Download Water Relation and Transport

TRANSPIRATION
What is Transpiration?
• An evaporation of water in the form of water
vapour from the surface of the plant to the
atmosphere
• Where does transpiration take place?
Transpiration
• Transpiration mainly takes place through
openings on leaves – STOMATA
• If the stomata is opened, then water vapour
will be lost into the atmosphere
• In some cases, transpiration also takes place
through the lenticels and cuticles
Transpiration
1) A thin film of water covers each mesophyll
cell
1
Transpiration
1) A thin film of water covers each mesophyll
cell
2) There are numerous air spaces between the
mesophyll cells
2
Transpiration
1) A thin film of water covers each mesophyll
cell
2) There are numerous air spaces between the
mesophyll cells
3) Water evaporates from the film of water
surrounding the mesophyll cells into the air
spaces (and eventually diffuses out of the
stomata into the atmosphere)
3
Transpiration
4) As water diffuses out of the cell, the water
potential within the cell will decrease. Due to
OSMOSIS, water from adjacent cells will be
drawn into the cell to replace the water loss
These adjacent cells will in turn draw water
from other neighboring cells
4
Transpiration
5) Water is drawn from the XYLEM vessels (in the
veins) into the neighboring mesophyll cells
There will be a water potential difference
between the xylem vessels and the mesophyll
cells. As water is drawn from the xylem
vessels, a suction pressure will develop and
this pressure will pull water up the xylem
vessels from the roots to the leaves
5
Transpiration
• The pressure that allows water to be pulled
from the roots to the leaves in the xylem
vessels is called TRANSPIRATION PULL
• It allows the transportation of water and
minerals in plants
Investigation #1:
Studying the loss of water by
a plant into the atmosphere
Procedure
polythene
bag
Discussions
1) What is the purpose of enclosing the pot and
lower part of the plants with a polythene bag?
Ans: The polythene bag prevents evaporation of
water from the soil, and also prevents
vapour released by soil microorganisms
from affecting the result of the experiment
Discussions
2) What do you think will happen in the two setups after 2 hours?
Ans: Water would be found condensed on the
bell jar with the leafy plant inside
Discussions
3) How can you show that it is water?
Ans: We can use anhydrous cobalt chloride
paper to test it. It will turn the paper from
blue to pink. Alternatively, we can use
anhydrous copper sulphate. Water will
turn it from white to blue
Discussions
4) Why is it better to use forceps instead of
fingers to hold cobalt chloride paper?
Ans: It is to avoid moisture on our fingers from
being absorbed by the cobalt chloride
paper
Discussions
5) How would you explain the results of this
experiment?
Ans: Transpiration occurs in plants through
their leaves
I have no trouble
absorbing sunlight, but I
just keep losing and
losing water. Is there any
way that I can prevent
excessive water loss?
Adaptations to Prevent Water Loss
1) Waxy layer of cuticle on the leaf’s outer
surface of the epidermis
Adaptations to Prevent Water Loss
1) A waxy layer of cuticle covers the outer
surface of the epidermis
2) More stomata are present in the lower
epidermis than the upper epidermis
3) Guard cells control the closing and opening
of stomata
Stomata
• Stomata are pores in the epidermis where
gaseous exchange takes place during
photosynthesis (or respiration)
Stoma
Guard cell
Epidermal cell
Opened Stoma
Closed Stoma
Guard Cells
• Each stoma is
surrounded by two
guard cells which
contain chloroplasts
• It is kidney-shaped
• The inner wall is
thicker than the
outer wall
Guard cell Stoma
How Guard Cells Control the Size of Stoma
• Guard cells contain chloroplasts that carry
out photosynthesis in the presence of light.
