HOMEOSTASIS IN
PLANTS
AKWASI ADU-GYIMAH JNR.
SKILLS
• Explain that stomata have daily rhythms of opening and closing
and also respond to changes in environmental conditions to
allow diffusion of carbon dioxide and regulate water loss by
transpiration;
• Describe the structure and function of guard cells and explain
the mechanism by which they open and close stomata;
• Describe the role of abscisic acid (ABA) in the closure of
stomata during times of water stress.
THE DAILY RHYTHMS OF STOMATA OPENING
AND CLOSING
Stomata tend to open in
 Stomatal opening during
daylight and close in the
the day maintains the
dark.
inward diffusion of carbon
dioxide as well as the
However, stomata show
outward diffusion of
daily rhythms of opening
oxygen as well as the
and closing and even when
outward diffusion of water
kept in constant light or
vapour in transpiration.
constant dark, these rhythms
persist.
This daily rhythm (pattern) is
however, over-ridden if and when
the plant becomes short of water
and starts to wilt.
Stomata open in response to:
increasing light intensity
low carbon dioxide concentrations
in the air spaces within the leaf.
WHAT CONDITIONS
INFLUENCE THE
OPENING OF STOMATA?
Stomata close in
response to:
darkness
high carbon dioxide
concentrations in the air
spaces in the leaf
WHAT CONDITIONS
INFLUENCE THE
CLOSING OF
STOMATA?
low humidity
high temperature
water stress, when
the supply of water
from the roots is
limited and/or there
are high rates of
transpiration.
The daily rhythm
of stomata closing
and opening
persists even when
plants are kept in
constant light b;
and in long periods
of constant dark c
STRUCTURE AND FUNCTION OF
GUARD CELLS
Each stomatal pore is
surrounded by two guard
cells.
Guard cells have unevenly
thickened cell walls;
The walls adjacent to the
pore (inner walls) are very
thick, whereas the wall
furthest from the pore
(outer walls) are thin.
NOTE:
The guard cells open
when they fill with
water and become
turgid
There are bundles of
cellulose microfibrils
arranged as hoops
around the guard cells
so that, as the cell
becomes turgid, they
mostly increases in
length and not in
diameter.
The ends of the two
guard cells are joined
and the thin outer walls
bend more readily than
the thick inner walls as
the cells become
turgid.
THE MECHANISM OF STOMATA
OPENING AND CLOSING
ATP-powered proton pumps in the
cell surface membrane of guard
cells actively transport H+ out of
the guard cells;
The low H+ concentration and
negative charge inside the cell
causes K+ channels to open;
K+ diffuses into the guard cell down
an electrochemical gradient;
Starch stored in the guard cells is
converted to organic acids,
particularly malate;
The high concentration of these
ions (K+ and malate) inside the
guard cells lowers the water
potential;
Water moves in by osmosis,
down a water potential gradient
The entry of water increases the
volume of the guard cells, so
they become turgid and expand.
The thin outer wall expands
most, so the cells curve apart.
ABSCISIC ACID AND CLOSURE OF STOMATA
Abscisic acid (ABA) is
a plant stress hormone
that is secreted by plants
in times of water stress
(very dry conditions);
ABA binds to receptors
on the cell surface
membrane of guard cells;
This inhibits proton
pumps so that H+ are not
pumped out of the guard
cells;
The result is a build up of
H+ inside the guard cells;
ABA stimulates the influx
of Ca2+ by opening Ca2+
channels;
Ca2+ acts as a second messenger and
encourages the efflux of K+ as well as
anions such as Cl-;
Thus water potential of guard cells
increases and water moves out of the
guard cells by osmosis;
The volume of the guard cells decreases
and the guard cells become flaccid;
 The stomata responds quickly by closing.
THE END!
•THANK YOU.