Plants exchange gases through specialized pores called stomata. Stomata are bordered by guard cells that regulate opening and closing. During photosynthesis, stomata open to allow carbon dioxide entry and oxygen release. However, this also causes water loss through transpiration. Plants have developed mechanisms to minimize water loss while allowing gas exchange, including closing stomata in response to hormones like abscisic acid during drought. The size of vacuoles in guard cells changes, controlling turgor pressure and stomatal position.

1. Leaves And Atmosphere

2. Gas Exchange in Plants
 In order to carry on photosynthesis, green
plants need a supply of carbon dioxide and a
means of disposing of oxygen.
 In order to carry on cellular respiration, plant
cells need oxygen and a means of disposing of
carbon dioxide.
Unlike animals, plants have no specialized
organs for gas exchange .

3. Reasons of Gas Exchange
• Each part of the plant takes care of its own gas exchange needs.
• Although plants have an elaborate liquid transport system, it does
not participate in gas transport.
Roots, stems, and leaves respire at rates much lower than are
characteristic of animals.
• Only during photosynthesis are large volumes of gases
exchanged, and each leaf is well adapted to take care of its own
needs.
The distance that gases must diffuse in even a large plant is not
great.
• Each living cell in the plant is located close to the surface. While
obvious for leaves, it is also true for stems.
• The only living cells in the stem are organized in thin layers just
beneath the bark. The cells in the interior are dead and serve only
to provide mechanical support.

4. • Most of the living cells in a plant have at least
part of their surface exposed to air. The loose
packing of parenchyma cells in leaves, stems,
and roots provides an interconnecting system
of air spaces. Gases diffuse through air several
thousand times faster than through water.
• Oxygen and carbon dioxide also pass through
the cell wall and plasma membrane of the cell
by diffusion.

5. Stomata
• A stoma (plural stomata) (occasionally called a stomate,
plural stomates) (from Greek "mouth“
• is a pore, found in the epidermis of leaves
• , stems and,
• other organs that is used to control the gas exchange.
• The pore is bordered by a pair of specialized parenchyma
cells known as guard cells.
• Air containing carbon dioxide and oxygen enters the plant
through these openings.
• Oxygen produced as a by-product of photosynthesis diffuses
out to the atmosphere through these same openings.
• Also, water vapor is released into the atmosphere through
these pores in a process called transpiration.

6. Stoma in cross section

7. Leaves
The exchange of oxygen and carbon
dioxide in the leaf (as well as the loss
of water vapor in pores called the
stomata.
 Normally Stomata open,
when the light strikes the leaf in
the morning
and close during the night

8. Stoma in a
tomato leaf
shown via
colorized
scanning
electron
microscope
image.

9. Location Of Stomata
• Stomata are present in the sporophyte
generation of all land plant groups except
liverworts.
• Dicotyledons usually have more stomata on the
lower epidermis than the upper epidermis
• Monocotyledons, on the other hand, usually
have the same number of stomata on the two
epidermes.
• In plants with floating leaves, stomata may be
found only on the upper epidermis;
• submerged leaves may lack stomata entirely.

10. Guard Cells
• Guard cells are specialized cells located in the leaf
epidermis of plants.
• Pairs of guard cells surround tiny stomatal airway
pores .
• These tiny pores in the leaf surface are necessary
for gas exchange into and out of the plant; carbon
dioxide (CO2) enters the plant allowing the carbon
fixation reactions of photosynthesis to occur.
• The turgor pressure of guard cells is controlled by
movements of large quantities of ions and sugars
into and out of the guard cell.

11. • Oxygen (O2) exits the plant as a byproduct of
photosynthesis. The opening and closing of the
stomatal gas exchange holes is regulated by
swelling and shrinking of the two surrounding
guard cells .
• Water evaporates through the stomatal
openings causing plants to lose water. Over 95%
of water loss from plants can occur by
evaporation (transpiration) through the
stomatal pores.

