Transcript 22.1.1 Water Conservation in Plants

Maintaining a Balance
Topic 21: Water Conservation in Plants
Biology in Focus, HSC Course
Glenda Childrawi, Margaret Robson and Stephanie Hollis
DOT POINT
 Describe adaptations of a range of terrestrial Australian
plants that assist in minimising water loss
Problems Facing Plants
The main form of water loss in
plants is by means of
transpiration – evaporation
of water from the stomata of
leaves. Transpiration serves
two functions – it lifts water
and dissolved ions up the stem
to the top of the plants in a
continuous transpiration
stream and it is a form of
evaporative cooling, a process
that is essential in regulating
temperature in plants.
bfwatercycle.blogspot.com
Problems Facing Plants
Those plants that live in areas
where water is in limited
supply (usually hot, dry areas)
must achieve a balance between
how much water the plant can
afford to lose for cooling
purposes and the risk of
dehydration. Xerophytes are
plants that live in such areas
and possess adaptations that
equip them to achieve this
balance and survive in their
hostile environment.
answerit.news24.com
Problems Facing Plants
Leaves of plants contain stomata and so they are the organs
where most transpiration occurs. Generally, stomata are most
abundant on the underside of leaves and are stimulated to open
in the presence of light and/or excess heat in well hydrated
plants. About 98% of water loss from plants occurs as a result
of transpiration
shoalwater.nsw.gov.au
Problems Facing Plants
Most of Australia is hot and dry, so
water becomes a limited resource
for plants, available in short supply
or only in sporadic bursts. Many
plants that live in hot and dry
conditions display complex
xerophytic adaptations, features
which have evolved and allow these
plants to minimise water loss while
maintaining functions such as
cooling the plant and
photosynthesis.
amodmag.com
Problems Facing Plants
Most of these adaptations are
evident as modifications of
leaves, but other organs may
also show modifications.
Xerophytes, such as some in
Australian plants minimise
water loss in four main ways.
tasteaustralia.biz
1. Reducing Internal Temperatures
Some plants have developed structural features and
physiological mechanisms to reduce their internal
temperature, allowing the plants to use less water for
evaporative cooling, but still keep their temperature within the
correct range for metabolism.
chemical-engineering.co
Reducing Internal Temperatures
For example:
 The saltbush has waxy leaves
that reflect heat and light
 Eucalypts and banksias have
coarse, leathery leaves with a
thick cuticle to protect them
from excessive sunlight.
Both of these features also ensures
that all the epidermal cells are
waterproof, preventing loss of
water by evaporation from these
surface cells to the outside.
anbg.gov.au
2. Reducing Exposure to Sunlight
Plant organs that have the most
abundant stomata have the greatest rates
of transpiration. Some plants reduce the
exposure of stomata to light by:
 Changing the orientation of leaves so
that stomata are not exposed to direct
sunlight.
 Reducing the surface area of organs
that have highest proportion of
stomata
 The complete loss of transpiring plant
organs.
en.wikipedia.org
Reducing Exposure to Sunlight
For example:
 Plants like Hakea and Acacias
(wattles) have leaves that become
reduced in size, where each leaf if
divided into leaflets. This reduces
surface area.
 Reduced size of flowers or having no
petals. Acacias have small clustered
flowers, reducing the energy and
water required to produce them.
 Shedding leaves; River gum
 Orientation of leaves; Eucalypts
oznativeplants.com
3. Reducing Water Gradient
The difference in water concentration between the plant and
the surrounding atmosphere determines how much water is
lost by transpiration. On a hot, dry day, the water
concentration in the air is much lower than the internal tissues
of the leaf and so more water is lost by transpiration than on a
cooler or more humid day.
flickriver.com
Reducing Water Gradient
Since plants cannot change the overall external environment,
many have adaptations that allow them to create their own
smaller ‘microclimate’ in the air immediately surrounding each
leaf.
britannica.com
Reducing Water Gradient
Structures such as hairy leaves
or rolled leaves trap water in
the immediate vicinity and in
this way they keep air around
the plant humid by preventing
the moist ait being swept away
by dry air currents and they
also create a barrier for
evaporation.
flickr.com
Reducing Water Gradient
Sunken stomata or stomatal pits occur in Hakea and in the
cladodes of she-oaks. The actual stoma (breathing pores) are
lower than the main surface of the leaf and this allows moist air
to be trapped in the pit, therefore reducing the difference in
water potential immediately outside the stoma (in the pit) and
inside the leaf.
vcbio.science.ru.nl
Reducing Water Gradient
Epidermal hairs trap a moist
layer of air, resulting in a
smaller difference between the
concentration of water in the
leaf tissue and the water
vapour in the layer of air
trapped by the hairs. For
example, the coastal banksia
has hairs on the under-surface
of leaves.
sydney.edu.au
Reducing Water Gradient
Porcupine grass has
curled/rolled leaves which
enclose a microclimate of
humid air to reduce the
difference in water potential.
These adaptations allow plants
to keep their stomata open for
a longer period of time, as
there is not as much water
being lost and so gaseous
exchange for photosynthesis
can occur freely.
flickr.com
4. Water Storage
Some plants like the Calandrinia, called succulents, have
adaptations such as fleshy stems or leaves which are able to
swell up and retain moisture when it is available; they then
survive by using this moisture during dry periods.
growinggroundsfarmsm.blogspot.com
Water Storage
Australia has some succulent species, including the desert plant
Calandrinia (parakeelya), an important food for Aboriginal
people. The leaves provide an excellent source of moisture in
desert environments and were eaten as a green salad leaf. The
word ‘parakeelya’ was derived from the word ‘periculia’, an
Aboriginal name for the bread-like seed meal that is made
from the cooked seed.
ausemade.com.au
Activity
-Students to complete DOT Point 3.15