lu5 transpiration
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STT 1043 Plant Physiology
LU 5: Transpiration
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• Transpiration = When a leaf’s guard cells shrink, stomata open, an
is lost.
• In turn, more water is pulled through the plant from the roots
Transpiration
• The rate of
transpiration is
directly related to
whether stomata
are open or closed
• Stomata account
for only 1% of a
leaf’s surface but
90% of the water
transpired
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• Transpiration is a necessary process and uses about 9
the water that enters a plant’s roots
• The other 10% is used in chemical reactions and in
tissues
Transpiration
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Importance
Transpiration
• Transpiration is responsible for:
I. Transporting minerals from the
soil throughout the plant
II. Cooling the plant through
evaporation
III. Moving sugars and plant
chemicals – photosynthesis
IV. Maintaining turgor pressure
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General Mechanism of Water Movement
Transpiration
• Plants depend on a vascular system connecting root to shoot
• Water is lost through transpiration
• A control feedback mechanism
(stomata) keeps absorption and
transpiration in balance
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General Mechanism of Water Movement
Transpiration
• Plants depend on a vascular
system connecting root to shoot
• Water is lost through transpiration
• A control feedback mechanism
(stomata) keeps absorption and
transpiration in balance
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Transpiration
• Water losses from leaf
when stomata open and
this process is called
transpiration
• Transpiration develops
pressure gradient
• Difference in osmotic
pressure drives movement
of water into the cells
General Mechanism of Water Movement
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Transpiration
• Leaves and green shoots are the
principal organs of transpiration
• Transpiration may occur through the
cuticle, lenticels or stomata
General Mechanism of Water Movement
• Cuticular, lenticular or stomatal transpiration
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Transpiration
• Cuticle = a layer of wax-like
covering on the epidermis of leaves and herbaceous stems.
It is meant to regulate
transpiration
• However, some water may be
lost through it
Cuticular Transpiration
• Cuticular transpiration accounts nearly 20% of the total water lo
plant
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Transpiration
• Loss of water in the form of water vapour taking place through the
present in woody stem and fruits
• It amounts 1-5 % of the total water loss by the plant
Lenticular Transpiration
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Transpiration
• Stomata are minute pores confined to epidermis of green shoot and
• Opening and closing of stomata are controlled by guard cells• Maximum loss (80-90 % of the total water loss) of water from the
tissues takes place through the stomatal openings
• The loss of water in the form of water vapour through the stomat
plants constitutes stomatal transpiration
Stomatal Transpiration
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Transpiration
• Stomata – stomatal opening causes evaporation and loss of
and induce water to flow
• Root – control is also through root absorption: if root tempera
cold or roots lack aeration, transpiration is reduced because
absorption produce water deficit that causes stomatal closure
Control of Water Movement
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Transpiration
• An invisible process, since the water is evaporating from th
surfaces, you don’t just go out and see the leaves “sweating”
• During a growing season, a leaf will transpire many times
water than its own weight
• An acre of corn gives off about 3,000 – 4,000 gallons (11
15,100 L) of water each day, and a large oak tree can tra
40,000 gallons (151,000 L) per year
How Much Water do Plants Transpire?
• 10% of the earth’s atmospheric
moisture – from plant transpiration
T i i
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Transpiration
Factors Affecting Transpiration
• Atmospher ic factors affecting transpiration
• The amount of water that transpire varies g
geographically and over time
I. Temperature
II. Relative humidity
III. Wind and air movement
IV. Soil-moisture availability
V. Type of plant
T i ti
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Transpiration
Factors Affecting Transpiration: Temperature
• Transpiration rates go up as the temperature goe
especially during the growing season, when the air is w
due to stronger sunlight and warmer air masses.
• Higher temperatures cause the plant cells which contr
openings (stoma) whether water is released to the atmos
to open, whereas colder temperatures cause the openin
close.
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T i ti
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Transpiration
Factors Affecting Transpiration: Wind and Air Movement
• Increased movement of the air around a plant will result in a
transpiration rate.
• This is somewhat related to the relative humidity of the air, in
water transpires from a leaf, the water saturates the air surro
the leaf.
• If there is no wind, the air around the leaf may not move very
raising the humidity of the air around the leaf
• Wind will move the air around, with the result that the
saturated air close to the leaf is replaced by drier air.
T i ti
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Transpiration
Factors Affecting Transpiration: Wind and Air Movement
Transpiration
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Transpiration
Factors Affecting Transpiration: Wind and Air Movement
Transpiration
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Transpiration
Factors Affecting Transpiration: Soil-moisture availabili ty
• A plant cannot continue to tra
rapidly if its water loss is not
up by replacement from the so
• When absorption of water b
roots fails to keep up with th
of transpiration, loss of
occurs, and the stomata close
Transpiration
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Transpiration
Factors Affecting Transpiration: Types of Plant
• Plants transpire water at different rates.
• Some plants which grow in arid regions, such as cac
succulents, conserve precious water by transpiring less wate
other plants.
Transpiration
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Transpiration
Mechanism of Transpiration
• Turgor pressure inside mesophyll cells of the leaf forces
outwards through the cell wall.
• Water is collected in the intercellular spaces. From interc
spaces, water diffuses out of the stomata into the atmosphere
Transpiration
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Transpiration
Mechanism of Transpiration
• When plant roots can no longer absorb enough water to
transpiration from its leaves, it begins to dehydrate
• At this point the stomates can over-ride photosynthesis and
down for certain parts of the day to avoid further water loss
• Under extensive water stress where the plant is unable to reh
at night, the stomates will remain closed all day
• While this is every effective in preventing water loss, it preve
absorption of CO2 and therefore prevent photosynthesis
Transpiration
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Transpiration
Consequences of Water Stress
• When transpiration exceeds water absorption by the roots, the
dehydrates
• This usually happens each day with the plant rehydrating
each night
• As the soil dries out this rehydration is not complete, resulting
plant becoming water stressed
Transpiration
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Transpiration
How Stomates Open and Close
• Each stoma is flanked by a pair of guard cells that are capachanging shape, thereby widening or narrowing the gap be
the two cells
• When guard cells take in water by osmosis, they become turg
swell
• Guard cells are not uniformly thick – this, along with a series of roriented cellulose microfibrils in the cell wall, cause the guard cell to
outwards
• As they swell, the gap between the guard cells widens
• If the plant loses water, the guard cells become flaccid and th
closes
Transpiration
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Transpiration
Mechanism of Stomatal Action
• The epidermal cells surrounding the guard cells are usually
pushing against the guard cells
• As the guard cells absorb water and expand, they tend t
outward, separating at the middle and causing the stomata to
Transpiration
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Transpiration
Environmental Factors that Influence Stomatal Opening and Cl
• CO2
Low partial pressure of carbon dioxide cause the sto
to open
High partial pressure of carbon dioxide cause the sto
to close
• Light
Typically, stomata close in darkness and open in light
Some exceptions, such as CAM plants that open
stomata at night, fix carbon dioxide into organic acids
dark, and close their stomata during the day
Transpiration
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Transpiration
Environmental Factors that Influence Stomatal Opening and Cl
• Water stress
As water potential decreases (water stress increases
stomata close
• Temperature
High – close
• Wind
High - close