12. Features Of Guard Cells
• A special feature of guard cells is that they can
increase or decrease their volume, thereby
changing their shape.
• This is the basis for the opening and closing of a
stoma, known as stomatal movement, which
controls gas exchange necessary for photosynthesis
and limits water loss.
• Such changes in vacuolar volume are quite rapid
and dramatic.
• This can be problematic because,
• unlike a quickly expanding balloon, biological
membranes are more limited in their elasticity and
do not allow over-stretching.

13. Function of Guard Cells
• Since guard cells control water loss of plant.
• Guard cells perceive and process environmental
and endogenous stimuli such as
• light,
• humidity,
• CO2,
• temperature,
• drought,
• and plant hormones to trigger cellular responses
resulting in stomatal opening or closure.

15. Mechanism and Regulation of
Stomata Movements

16. Mechanisms
• Stomata are functional units of the epidermis
serving the exchange of gases between the
intercellular spaces of the plant and its surrounding
• They are especially common – and of characteristic
shape –
• at the epidermis of the leaf’s underside of most
species.
• Their development differs from plant group to
plant group, but unequal cell divisions are always
involved.

17. • A functional unit consists of the guard cells
themselves that contain nearly always
chloroplasts, and of their neighbouring
subsidiary cells that are usually devoid of
chloroplasts.
• It is well-known that stomata open in a humid
surrounding, and close when it is dry.
• J. HEDWIG was the first researcher of the 18th
century who understood that they work as
‘transpiration openings.

18. • The wall between guard and subsidiary cell and
the wall that makes up the opening are rather
thin and are easily stretched.
• This asymmetry of the cell’s structure and of the
wall thicknesses explains the directed
movement caused by the turgor.
• The mechanics of the guard cells becomes thus
understandable.
• Water proved to be an important, though not
the only controlling factor of guard cell
movements.

19. Water
• Guard cells can emit water into three different directions:
• outwards,
• into the neighbouring subsidiary cell, and
• into the respiratory cavity that is a part of the intercellular
system lying beneath the guard cells.
• Plants profit from the concentration gradient (that is a
gain of energy for them),
• while the closing movements of the stomata exert a
decisive, regulating influence. They close when too much
water is lost, or when not enough supply exists.

20. Light
• The stomata of most plant species are closed in darkness.
• Light stimulates opening.
• The action spectrum is similar to that of photosynthesis.
Blue light is especially effective.
• The stomata of CAM-plants, like Crassulaceans, are
opened during the night.
• They depend on the accumulation of carbon dioxide
during the night.
• These plants store the carbon dioxide as malate or
aspartate and feed it into the CALVIN cycle during daytime
• Opened stomata would cause intolerable transpiration
losses in the areas that CAM-plants live in.

21. Carbon dioxide
• A low concentration of carbon dioxid causes the
stomata to open,
• a high concentration leads to their closing.
Photosynthesis starts with the first light of the day,
• because enough carbon dioxide has been
accumulated.
• Photosynthesis takes place in guard cells, too, since
they contain chloroplasts – in contrast to the subsidiary
cells.
• This activity again is related to the rise of the osmotic
value and thus also to the opening of the stomata.

22. Potassium ions
• Potassium ions are actively pumped (by a
potassium pump) from the subsidiary cells into
the vacuoles of the guard cells.
• At the same time, anions (chloride, malate)
accumulate within the vacuoles.
• Protons are given off to the subsidiary cells.
• The ion flows are quantitatively enough to
explain a rise of the turgor that is large enough
for the guard cell movements, but the primary
stimulus is still not clarified.

23. What is the reason for the potassium
pump’s sudden rise in activity?
• The actual importance of the potassium pump for the guard
cell movement is best demonstrated with a fungal toxin
called Fusicoccin (from the fungus Fusicoccum amygdali).
• This toxin activates the potassium pump.
• If consequently the toxin is applied to the stomata, then the
loss of water becomes higher than its supply resulting in
withering.
• The biological advantage for the fungus lies in the open
stomata since they are, beside wounds, the only places
where its hyphes can penetrate the leaf tissue.
• The total water balance is hardly or not at all influenced, as
long as the effect of Fusicoccin remains a local one.
• In principle, one open stomata is enough for a hype.

