Download Animal and plant orientation

What is the Abiotic Environment?
Planet Earth reveals its unique (in our solar system anyway) features that
support life. A thin layer around our planet provides light and water to
sustain life. We call this layer the Biosphere. Fortunately, this layer
provides a narrow range of the conditions needed for life to exist temperature does not change too much over a day or even a year
anywhere on Earth.
It is the atmosphere that buffers and determines the climate at the
Earth's surface. Without it the Earth would be a lifeless, desolate planet
like Mars, or the Moon.
We use the word environment to describe the set of conditions that affect
an organism in its habitat.Obviously, these conditions vary according to
where we are on Earth and they determine where organisms are found.
The most obvious factors of the environment are non-living, we call
these abioticfactors. Examples are: temperature, light, wind, water,
substrate, pH, minerals and current speed.
Abiotic factors generally determine whether an organism is successful in
any habitat. This is because most species can only tolerate a narrow
range of any environmental factor. A typical tolerance range for an
organism is shown below.
If temperature (or any other abiotic factor) strays too much from
the optimum range it puts the organism under stress. If it moves even
further beyond thelimits of tolerance the organism will die. For this
reason organisms have developed adaptations to reduce the stress and
prevent the death. Many vertebrates show homeostatic
mechanisms which attempt to keep the internal environment constant
even though their outside world is changing.
Most invertebrates (and vertebrates) have developed behaviours that
move them out of the conditions causing stress. We call this orientation or migrationif the response is to seasonal climatic change.
Organisms also affect each other. So living things are also part of the
environment, we call them bioticfactors. The relationships between
organisms can be complex and they are the subject of the section entitled
'behaviour'.
What stimuli are responded to?
All organisms are affected by their abiotic environment. Factors include
temperature, wind, light intensity, humidity, pH, mineral availability,
water, chemicals, gravity, and sometimes touch.
The survival of an organism depends on their ability to find places that
offer the best conditions for them. Many terrestrial invertebrates - such
as slaters and earthworms - must avoid conditions that will lead to them
drying out. So they have the ability to detect differences in light
intensity, humidity and temperature. They orientate accordingly.
Plants depend on their leaves receiving enough sunlight to carry
out photosynthesis. They need this to supply energy to stay alive. For
this reason, plant shoots grow toward the light. Similarly, their roots must
seek out and absorb water - in fact the roots will grow towards water. The
roots are also important in stabilising the plant in the soil, so roots detect
and grow down towards gravity.
Sometimes movements are not due to an animal directly sensing
favourable conditions. It ofen seems as thought they have anticipated
favourable conditions, such as by migration. However, the animal does
detect an external stimulus - usually daylength - and this sets off internal
changes that lead to the migratory movement.
The types of abiotic factor (stimuli) that organisms respond to are listed
below with the prefix that is used to indicate what caused the response.
Stimulus
Prefix
Light
photo-
Temperature
thermo-
Water
hydro-
Current
rheo-
Chemicals
chemo-
Touch
thigmo-
Gravity
geo- or gravi-
Sound
Pressure
Wind
Substrate
How do Organisms Detect Stimuli?
In most animals stimuli are detected by nerve endings or specialised cells
called receptors. These receptors cause electrical signals to travel
along sensory nervesto the central nervous system.
Receptors are often clumped to form sense organs (e.g. the eye). The
central nervous system is a co-ordinating centre which interprets the
information and sends out another electrical signal along motor nerves.
Motor nerves end at effectors which are usually muscles or glands. They
respond to the stimulus, often by movement but also by chemical
secretions and pigment changes.
Used with permission from Eric Chudler.
Nerves are strange-looking, specialised cells that are designed to carry an
electrical impulse very rapidly.
The strength of a stimulus is determined by thefrequency that it 'fires'
electrical impulses. A weak stimulus will not produce a response.
A weak stimulus (above) and a strong stimulus (below).
Most complex animals have a central nervous system that coordinates
activity in some way. This is important because animals need to be able
to determine direction. This can be done in one of two ways:


the central nervous system compares nerve impulses from different
receptors.
the central nervous system compares nerve impulses from one
receptor over a period of time as it changes position.
Used with permission from Freedman and Sinnaeur Associates.
Such complexity is of course not needed for unicellular organisms like
amoeba.
What senses are used?
Animals use a variety of senses to view their world, often with greater
sensitivity than us as you will see as you look at the examples shown
below.