Carbohydrates are formed and thus lower the
water potential of the cell
• Water enters the guard cells from adjacent
cells by osmosis and guard cells become
turgid
How Guard Cells Control the Size of Stoma
• The inner wall is thicker than the outer wall,
so the cell stretches to the outer side and
stoma is opened
• At night, there is no photosynthesis. Guard
cells become flaccid and so they return to the
original shape and stoma is closed
Distribution of Stomata in Leaves
Distribution of Stomata in Leaves
1) Normal Plants
- Mainly on the lower surface of plants
2) Floating plants
- Mainly on the upper surface of plants
- Leaves may also have air sacs to keep them
afloat. These sacs can be used in gaseous
exchange
Distribution of Stomata in Leaves
3) Submerged Aquatic Plants
- No stomata (not required since gaseous
exchange can be carried out by diffusion
though the leave surface)
- No cuticle (the primary function of cuticle is
to prevent excess water transpiration which
is not present in aquatic plants)
Distribution of Stomata in Leaves
4) Plants in dry and hot conditions
- usually have much less stomata to reduce
the amount of water loss
Investigation #2:
Investigating stomatal
distribution in a leaf by using
cobalt chloride paper
cobalt chloride paper
sellotape
Which piece of cobalt chloride paper will turn pink first?
Ans: The piece of cobalt chloride paper attached to the
lower epidermis of the leaf will turn pink first.
Investigation #3:
Comparing the abundance of
stomata on the upper and
lower surfaces of a leaf
Introduction to Investigation
• This investigation allows us to compare the
amount of stomata present on the upper and
lower surfaces of a leaf by putting the leaf in
hot water and observing the amount of
bubbles appeared
Procedure
forceps
hot water
leaf
Discussions
1) Which surface has more air bubbles coming off?
Ans: There should be more air bubbles appearing
on the lower surface of the leaf
Discussions
2) Why do air bubbles appear on the leaf surfaces?
Ans: Air in the air spaces between the mesophyll
cells in leaf expands on heating and passes
out through stomata of the leaf, forming air
bubbles
Discussions
3) What does the result show?
Ans: The result shows that more stomata are
present on the lower epidermis of a leaf
Investigation #4:
Comparing the abundance of
stomata on the upper and lower
surfaces of leaves by weighing
Introduction to Investigation
• As water is lost by evaporation, the weight of a
detached leaf will decrease with time. In this
investigation, leaves will be treated differently
(with vaseline) and the loss in weight will then
be compared among the leaves.
Procedure
A
Smear with
vaseline on
both surfaces
of the leaf
B
Smear with
vaseline on
lower surface
of the leaf only
C
Smear with
vaseline on
upper surface
of the leaf only
D
Do not apply
vaseline on
the leaf
Discussions
1) What is the function of vaseline in this
experiment?
Ans: It is used to block the stomata so as to
prevent water being lost by evaporation
Discussions
2) Which leaf would show the greatest/least
change in weight?
Ans: Leaf D > Leaf C > Leaf B > Leaf A
Discussions
3) What do the results of leaves B and C indicate?
Ans: The change in weight of leaf B is less than
that in leaf C, which indicates that more
water is lost in leaf C. This suggests that
there are more stomata present in the lower
surface than the upper surface of leaves
Xerophytes
Plants living in hot and dry environment
Adaptations of Xerophytes
1) They have numerous epidermal hair
- trap moisture
Adaptations of Xerophytes
1) They have numerous epidermal hair
- trap moisture
2) Some have sunken stomata
- also trap moisture
Adaptations of Xerophytes
1) They have numerous epidermal hair
- trap moisture
2) Some have sunken stomata
- also trap moisture
3) Some have rolling leaves which enclose the
stomata – reduce contact between stomata
and the environment
Adaptations of Xerophytes
1) They have numerous epidermal hair
- trap moisture
2) Some have sunken stomata
- also trap moisture
3) Some have rolling leaves which enclose the
stomata - reduce contact between stomata
and the environment
4) Some have small, needle-like or spiny
leaves, which have a small surface area
Adaptations of Xerophytes
1) They have numerous epidermal hair
2) Some have sunken stomata
3) Some have rolling leaves which enclose the
stomata
4) Some have small, needle-like or spiny
leaves, which have a small surface area
5) Some have fleshy stems or leaves to help
store water
Adaptations of Xerophytes
• Some trees also
shed their leaves
during dry seasons
(e.g. autumn) in
order to reduce the
rate of transpiration
Investigation #5:
Comparing the rates of transpiration
of a leafy shoot under different
environmental conditions using a
bubble potometer
Introduction to Investigation
• Bubble potometer can be used for estimating
the rate of water uptake by a plant. That is, it
can be used as an indirect method in
measuring the rate of transpiration. In this
investigation, the transpiration rates of a
plant under different conditions will be
compared
Procedure
graduated
capillary tube
leafy shoot
reservoir
tap
air/water
meniscus
Results Table
Condition
Normal
condition
Under sunlight
Near a fan
Covered with
plastic bag
Position of
Bubble (Start)
Position of
Bubble
(20 mins.)