24. Vacuole Size Changes Are Correlated
with Stomatal Movements

25. • During photosynthesis, leaves take in atmospheric CO2 and
release O2 through stomata, microscopic pore structures in
the leaf epidermis.
• A pair of guard cells surrounds each stoma, and these cells
control the opening and closing of the stomatal pore
between them.
• Guard cells regulate this opening and closing in response to
a wide variety of environmental signals, such as day/night
rhythms, CO2 availability, and temperature.
• The primary reason is that stomata also regulate the
passage of water molecules.
• If the stomata were constantly open, plants would lose too
much water via evaporation from the leaf surface, a process
called transpiration.

26. How Do Vacuoles Change During
Stomatal Opening and Closing?
• Plant vacuoles are ubiquitous organelles that are essential
to multiple aspects of plant growth, maintenance and
development.
• Their key role in stomatal movements underscores their
importance in fundamental gas exchange for plant
• .In addition to their role in controlling photosynthetic gas
exchange, vacuoles also store compounds that may help
to protect photosystems in the chloroplast from damage
caused by excess light.
• Vacuoles are important compartments in plant cell
metabolism. An intact vacuole is necessary for many plant
functions.

27. Abscisic Acid
• The plant starts an enhanced production of
abscisic acid in case of a water shortage.
• The abscisic acid is transported to the guard cells,
where it is stored. It inhibits the potassium pump,
hinders the production of an osmotic pressure, and
does thus cause the closing of the stomata.
• The water and the carbon dioxide cycle may
compete in case of closed stomata, since carbon
dioxide is usually a limiting factor in
photosynthetically active tissues.

28. • The Rate of photosynthesis
decreases to a low level,
• though it does not stopped at all due to the
carbon dioxide,
• that is repeatedly produced a new in
amounts by the respiratory processes within
the plant.

29. Stomatal Role In Transpiration

30. Transpiration
• Transpirationis the process of water movement through a
plant and its evaporation from aerial parts especially from leaves
but also from stems and flowers.
• Leaf surfaces are dotted with pores which are called stomata, and
in most plants they are more numerous on the undersides of the
foliage.
• The stomata are bordered by guard cells and their stomatal
accessory cells (together known as stomatal complex) that open
and close the pore.
• Transpiration occurs through the stomatal apertures, and can be
thought of as a necessary "cost" associated with the opening
of the stomata to allow the diffusion of carbon dioxide gas from
the air for photosynthesis.
• Transpiration also cools plants, changes osmotic pressure

31. Regulation of Transpiration
Through Stomata
• Plants regulate the rate of transpiration by the
degree of stomatal opening.
• The rate of transpiration is also influenced by
the evaporative demand of the atmosphere
surrounding the leaf such as humidity,
temperature, wind and incident sunlight.
• Soil water supply and soil temperature can
influence stomatal opening, and thus
transpiration rate.

32. • The amount of water lost by a plant also
depends on its size and the amount of water
absorbed at the roots.
• Transpiration accounts for most of the
water loss by a plant, but some direct
evaporation also takes place through the
cuticle of the leaves and young stems.

33. Techniques of Transpiration
• Transpiration rates of plants can be measured by a
number of techniques, including,
• potometers,
• lysimeters,
• porometers,
• photosynthesis systems and
• heat balance sap flow gauges.
• Isotope measurements indicate transpiration is the
larger component of evapotranspiration.[3]
• Desert plants have specially adapted structures, such as
thick cuticles, reduced leaf areas, sunken stomata and
hairs to reduce transpiration and conserve water.

34. • Many cacti conduct photosynthesis in
succulent stems, rather than leaves, so the
surface area of the shoot is very low.
• Many desert plants have a special type of
photosynthesis, termed crassulacean acid
metabolism or CAM photosynthesis,
• in which the stomata are closed during the
day and open at night when transpiration will
be lower.

35. Functions Of Stomata

36. How do plants breathe
through stomata?
• Like animals, plants breathe.
• The gas exchange into and out of a plant leaf occurs at the
underside of leaves, and the process is precisely regulated.
• The main energy-producing biochemical process in plants
is photosynthesis,
• a process that,
• initiated by energy from the sun,
• converts CO2 and
• water into carbohydrate energy molecules for the plant
and releases O2 back into the atmosphere.
• In this process, leaves take in atmospheric CO2 and release
O2 back into the air.