1. Photoreceptors
The simplest eye is nothing more than a light detector but insects
and crustaceans have developed a compound eye.
Used with permission.
Having so many separate 'eyes' within a compound eye means they are
very good at detecting movement, but they do not form a clear image.
Thevertebrates have a lense to help form a clear image.
Used with permission from Eric Chudler.
We see colour because our retina has cones as well as rods. Fish, birds,
reptiles and insects all have species that are able to see in colour, but
many mammal species cannot. Primates have good colour vision.
Used with permission from Eric Chudler.
Colour has become very important to many animals as a means of
attracting a mate or initiating a feeding response in seabirds.
In the case of the monarch butterfly the coloured patterns signal to
predators that they are poisonous.
Vision is very important in the life of hunting birds like the eagle. By
having both eyes facing forward (binocular vision) they are able to better
judge distance. They also have more rods and cones and a thicker retina
for better vision. Also, they have more sensory cells in the upper half of
the retina so they see better looking down (at their prey). Like us their
sharpest vision is at the fovea (which is packed with sensory cells) and
the eagle has two fovea compared to our one.
Bees see ultraviolet light but our eyes cannot. This means they have a
very different view of some flowers, which are designed to attract the bee
(not humans) by having markings in ultraviolet.
Light is also important in the lives of plants. Apart from the obvious need
for photosynthesis, light causes bending and flowering in plants. Pigments
in the plant such as phytochromes and riboflavins absorb light and
somehow causes a response. The details of how these pigments lead to a
response are not yet known.
What senses are used?
Animals use a variety of senses to view their world, often with greater
sensitivity than us as you will see as you look at the examples shown
below.
2. Thermoreceptors
Used with permission from Infrared Processing and Analysis Center (IPAC/NASA).
Just beyond the red light end of the visible spectrumlies infra-red, which
is a form of heat. All living organisms radiate heat and a few animals can
detect the heat with infra-red sensors.
Mosquitos can home in on their warm-blooded prey using
thermoreceptors. Snakes like the rattle snake and pit viper can use infrared to detect their prey so accurately that they can strike in total
darkness.
What senses are used?
Animals use a variety of senses to view their world, often with greater
sensitivity than us as you will see as you look at the examples shown
below.
3. Chemoreceptors
Smell and taste detect chemicals that are diffusing through the air or
water or that have been absorbed by the animal. Not all animals have
their receptors concentrated in the nose or tongue. Many have sensory
hairs on their feet or receptors on their antennae.
Many insects such as ants leave trails of chemicals
called pheromones which other ants follow by detecting the chemical.
Some moths will detect sex-attractant pheromones up to 5 km away.
Plants also release volatile chemicals that are characteristic of their
species. This is how many nocturnal insects identify which plant to visit,
especially for laying eggs.
The octopus has chemoreceptors on its tentacles so it can detect
chemicals without leaving home. The earthworm's entire body is covered
with chemoreceptors to 'taste' their world.
Some animals use chemicals such as urine to mark their territory. They
have very well-developed sense of smell. In fact the dog has 150 cm2 of
membrane for smell compared to humans with 4 cm2.
There is some evidence that humans may respond to chemical signals
from other people. Women that had lived in the same dormitory for some
time developed fairly synchronised menstrual cycles even though they
were initially randomly scattered. However, humans are very difficult to
study.
What senses are used?
Animals use a variety of senses to view their world, often with greater
sensitivity than us as you will see as you look at the examples shown
below.
4. Mechanoreceptors
It is very important for animals to know how they are orientated with
respect to gravity and to be able to co-ordinate their body movements.
Mechanoreceptors help an animal achieve this by sensing touch, pressure,
stretch and gravity.
Many invertebrates can detect minute air movement or water currents
with hairs on their limbs. Many invertebrates also have a special gravity
detector called a statocyst (or statolith). This is a chamber containing
a granule (often sand) resting on sensitive hairs, and numerous nerve
cells. The movement of the granule tells the animal it has changed
position.
Used with permission.
Plants appear to be able to do something similar, that is to detect gravity.
Starch-containing organellescalled amyloplasts (or statoliths) detect
gravity in the cells of the root cap. They are denser than other cells so
they sink to the bottom of the cell and their distribution in the cell may
indicate how the cell is oriented. These cells are called statocysts and
they can also be found all the way up the stem. They allow a plant stem
to grow upwards, away from gravity, and the plant roots to grow
downwards toward gravity.