Distance
Traveled by
Bubble
Discussions
1) Why is it important to cut the leafy shoot under
water?
Ans: This prevents air from entering the xylem
vessels of the stem and blocking the water
uptake
Discussions
2) What is the relationship between transpiration
rate and the distance traveled by the bubble?
Ans: The rate of movement of the bubble is
proportional to the transpiration rate of the
plant. Under normal conditions, the rate of
water absorption of a plant is equal to the
rate of transpiration
Discussions
3) Compare the transpiration rate of the leafy
shoot under the different conditions.
Under Sunlight
• The plant has faster transpiration rate when it
is placed under sunlight. Stomata open wider
when the plant is placed under sunlight.
Therefore the diffusion rate of water vapour to
the atmosphere, and thus the transpiration
rate, increases. High light intensity also
increases the atmospheric temperature
which in turn increases the rate of
transpiration
Near a Fan
• The plant has faster transpiration rate when it
is placed near a fan. Greater air movement
carries water vapour away from the surface of
the leaf at a faster rate. This keeps the
concentration of water vapour around the
stomata at a low level. Hence, water vapour
diffuses out of the leaf faster and the rate of
transpiration is increased
Covered with Plastic Bag
• The plant has a slower transpiration rate
when it is covered with a plastic bag. High
humidity in the surrounding air decreases the
concentration gradient of water vapour
between the leaf inside and that of the
atmosphere. Hence, water vapour diffuses out
of the leaf more slowly and the rate of
transpiration is decreased
Limitations
• The potometer can only measure the rate of water
uptake by the leafy shoot but cannot directly
measure the rate of transpiration. If a dehydrated
plant is used, the rate of water absorption is higher
than the transpiration rate. Under dry condition, the
transpiration rate of the plant may exceed its rate of
water absorption.
• Therefore, when using a bubble potometer, assume
that the rate of water absorption by the plant is
equivalent to the rate of transpiration
Investigation #6:
Comparing the rates of transpiration
of a leafy shoot under different
environmental conditions using a
weight potometer
Introduction to Investigation
• Another way to measure the a plant’s rate of
transpiration is to measure its loss in weight
using an electrical balance. This is called a
weight potometer. In this investigation, the
transpiration rates of a plant under different
conditions will be compared
Procedure
Discussions
1) What is the function adding a layer of oil in the
set-up?
Ans: The layer of oil can prevent evaporation of
water in the flask, which can affect the result
Discussions
2) What is the relation between the transpiration
rate and the change in weight of the plant?
Ans: Since the water loss from the set-up is due to
transpiration only, the change in weight of
the set-up is directly proportional to the
transpiration rate of the plant
Discussions
3) What limitations may lead to inaccurate results
in the experiment?
Ans: The initial weight of the set-up may not be
accurate because water may be present on
the wall of the apparatus and also on the
leafy shoot
Discussions
4) Compare the weight potometer and the bubble
potometer. Which one is easier to use? Which
one is more accurate?
Ans: A weight potometer is easier to use and is more
accurate to measure the transpiration rate of plants.