37. CO2 Gain And Water Loss
• Carbon dioxide, a key reactant in photosynthesis, is
present in the atmosphere at a concentration of about
390 ppm .
• Most plants require the stomata to be open during
daytime.
• The problem is that the air spaces in the leaf are
saturated with water vapour, which exits the leaf
through the stomata (this is known as transpiration).
Therefore,
• plants cannot gain carbon dioxide without
simultaneously losing water vapour.

38. CAM Plants
• A group of mostly desert plants called "CAM" plants
(Crassulacean acid metabolism, after the family
Crassulaceae,
• which includes the species in which the CAM process was
first discovered) open their stomata at night (when water
evaporates more slowly from leaves for a given degree of
stomatal opening),
• use PEPcarboxylase to fix carbon dioxide and store the
products in large vacuoles.
• The following day, they close their stomata and release the
carbon dioxide fixed the previous night into the presence of
RuBisCO.
• This saturates RuBisCO with carbon dioxide, allowing
minimal photowhen water is severely limiting.

39. Inferring Stomatal Behavior
From Gas Exchange
• The degree of stomatal resistance can be determined by
measuring leaf gas exchange of a leaf.
• The transpiration rate is dependent on the diffusion
resistance provided by the stomatal pores, and also on the
humidity gradient between the leaf's internal air spaces and
the outside air.
• Stomatal resistance (or its inverse, stomatal conductance)
can therefore be calculated from the transpiration rate and
humidity gradient.
• This allows scientists to investigate how stomata respond to
changes in environmental conditions,
• such as light intensity and
• concentrations of gases such as water vapor,
• carbon dioxide, and
• ozone.

40. Here
ei and ea is the partial pressure
of
leaf water,
P is the atmospheric pressure
r is the Stomatal resistance.

41. Opening stomata
• The increase in osmotic pressure in the guard cells is
caused by an uptake of potassium ions (K+).
• The concentration of K+ in open guard cells far exceeds
that in the surrounding cells. This is how it accumulates:
• Blue light is absorbed by phototropin which activates
• a proton pump (an H+-ATPase) in the plasma membrane of
the guard cell.
• ATP, generated by the light reactions of photosynthesis,
drives the pump.
• As protons (H+) are pumped out of the cell, its interior
becomes increasingly negative.
• This attracts additional potassium ions into the cell, raising
its osmotic pressure.

42. Closing stomata
• Although open stomata are essential for
photosynthesis, they also expose the plant to the risk of
losing water through transpiration. Some 90% of the
water taken up by a plant is lost in transpiration.
• In angiosperms and gymnosperms (but not in ferns and
lycopsids), Abscisic acid (ABA) is the hormone that
triggers closing of the stomata when soil water is
insufficient to keep up with transpiration (which often
occurs around mid-day).
• The mechanism:
• ABA binds to receptors at the surface of the plasma
membrane of the guard cell.

43. • The receptors activate several interconnecting pathways
which converge to produce
– a rise in pH in the cytosol;
– transfer of Ca2+ from the vacuole to the cytosol.
• These changes
– stimulate the loss of negatively-charged ions
(anions), especially NO3
− and Cl−, from the cell and
also
– the loss of K+ from the cell.

44. Opening And Closing of Stomata

45. Density of stomata
• The density of stomata produced on growing
leaves varies with such factors as:
• the temperature, humidity, and light intensity
around the plant;
• and also, as it turns out,
• the concentration of carbon dioxide in the air
around the leaves. The relationship is inverse; that
is,
• as the concentration of CO2 goes up,
• the number of stomata produced goes down, and
vice versa.