Used with permission.
Some plants can also respond to touch. The Venus Flytrap is a well known
example. There are trigger hairs on the leaves that detect a touch and
convert it into an electrical signal that sets off the trap.
Fish have a lateral line, a system of sense cells running the length of
their body. It senses movement, pressure changes and may be used to
locate prey.
What senses are used?
Animals use a variety of senses to view their world, often with greater
sensitivity than us as you will see as you look at the examples shown
below.
5. Sound Receptors
Used with permission
Sound waves are produced whenever objects vibrate. The waves must
pass through a medium like water, air or soil. The human ear picks up
these waves and begins to vibrate at the eardrum. These vibrations are
then transmitted across the middle ear by bones to the cochlea, where
they set up electrical impulses in the auditory nerve. Our brain 'hears'
these, but only if the frequency of vibrations are between 20 Hz and 20
000 Hz.
Sounds with frequencies above this range are called'ultrasound' and
frequencies below this range are called 'infrasound'. Many animals can
hear well beyond our range of hearing.
Elephants can hear infrasound which they use to communicate over great
distances (50-100 square meter range) in the early evening. Low
frequency sounds do not dissipate as much at this time of day, they travel
over long distances. Elephant communication has been found to be quite
complex ranging from warning calls to a female signalling oestrus.
Used with permission
Bats can send out ultrasonic waves which bounce off objects as an echo.
They can tell the size shape and position of the object from the echo. This
process is called echolocation and bats use it to find food in the dark.
Dolphins and whales also use echolcation because these waves travel so
well under water.
Sound is also used to form songs which may convey messages.
Used with permission
Humpbacks are the only whales known to "sing"--it's one very specific
type of communication not related to echolocation or other forms of
communication. Humpback Whales sing the most during mating season.
They sing long complicated songs with repeating patterns. These songs
last a few minutes to more than half an hour and can be heard up to 100
km away.
Bird song is especially important as it is used to mark territories
(especially by the males) and to identify species in courtship rituals. A
sonogram is a visual representation of a song, as shown below.
Used with permission from Paul Kenyon.
Songs can be very specific to one species, allowing them to identify
themselves as shown by the sonograms of different sparrows below. The
mountain and coastal (Nuttal's) sparrows are the same species and have
very similar songs while the swamp sparrow is a different species.
What senses are used?
Animals use a variety of senses to view their world, often with greater
sensitivity than us as you will see as you look at the examples shown
below.
6. Magnetic Receptors
It appears that some animals can detect the Earth's magnetic field and
navigate with it.
Used with permission
Some bacteria have been found to have particles of magnetic minerals
(usually Fe3O4 which is called magnetite) enclosed in a membrane. Such
particles are called magnetosomes. Rows of magnetosomes form a type
of magnet with two poles (a dipole) and this allows such bacteria to
navigate in a magnetic field.
Used with permission.
Similar mechanisms have been suggested for navigation in animals such
as the pigeon. It is believed that birds may use particles of magnetite to
detect the magnetic field of the Earth. There is also evidence suggesting
that changes in a magnetic field might alter the energy of certain pigment
molecules which could then be detected by part of the retina. In other
words, the visual system is detecting magnetic field changes.
7. Electroreceptors
The ability to perceive and interpret electrical fields appears to be present
in all aquatic vertebrates. The fish below emits an electrical field from an
organ and then interprets the field with electroreceptors.
Used with permission.
Any object coming into the electric field causes avoltage drop and a
distortion to the field, which the fish can detect and interpret.
Used with permission.
Sharks have been shown to locate their prey, such as flounder buried
beneath the sand, using electrical signals.
Used with permission.
In some cases complex social communication has developed so that
individuals signal aggressive and sexual behaviour with their unique
electrical signals.
Electric eels have developed the ability to deliver the electrical charge in
the form of an electric shock of up to 650 volts, which stuns the prey.
How do Plants respond?
All living things are sensitive - they detect and respond to their
environment. Of course, this includes plants even though their responses
are usually much slower than animals. This is because plants
must grow to respond, they cannot get up and move like an animal.
Young stems grow up toward the light so that they can carry
out photosynthesis, while the main root grows down towards gravity so it
can anchor the plant firmly in the ground. Other roots grow towards
water so the plant will not dry out. How do we know that plants respond
by growth?
In order to answer this question we must first understand where plants
grow.
