LU5 Transpiration

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STT 1043 Plant Physiology

LU 5: Transpiration



• 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



• 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



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



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




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



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




Transpiration

• Leaves and green shoots are the

principal organs of transpiration

• Transpiration may occur through the

cuticle, lenticels or stomata


• Cuticular, lenticular or stomatal transpiration



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



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



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



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



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



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



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.



T i ti



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



Transpiration


Transpiration



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



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



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



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



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



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



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



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



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

LU5 Transpiration

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