46. Some evidence
– Plants grown in an artificial atmosphere with a
high level of CO2 have fewer stomata than
normal.
• Herbarium specimens reveal that the number
of stomata in a given species has been
declining over the last 200 years —
• the time of the industrial revolution and
rising levels of CO2 in the atmosphere

47. Stomata reveal past carbon dioxide
levels
• Because CO2 levels and stomatal index are inversely
related, could fossil leaves tell us about past levels of
CO2 in the atmosphere, his study of the fossil leaves of
the ginkgo and its relatives shows:
• their stomatal indices were high
– late in the Permian period (275–290 million years ago) and
again
– in the Pleistocene epoch (1–8 million years ago).
• Both these periods are known from geological
evidence to have been times of
– low levels of atmospheric carbon dioxide and
– ice ages (with glaciers).

48. Factors Affecting
Stomatal Regulation

49. Number of leaves
• More leaves (or spines, or other
photosynthesizing organs)
• means a bigger surface area and
more stomata for gaseous
exchange.
• This will result in greater water
loss.

50. Number of stomata
•More stomata will provide
more pores for
transpiration.

51. Size of the leaf
• A leaf with a bigger surface
area will transpire,
• faster than a leaf with a
smaller surface area.

52. Presence of plant cuticle
• A waxy cuticle is relatively impermeable to
water and water vapour and reduces
evaporation from the plant surface except via
the stomata.
• A reflective cuticle will reduce solar heating and
temperature rise of the leaf, helping to reduce
the rate of evaporation.
• Tiny hair-like structures called trichomes on the
surface of leaves also can inhibit water loss by
creating a high humidity environment at the
surface of leaves.

53. Light supply
• The rate of transpiration is controlled by
stomatal aperture, and these small
pores open especially for
photosynthesis.
• While there are exceptions to this (such
as night or "CAM photosynthesis"),
• in general a light supply will encourage
open stomata.

54. Temperature
• Temperature affects the rate in two
ways:
• 1) An increased rate of evaporation
due to a temperature rise will hasten
the loss of water.
2) Decreased relative humidity outside
the leaf will increase the water
potential gradient.

55. Relative humidity
•Drier surroundings gives a
steeper water potential
gradient, and so increases
the rate of transpiration.

56. Wind
• Wind blows away much of this water vapor
near the leaf surface, making the potential
gradient steeper and speeding up the
diffusion of wateEven in wind, though, there
is some accumulation of water vapor in a thin
boundary layer of slower moving air next to
the leaf surface.
• The stronger the wind, the thinner this layer,
and the steeper the water potential gradient.

57. Water supply
•Water stress caused by
restricted water supply
from the soil may result in
stomatal closure and
reduce the rates of
transpiration.

58. Response of Stomata to
Environmental Factors
• Photosynthesis,
• plant water transport (xylem) and
• gas exchange are regulated by stomatal function which is
important in the functioning of plants.
• Stomatal density and aperture (length of stomata) varies
under a number of environmental factors such as
atmospheric CO2 concentration,
• light intensity,
• air temperature and
• photoperiod (daytime duration).
• Decreasing stomatal density is one way plants have
responded to the increase in concentration of atmospheric
CO ([CO ] ).

59. Future Adaptations during Climate
Change
• The gene HIC (high carbon dioxide) encodes a
negative regulator for the development of stomata in
plants.
• Research into the HIC gene using Arabidopsis thaliana
found no increase of stomatal development in the
dominant allele
• but in the ‘wild type’ recessive allele showed a large
increase,
• both in response to rising CO2 levels in the
atmosphere.
• These studies imply the plants response to changing
CO2 levels is largely controlled by genetics.

60. Agricultural Implications
• Predicting how stomata performs during
adaptation is useful for understanding the
productivity of plant systems for both natural and
agricultural systems.
• Plant breeders and farmers are beginning to
work together using evolutionary
• and participatory plant breeding to find the best
suited species such as heat and drought resistant
crop varieties that could naturally evolve to the
change in the face of food security challenges.

61. Stomata as pathogenic pathways
• Stomata are an obvious hole in the leaf by which,
as was presumed for a while, pathogens can
enter unchallenged.
• However, it has been recently shown that
stomata do in fact sense the presence of some,
• if not all, pathogens. However,
• with the virulent bacteria applied to Arabidopsis
plant leaves in the experiment, the bacteria
released the chemical coronatine, which forced
the stomata open again within a few hours.