Growth in plants occurs at special regions at the tip of shoots and roots
called the apical meristem (in roots this is protected by the root cap). It
occurs in three stages:



cell division at the tip
cell elongation just below the tip
differentiation into specialised cells.
The same regions are found in shoot tips.
It has long been known that plants will bend when exposed
to unidirectional light.
In order to show plants responded to light by growth the following
experiment was carried out. Young seedlings were marked at even
intervals up the stem and then grown under the different conditions
shown below.
The bending occurred where the elongation of cells happens. This shows
that plants 'grow' in response to a stimulus. The cell elongation is what
pushes the stem up into the air and the root down into the soil. If the
elongation is the same on both sides of the stem then growth is straight
up. However, if elongation is uneven, bending occurs. But how do cells
elongate?
Cell elongation or enlargement happens as water is taken into the cell
and vacuoles form. The cell wall stretches and grows to accommodate
this enlargement.
This cell elongation is under the control of a growth regulator chemical
called auxin. This chemical makes the cell walls more easily stretched by
the extra water inside the cell.
Such a growth movement by a plant is called atropism. A definition of a
tropism is,
'a growth movement by a plant in response to an external stimulus where
movement is towards or away from the stimulus.'
Different parts of a plant may respond differently to the same stimulus.
What kinds of tropisms do plants show?
A tropism is a growth movement by a plant in response to an external
stimulus where movement is towards or away from the stimulus.
If growth is towards the stimulus the response ispositive and if it is away
from the stimulus it isnegative. We use the standard prefixes added to
the word tropism to indicate the nature of the stimulus that caused the
tropic response.
1. Phototropism
Seedlings and mature plants grow towards the light so
the stem is positively phototropic. The seedling on the left was placed
in even light while the seedling on the left was placed
in unidirectional light from its left.
2. Gravitropism (geotropism)
Roots are positively gravitropic (or geotropic) because they grow
towards gravity. However, if you germinate seeds on a petri dish lined
with cotton wool and rotate the dish frequently the seedlings are
'confused' about the direction of gravity. They grow horizontally instead of
down. The petri dish on the right (below) was not turned as a control.
Used with permission.
Stems are negatively gravitropic (geotropic), because they grow away
from gravity. The coleus plant on the left (above) grew normally while the
plant on the right was was left in this position growing on its side. It bent
to grow upwards, away from gravity.
Used with permission.
However, if the coleus plant was grown on its side and turned regularly it
did not bend upwards because it was 'confused' about the direction of
gravity. Notice that it grew straight ahead, or horizontally as it was
unable to sense the direction of gravity.
We can carry out such experiments without hand turning of the plant if
we have a clinostat or klinostat (shown below), which rotates very
slowly and automatically. This confuses the plant about the direction of
gravity.
The Clinostat has been used by biologists for over a hundred years to
study what effects the force of gravity has on plant and animal
development and behaviour. The clinostat is a simple device which places
a plant, a small organism, or cell growing in culture on a rotating
platform. The rotation causes the organism under test to be subjected to
the gravity vector from "all" directions.
The diagram above shows a very old kind of clinostat that was used to
show that the stem is negatively gravitropic. When constantly rotated
there was no bending.
A modern clinostat that you would find in a school laboratory is shown
above. If seeds are germinated whilst pinned to the rotating cork platform
of the clinostat the roots grow horizontally. However, if they are not
rotated they grow downwards (below).
3. Thigmotropism
Climbing plants such as sweat peas and passionfruit vine need support
from some object such as a stake,trellis or host plant. They bend around
the object that they touch, moving towards the stimulus (touch) so
the stems are positively thigmotropic. Tendrils on climbing plants are
especially sensitive to touch and serve to hold onto the supporting object
by winding around it.
Used with permission.
Roots are negatively thigmotropic because they grow away from
objects that they touch.
4. Chemotropism
Pollen grains land on the stigma during pollination and begin to grow
downwards through the style toward the egg cell. The pollen tube can
detect chemicals from the egg cell in the ovary and grow towards it.
Thepollen tube is therefore positively chemotropic.
Plant roots grow towards some chemicals and away from others.
5. Hydrotropism
It has always been thought that many roots will grow towards water,
therefore they must be positively hydrotropic. If pot plants are not
watered deeply their roots will grow to the surface where the water is but
this may be random growth. Evidence is still inconclusive for
hydrotropism.
