of Opening And Closing Of
Stomata:
 In general it is considered that stomata open at day and closed
at night. Stomatal movements are regulated by the change of
turgor pressure in guard cells.
 When water enters the guard cell, it swells and its unevenly
thickened walls stretch up resulting in the opening of stomata.
 This is due to concave non-elastic nature of inner wall pulled
away from each other and stretching of the convex elastic
natured outer wall of guard cell.

8. Theories:
Different theories have been proposed regarding opening
and closing of stomata. The important theories of stomatal
movement are as follows,
1. Theory of Photosynthesis in guard cells
2. Starch – Sugar interconversion theory
3. Active transport potassium ion concept

9. Theory of Photosynthesis in guard cells:
Von Mohl (1856) observed that stomata open in light and close in
the night. According to him,
 The mechanism of the closing and opening of the stomata
depends upon the presence of sugar and starch in the guard
cells.
 During day time or in the presence of light, the guard cells of
the stomata contain sugar synthesized by their chloroplasts.
 The sugar is soluble and increases the concentration of the sap
of guard cells.

10.  Due to higher concentration of the cytoplasm of guard cells, the
water comes to them from the neighboring cells by osmosis and they
become turgid. With the result the stomata remain open.
 In the night or in the absence of light the sugar (glucose) present in
guard cells converts into the starch. The starch is insoluble, and this
way the cell sap of the guard cells remains lower concentration than
those of neighboring cells, and the neighboring cells take out the
water from the guard cells by osmosis making them loose and the
stomata closed.

11. Demerits:
 Chloroplast of guard cells is poorly developed and
incapable of performing photosynthesis.

12. Starch – Sugar Interconversion theory:
 According to Lloyd (1908), turgidity of guard cell depends
on interconversion, of starch and sugar.
 It was supported by Loftfield (1921) as he found guard cells
containing sugar during the daytime when they are open and
starch during the night when they are closed.
 Sayre (1920) observed that the opening and closing of
stomata depends upon change in pH of guard cells.

13.  According to him stomata open at high pH during day
time and become closed at low pH at night. Utilization
of CO2 by photosynthesis during light period causes an
increase in pH resulting in the conversion of starch to
sugar
 Sugar increase in cell favours endosmosis and increases
the turgor pressure which leads to opening of stomata.
Likewise, accumulation of CO2 in cells during night
decrease the pH level resulting in the conversion of
sugar to starch. Starch decreases the turgor pressure of
guard cell and stomata close.

14.  The discovery of enzyme phosphorylase in guard cells by
Hanes (1940) greatly supports the starch-sugar
interconversion theory. The enzyme phosphorylase
hydrolyses starch into sugar and high pH followed by
endosmosis and the opening of stomata during light. The
vice versa take place during the night.

15.  Steward (1964) proposed a slightly modified scheme of
starch-sugar interconversion theory. According to him,
Glucose-1-phosphate is osmotically inactive. Removal of
phosphate from Glucose- 1-phosphate converts to Glucose
which is osmotically active and increases the concentration
of guard cell leading to opening of stomata.

16. Objections to Starch-sugar interconversion theory:
 In monocots, guard cell does not have starch.
 There is no evidence to show the presence of sugar at a
time when starch disappears and stomata open.
 It fails to explain the drastic change in pH from 5 to 7 by
change of CO2.
 The conversion of starch into sugar is not fast process as
compared to the opening and closing of stomata.

17. Active Potassium (K+) ion Theory:
 Role of potassium K+ in stomatal opening is now universally accepted. This was
observed for the first time by Fujino (1967) that opening of stomata occurs due to
the influx of K+ ions into the guard cells.
 The sources of K+ ions are subsidiary and epidermal cells, thereby increasing the
concentration from 50 mM to 300 mM in guard cells.
 ATP helps in entry of K+ ions into the guard cells.
 In the presence of light the pH increase due to absence of CO2 which is used in
photosynthesis and the starch is converted to a 3 C compound phosphenolpyruvic
acid .

18.  The PEP combines with CO2 to produce oxaloacetic acid
which is converted to malic acid.
 The malic acid being a weak acid dissociates into H+
ions and malate ions.
 K+ and malate ions combines to form potassium malate
that increase the solute potential and lowers the water
potential of the guard cells.
 The water move into the guard cells to make them turigd
and the stomata open.

19.  In dark, CO2 produced during respiration accumulates
and pH falls resulting in conversion of malic acid to
starch.
 The k+ ion and water move out of the guard cells lowering
the osmotic potential of the cells.
 Loss of turgor result in closure of stomata.
 Also ABA ( abcissic acid) produced under water stress
conditions and plays a role in closure of the stomata.

20. Opening and closing of stomata

21. References:
 https://www.biologydiscussion.com/plants/stomatal-
mechanism/stomatal-mechanism-in-plant-cells-botany
 http://www.brainkart.com/article/Mechanism-of-Stomatal-
Movement--_33071/