It is because the weight potometer can measure the
rate of transpiration directly whereas the bubble
potometer can only measure the rate of water
uptake of plants
Environmental Factors Affecting
the Rate of Transpiration
• There are FIVE environmental factors which
affect the rate of transpiration. They are:
(I) Light Intensity
(IV) Wind Speed
(II) Temperature
(V) Water Supply
(III) Humidity
Light Intensity
 More stomata open wider in light, so plants can get
enough carbon dioxide from atmosphere for carrying
out photosynthesis
 Light will increase temperature
 Rate of diffusion/evaporation of water vapour
through stomata will increase
 Rate of transpiration increases
Effect of Light Intensity
on Rate of Transpiration
Rate of
transpiration
Maximum rate
Light intensity (lux)
Temperature
Temperature
Rate of diffusion of water
vapour from intercellular
space in leaf to outside
Rate of evaporation
of water from
mesophyll cells
Ability of air to hold
water vapour
Rate of
transpiration
Effect of Temperature
on Rate of Transpiration
Rate of
transpiration
Temperature (0 C)
Humidity
Humidity
Rate of
Outside
Transpiration
A decrease in humidity makes the diffusion
gradient of water vapour from the moist
intercellular space of a leaf to the external
atmosphere steeper, therefore the rate of
diffusion of water vapour increases

Effect of Relative Humidity
on Rate of Transpiration
Rate of
transpiration
Relative humidity (%)
Wind Speed
Water vapour around
Under very windy
conditions, stomata
will be closed to
reduce water loss
the leaf is swept away
Transpiration rate
Wind blows
increases
Diffusion gradient between
intercellular space in leaves
and outside becomes steeper
Effect of Wind Speed
on Rate of Transpiration
Rate of
transpiration
Wind speed (km/hr)
Availability of Water
Lack of water (plants become
dehydrated)
Soil dries, plant wilts
and stomata close
Transpiration rate decreases
Wilting – the leaves and stems
become flaccid due to dehydration
Enrichment Reading 9.1
Daily Changes in Transpiration Rate
Plant Tissues
Plant tissues fall into 3 fundamental
categories:
• Dermal tissues
• Ground tissues
• Vascular tissues
＊These tissues can be found in roots, stems
and leaves
Dermal Tissues
• Provides a protective layer around the plant
• Exists as a single layer of cells called the
epidermis
• No chloroplasts
Dermal Tissues
• In the shoot system the epidermis secretes a
waxy layer called the cuticle (a protective
barrier to retard water loss and to prevent
infection)
• Stomata are found on the epidermis to allow
gas exchange to occur
• Guard cells control the closing and opening of
stomata
• Guard cells are the only cells in epidermis that
contain chloroplasts
Ground Tissues
• Make up the bulk of the plant
• Ground tissues are needed for storage,
mechanical support and energy production
• They can be classified into four broad
categories – parenchyma, chlorenchyma,
collenchyma and sclerenchyma
Parenchyma Cells
•
•
•
•
•
The most abundant ground tissues
Loosely arranged with intracellular spaces
Have thin walls and large vacuoles
Metabolically active
Perform a variety of functions, such as
photosynthesis, repair, food storage and
secretion
Parenchyma Cells
Vascular Tissues
• Continuous throughout the plant
• Usually embedded in ground tissues
• Composed of two complex conducting tissues
which form the vascular bundles
• Xylem + Phloem = Vascular bundles
Xylem
• Conduct water and minerals (one way only:
from roots to shoot)
• Provide support to plant
• The cell walls of xylem cells derive most of
their strength from lignin, a chemical
compound produced only by plants
• It is composed of tracheids, vessel elements,
fibers, and parenchyma cells
Xylem
1) Tracheids – long, thin cells with closed ends
and are dead at maturity. Contain numerous
pits through which water moves
2) Vessel elements – similar to tracheids but
contain holes at each end and are joined
end-to-end forming vessels. They are thickwalled and non-living (no cytoplasm and no
nuclei)
Phloem
• Transport organic materials (glucose)
synthesized by the plant from leaves to the rest
of the plant
• It is composed of sieve tube members,
companion cells, fibers and parenchyma cells
Phloem
1) Sieve tube members - a sieve tube, like
xylem vessels, is a series of cells (sieve
elements) joined end to end. The cross
walls between successive sieve elements
are perforated, forming sieve plates. The cell
walls are thin. Although the cells are living,
they lack a nucleus. Unlike xylem vessels,
the cells walls are not thickened by lignin
Phloem
2) Companion cells – specialized parenchyma
cells that develop alongside a sieve tube
member. They are elongated, thin-walled
and possess a nucleus. Companion cells
are linked with the sieve tubes by small
canals filled with cytoplasm, which are
smaller than pits. Companion cells help to
regulate the metabolic activities of sieve
tube elements, and help to load and unload
the food for transport
Different Parts of a Dicot Plant
•
Let’s examine the distribution of different
tissues in various parts of a dicotyledonous
plant:
1) Root
2) Stem
3) Leaf
Cambium
• Layer of thin-walled cells
between xylem and
phloem
• The cambium produces
new layers of phloem on
the outside and new
layers of xylem on the
inside, thus increasing
the diameter of the stem
Structure of Dicot Root
• Outermost layer:
epidermis – no cuticle
• Cortex – thin walled
parenchyma allow
movement of water and
minerals
• Vascular tissues in
centre of root
• Tip: root cap
Root Cap
• A protective layer at the
very tip of root
• To protect the delicate
cells of root from being
damaged as the root
grows down through the
soil
• Growing point is behind
root cap – by active cell
division
Functions of Roots
1) Water and minerals absorption
2) Anchorage
Adaptations for Absorption
of Water and Minerals
• Extensive branching system – what is
advantage?