6. Heliotropism
Some plants take the response to light a step further, by tracking the sun
as it moves through the course of the day. The benefits to the plant must
be greaterphotosynthesis and improved vigor. The mechanism is similar
to phototropism although more subtle. The sunflower shows heliotropism,
or sun-tracking.
How do plants bend?
Plants that are grown in the dark are thin and weak-stemmed with tiny
yellowed leaves. We say they are etiolated.
The radish seedlings on the left were grown in full sun while those on the
right were grown in a box in the dark. They are etiolated. Seedlings
grown in full light are shorter with much sturdier stems and their leaves
are a dark green colour.
So why are plants in the dark bigger?
We already know that light is essential forphotosynthesis and we have
shown that plants bend towards the light but why are plants grown in the
dark taller? This suggests that light somehow inhibits stem growth. How
is this possible? To answer this questions scientists used oat seedling
coleoptiles.
What are oat seedling coleoptiles?
A coleoptile is a sheath of tissue surrounding the shoot of a germinating
grass seedling, such as oat. They are convenient for study because there
are no leaves orbuds at this stage.
Many scientists have carried out a series of experiments using oat
coleoptiles. The main ideas to come from these are shown below.
The experiment above showed that light is needed for bending of the
stem. The light source was from the left. But which part of the stem
detects the light?
The experiment above shows that the tip of the shoot detects the light
which causes bending. The light source was from the left. The seedlings
were covered (in part) with aluminium foil to block light from certain parts
of the coleoptile.
If light is detected at the tip of the stem and growth (elongation of cells)
occurs further down then a chemical messenger must be involved
between the two areas. More experiments were carried out to investigate
this chemical messenger.
The experiment above showed that the chemical messenger was watersoluble as it passed through the gelatin layer and caused bending. It also
showed that the tip could be removed without harming the tropic
response. But to be sure that a chemical had to get down to the zone of
elongation the next experiment was carried out.
The mica plate did not allow any chemicals through to the zone of
elongation and no bending occurred. So this shows that a chemical must
move from the tip to the zone of elongation for a tropic response.
What is the chemical?
The chemical causing elongation of cells was isolated and named auxin,
from the Greek for 'to grow'. Auxin is a class of hormone that causes cell
elongation by allowing the cell wall to stretch more easily as the cell
expands with water. The main auxin found in plants is a chemical
called indoleacetic acid, or IAA.
Auxin is made in the dividing cells at the tip of the root and stem and
then transported to the regions of growth just behind the tip. Here, it
causes the cells to elongate.
How does light affect auxin?
If auxin causes cell elongation then light must lower the concentration of
auxin because plants do not grow as tall in the light as they do in the
dark.
There are several ways that this might happen:
Light might destroy auxin.
Less auxin might be made in light.
Auxin might move away from the light.
So an etiolated seedling grows taller because more auxin gets to the zone
of elongation, causing more cells to become elongated. The advantage of
etiolation to the plant is that it may find light for photosynthesis by
growing taller.
So much for the faster growth in the dark but what about bending? What
causes the bending in phototropism?
What causes bending in phototropism?
If light comes from one direction it will cause lessauxin to be released on
the side closest to the light. When the auxin moves down to the zone of
elongation it will cause elongation of cells. This elongation will be uneven,
cells will be smaller on the side closest to the light because they get less
auxin.
It is this uneven growth that causes the bending.
More Experiments with coleoptiles
The following experiment was carried out to show auxin moved down the
dark side of the stem.
If the movement of auxin down the dark side is blocked by mica there is
no bending. This shows that auxin must move down the dark side to
cause elongation and bending. A control for this experiment is shown
below.
If the mica block is put into the side closest to the light there is no affect
on the bending. This is because the auxin is able to move down the dark
side of thecoleoptile.
In fact bending can be made to occur without the light stimulus. If a
coleoptile tip is cut off and placed on one half of the stump it will send
auxin down that side of the coleoptile, causing bending.
Whichever side it was placed it caused bending away from that side, even
in the dark.
Is it the same for plants as it is for coleoptiles?
Coleoptiles are unusual in that they have no lateral buds or leaves and
also they have been germinated in dark conditions so they are very
sensitive to light. So does what we have said about phototropism in
coleoptiles apply to 'real' plants with green, leafy shoots?
The answer is yes, the same model applies.
How does gravity cause bending?
If a seed is placed in the soil the primary root always grows downwards,
regardless of how the seed is placed. Similarly, the stem always grows
upwards. This suggests that the stem and root can detect gravity.
How does the plant detect gravity?
The root cap is needed for the gravitropic response so it appears to be
the site of gravity detection. The cells responsible are
called statocytes and they contain large starch grains (statoliths) inside
membranes called amyloplasts. These cells are also found all the way up
the stem.
The amyloplasts roll like marbles in a bag when the root is tipped, falling
to the bottom of the cell. It is actually the membrane they fall against
that is important. Somehow, this triggers auxin release and
subsequent cell elongation.
Used with permission.
How does auxin cause the bending?
If a seedling is placed on its side as shown below the stem grows up and
the root grows down. How does auxin cause this?
When the levels of auxin are measured they are found to be higher in the
lower surface of the seedling above.
If the distribution of amyloplasts causes auxin to accumulate on the lower
surface of the stem and root how is it that one grows up and the other
down?
The answer seems to lie in the different sensitivity of the stem and root
tissues to auxin concentration. The graph below shows the difference.
Used with permission.
Higher levels of auxin stimulate stem growth as expected so the lower
side of the stem should elongate more and bend upwards. However,
those same high levels of auxin inhibit root growth. This means the upper
surface of our horizontal seedling will elongate faster causing the root to
bend downwards.
Summary of the model for gravitropism
What happens in other tropisms?
Used with permission.
Plant roots are negatively thigmotropic, making it easier for them to
work their way through the soil by avoiding objects. Climbing plants
show positive thigmotropism, especially at special structures
calledtendrils, which coil around objects for support.
The climbing bean above does not grow straight up along a vertical line.
Instead the tip follows a helical path increasing its chances of coming into
contact with a supporting object. This is an automatic process (i.e. not
driven by external stimuli) and is calledcircumnutation.
Once a tendril comes into contact with an object there is a positive
thigmotropic response. Several hormones seem to be involved
although auxin does not seem to be important. At least four genes have
been identified that produce hormones (such as jasmonates). These
chemicals move across to the outside of the tendril/stem causing the cells
there to elongate. The tendril/stem then bends.
What are nastic responses?
Plants also make movements in response to stimuli that have no relation
to the direction of the stimulus (unlike tropisms). Such a response is
called a nastic response. These responses are often slower than tropisms.
Many plants show what could be called 'sleep movements' such as closing
their flowers or droopingtheir leaves at night. This movement is
calledphotonasty because it is response to light, but the movement is not
affected by the direction that light came from.
Leaves and flowers can also respond to temperature in a response
called thermonasty. An example is the opening (above 16oC) and closing
(below 16oC) of tulip or crocus flowers.
Plants with compound leaves usually fold their leaves up at night to
prevent them being eaten, reduce water loss and perhaps to stop
moonlight inducing unwanted responses.
Such responses are due to changes in the water content of special cells in
a chamber at the base of the flower or leaf. When these cells lose water
(become flaccid) they become weak causing a movement. The diagram
below shows how this causes a dandelion flower to open and close.
Some plants show much more rapid nastic movements which are
designed to either protect the easily damaged leaves or to capture
insects.
Used with permission.
The 'sensitive plant' (Mimosa pudica), above, has compound leaves which
will close when touched. The leaves also fold up at night for protection.
This is calledthigmonasty.
Once again it is a change of water pressure inside special cells at the base
of the leaf stalk that causes the 'collapse' or folding of the leaves.
What is more amazing is that if only one part of a leaf is touched all of the
leaves will fold! An electric charge is generated when the leaflet is
touched which can travel at a rate of 3 cm per second. This signals the
other leaflets to fold.
The Venus Fly Trap is another well known plant that shows thigmonasty.
These plants grow in conditions that are short of nitrogen so they have
evolved a way of getting their nitrogen by capturing and digesting insects.
Special 'trigger hairs' sense the insect and send an electrical signal to the
base of the trap causing changes to the water pressure of cells at the
base. This closes the trap.
Nastic responses are reversible because they are the result of water
movements in and out of special cells.
What are plant hormones?
Plants produce growth regulators, or hormones, which control their
growth and development. There are several different classes of hormones
produced.
1. Auxin
This is an important group of hormones which are made in the young
dividing cells at the root and shoot tips and in expanding leaves. They are
transported back to the growing region just behind the tip where they
cause elongation of cells. They do this by promoting stretching of the cell
wall as cells fill with water and vacuoles get bigger.
If the cell elongation is asymmetrical, bending occurs as happens
in phototropism and gravitropism.