• Outermost layer consists of epidermal cells
that lack cuticle – what is advantage?
• Some epidermal cells near the root tip have
root hairs – what is advantage?
Adsorption of Soil Water by Root Hairs
• Soil water is a dilute solution of salts – it is
more dilute than the cell sap and cytoplasm of
root hair
• Water will pass by OSMOSIS into root hair
through cell wall and cell membrane
Absorption of Minerals by Root Hairs
• Concentration of minerals in soil is usually
lower than that inside the root epidermal cells
• Can minerals be taken up by osmosis???
Water and Minerals Transportation
• Can you design an
experiment to show that
water and minerals are
transported along the
xylem vessels only?
Capillary Action
• Adhesion
• Cohesion
• Against gravity
Root Pressure
• Minerals are actively transported from root to
xylem
• If transpiration rate is low, salts accumulate
in xylem and thus water potential is lowered
• Therefore, water enters xylem by osmosis
• Pressure builds up, pushing the content in the
xylem upwards
• If stem is cut, water can be seen gushing out
(“stem-bleeding”)
Transportation of Organic Substances
• Mesophyll cells in
leaves (high [ ] of
carbohydrates) ->
sieve tubes of
phloem (low [ ] of
carbohydrates) ->
active growing
areas (e.g. root tips)
/storage areas (e.g.
fruits)
Why do plants need a support system?
Why do plants need a support system?
1) Prevent leaves from shaded by other plants
2) Allow leaves to receive maximum amount of
sunlight for photosynthesis
3) Display flowers as to facilitate dispersal of
pollen or seeds
Support in Young Dicot Plants
Support in young plants or non-woody
parts of plants is contributed mainly by
turgidity of the thin-walled cells
(parenchyma) in the cortex and pith
Support in Young Dicot Plants
• Thin-walled cells in
cortex and pith with
large central vacuole
• Water enters the cell
vacuole by osmosis
• Pressure exerted on the
tough epidermis
• Stem becomes hard and
upright
Support in Young Dicot Plants
Water absorbed
by osmosis
turgid
Water lost by
osmosis
flaccid
Wilting
Support in Young Dicot Plants
• Xylem vessels contain cells walls with lignin –
thick and rigid
• Vascular bundles arranged in a ring near the
epidermis – prevent bending
• In roots – vascular bundles in centre to give
roots more penetrating power and more
resistance to stretching (prevent uprooting)
Support in Old Dicot Plants
• Majority of xylem is thick-walled cells (e.g.
xylem vessels) – make stem rigid
• Plant no longer rely on turgidity for support
• Support is now contributed mainly be rigidity
of the lignified cells in xylem
Support in Old Dicot Plants
• Thin-walled cells, cambium, found between
xylem and phloem
• As plant matures, cambium cells divide to
form new cells -- inner side -> new xylem
-- outer side -> new phloem
• Accumulation of xylem tissues
-> stem becomes woody and increases in
diameter
Support in Old Dicot Plants