Used with permission.
The effect of auxin depends upon the concentration and the target tissue.
The graph below shows the differences.
Used with permission.
Notice that very high levels of auxin inhibits growth in plants, i.e. very
high levels of auxin are toxic to plants. This property has been used to
produce weedkillers. These weedkillers kill only the broadleaf weeds and
leave the grasses alone. The two best known examples of these
herbicides are 2,4-dichlorophenoxy acetic acid (2,4-D) and 2,4,5trichlorophenoxy acetic acid (2,4,5-T). They were mixed to produce 'agent
orange' which was sprayed on the jungle during the Vietnam War.
Used with permission.
Used with permission.
Have another look at the graph showing how auxin concentration affects
various parts of the plant. Notice that the lateral buds are inhibited by
high levels of auxin.
Used with permission.
Auxin from the apical bud (terminal bud) moves to the lateral buds which
are inhibited by it. This means the plant will continue to grow up (vertical)
rather than out (laterally). This is called apical dominance, because
the apex is shutting down lateral growth.
An experiment to demonstrate apical dominance is shown below. When
there is an active apical bud auxin is produced which shuts down lateral
growth.
When the apical bud is removed but auxin is applied in its place the
lateral buds are still inhibited.
However, when the apical bud is removed so there is no auxin being
produced at the tip then lateral buds grow.
Apical dominance explains the shapes of plants. Many trees have a classic
pyramid (or triangle) shape because the auxin concentration decreases as
it goes down a tree. This means lateral growth is stronger lower down the
tree because it is not being inhibited as much.
Auxin also stimulates the growth of roots, even on the stem or leaves. For
this reason, synthetic auxins are used commercially as 'rooting hormones'
to encourage roots to grow when cuttings are made.
Normal growing leaves produce lots of auxin but when they age or
become damaged the auxin production stops and senescence (aging)
begins. The high auxin levels somehow stops the aging process.
Used with permission.
The drop in auxin levels may lead to changes in theabscission zone at the
base of the leaf or fruit.Ethylene is released which causes enzymes to be
made that digest the cellulose and pectin of this area. Once digested this
area becomes weak and the leaf or fruit falls off the plant.
Used with permission.
Fruit growers use this by spraying auxin onto their fruit trees to delay the
fall of fruit so it can ripen fully on the tree.
Auxin also stimulates secondary growth or woody tissue in the stem. This
gives a plant more support. Auxin also causes flowers to form fruits
without fertilisation.
What are plant hormones?
2. Cytokinins
Cytokinins are hormones that are produced in dividing cells and
they promote cell division. They are most abundant in the roots, seeds
and fruit.
They also promote lateral growth as the experiment below shows. When
the apical bud is dominatinglateral growth is inhibited.
Used with permission (modified).
However, if no auxin is added the apical dominance is removed and
lateral buds grow. This is because the cytokinin has not been masked by
auxin.
Used with permission (modified).
It appears to be the relative amounts of auxin and cytokinin that
determine the shape of the plant. Cytokinin promotes lateral growth while
auxin promotes vertical or apical growth.
Cytokinins also prevent senescence.
The ability of cytokinins to promote cell division has been exploited
in tissue culture. When a small amount of mature plant tissue is placed
on agar jelly with auxin and cytokinin cell division occurs to produce a
lump of tissue called a callus. By adjusting the ratio of auxin to cytokinin
this callus may be forced to either continue dividing, grow roots or to
grow leaves.
Used with permission (modified).
A callus is shown above. This technique has become very important in the
mass production of commercial plants.
Bonsai growers prune the roots off their plants so they stop producing
cytokinins, as they would cause lateral growth. This would make the plant
too big for container growth.
Used with permission.
3. Gibberellins
Japanese rice growers observed a disease that caused their plants to
grow very tall so that they fell over and died. The fungus causing this was
found to release a chemical called gibberellic acid. The disease occurred
because gibberellic acid causes stem elongation by increasing the
internode length.
Used with permission.
Dwarf plant species (those that naturally grow smaller than normal) were
found to have no gibberellin. When gibberellin is added to dwarf species
there is a huge increase in internode length. This is shown below for
dwarf lettuce.
When this experiment is repeated with dwarf peas the internode
elongation is more measurable and the differences more obvious.
The internode lengths were measured for a treated sample and a control
sample of peas and the results are in the table below.
Internode length of GBA treated
plants (mm)
Internode length of control plants
(mm)
60
12
So gibberellin makes plants grow bigger.
Gibberellin also breaks dormancy in plants (buds) and seeds. Many plants
have a period of 'rest' usually during the winter months when it is very
cold. They may lose their leaves and form a dormant bud or they may
produce seeds that 'sit out' the conditions as a dormant seed. It is
important that the new sensitive growth does not occur until conditions
become favourable so each species has evolved a means of ensuring this.
Some need a minimum cold (or chilling) period before they will develop,
others require exposure to light.
Whatever the environmental cue is for growth it appears that gibberellin
is the link. Once the cue has been detected gibberellin is produced and it
causes food stores (starch) to be broken down so they are available for
the growing embryo or bud. The diagram below shows how it breaks the
dormancy in seeds.
Gibberellin is applied to grain seeds to get an evengermination in
commercial brewing operations. They obviously want all their seeds
sprouting at the same time when brewing beer.
Gibberellin also increases the size of some fruit, like seedless grapes. It is
sprayed on grapes for this purpose.
Gibberellin also stimulates flowering in plants that may have dormant
flower buds. It is routinely added to calla tubers to increase flowering by
100 - 400 %.
What are plant hormones?
4. Abscisic acid
Abscisic acid (ABA) is a growth inhibitor that acts against the other
growth stimulators (auxin,gibberellin and cytokinin). It is high in
concentration indormant buds and seeds but its role in dormancy is not
clear.
Used with permission.
Abscisic acid does control water loss by keeping thestomata closed.
What are plant hormones?
5. Ethylene
Ethylene is a very simple molecule. It is a gas with the structure shown
below.
It is produced by fruits as they ripen and it speeds up the ripening
process. A rotten apple will release ethylene which will cause fruit around
the apple to ripen faster.
Orchadists make use of this property. Bananas are picked before they are
ripe and shipped to theirmarkets green. This protects them from damage
during handling. They are then ripened by spraying with ethylene gas.
Similarly, apples are picked green and cool stored so they will not ripen.
They can then be sold much later when there is a shortage of treeripened apples. They are simply sprayed with ethylene to ripen them on
demand.
Ethylene also acts to inhibit growth and is implicated inthigmotropism. It
has a definite role in abscission - leaf fall. Deciduous trees lose their
leaves after environmental cues (daylength shortening or temperature
decreasing) signal that winter approaches. Leaves would lose water at a
time when it is impossible to replace as the ground becomes frozen.
One of the first signs of leaf fall is the colour changes in the leaf. The
leaves stop making chlorophyll and often make new yellow, orange or red
pigments. This produces the rich colours of the Northern Hemisphere'fall'.
Used with permission (modified).
The leaf stalk separates from the stem in a region called the abscission
layer. This area is made up of small cells with thin walls that become
weak when ethylene signals digestive enzymes to break down the walls
of these cells. The layer becomes so weak that the leaf falls under its own
weight or maybe with a little help from the wind. But why don't the leaves
fall at other times? The anwser is the hormone auxin. Auxin maintains the
leaf during the year by workingantagonistically to ethylene.
Cherry growers use this property to harvest their fruit. The cherry trees
are sprayed with a compound that releases ethylene to induce abscission
of the fruit. Then the tree is shaken by machine to make the cherries fall.
What are plant hormones?
6. Florigen
Florigen is the name given to the hormone believed tocause flowering in
plants, however, it has never been isolated. It may even be a mixture of
hormones.
If all of the leaves are removed from a plant it will not flower. If just one
leaf is left on the plant it will flower. This suggests a chemical (florigen) is
made in the leaf and moves to the flower bud causing flower
development.
A series of experiments were carried out with Cocklebur, a plant (like
many others) that requires a certain daylength before it will flower.
Used with permission (modified).
When all leaves were removed plants did not flower even in suitable
daylength conditions. But the presence of one leaf was enough to produce
flowering.
Used with permission (modified).
This showed that a hormone is produced in the leaf that causes flowering.
This hormone could be made to pass to another plant by grafting two
plants together. The experiment is shown below.
Used with permission (modified)
Summary of plant hormones.
This page gives a summary of the plant hormones by region of the plant.
Use your mouse to find out which hormones are active in a particular
region.
http://www.easyscience.co.nz/ubbiology/orientation/lesson7f.htm
Notice how many responses are the result of the interaction of more than
one hormone. It is the balance of hormones that is important.