Download PLANT HOMEOSTASIS

PLANT HOMEOSTASIS
Plant responses to environmental stimuli.
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Tropisms, nasties, taxes and more.
Auxins: plant growth hormones
A Table of plant tropisms
More plant hormones and responses
Movement of substances in & out of leaves
Gas exchange and saving water in leaves.
Guard cells
Summary of major plant adaptations
Plants may not be able to move in the way that animals do, but it is still possible for them
to respond to environmental stimuli.
Sometimes the response will involve the movement of a part of the plant.
Tropisms, nasties, taxes and other responses to the environment.
One type of movement seen in plants is called tropism (direction of stimulus determines
direction of response). If the movement is towards the stimulus, then it is said to be
positive tropism, but if the movement is away from the stimulus, then it is said to be a
negative tropism.
Another type of plant movement is called nastic movement (plural nasties) (direction of
response independent of direction of stimulus).
Small algae such as Euglena and Chlamydomonas can exhibit movements of the whole
organism and such movements are called taxes.
Plants can also respond to environmental stimuli without moving, as is the case with the
photo periodic response, flowering and vernalisation.
Auxins: an important class of plant growth hormones
Plant growth, like animal growth is controlled by hormones.
A plant hormone is a chemical which is made by one part of the plant, but then
travels to another part where it has some effect. (Is this different to the definition of an
animal hormone?)
Auxins affect plant growth. The main auxin is indole acetic acid (IAA). Auxin controls
phototropism (plant shoots bending towards light) and gravitropism - (roots growing
down into soil). The stimulus/response sequence for auxin action is as follows:
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Auxin is made by the growing tips, or meristem, of shoots and roots.
The stimulus for its production in the shoot is uneven light distribution.
In the root, slight variation in the force of gravity and also light seem to be the
stimuli.
The auxin diffuses to the area of elongation just behind the tip.
Here newly formed cells are developing their vacuoles and becoming bigger.
Auxins change the flexibility of cell walls and this allows for expansion of the
cells.
Because the distribution of the auxin is not uniform in the shoot or root, the
response is not equal on each side of the tissue.
This leads to greater expansion of cells on one side than the other. The response
depends on the amount of auxin received.
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So the side with less expanded cells is smaller, the opposite side's more
expanded cells are larger, and bending of the growing tissue results.
Biochemists have discovered some interesting relationships between auxin
concentration and tissue response in shoots and roots. There is not a simple
'more auxin: more response' relationship in either tissue, and roots appear to be
much more senstive to low levels of auxin than are shoots. ... The research
continues!
Here is a classic diagram of a shoot bending towards the
light. Note how auxin released only on the dark side of the
shoot is able to cause cell elongation on that side. It is the
uneven cell growth which causes the shoot to bend towards the light in this case to optimise the exposure to
the light for photosynthesis.
TABLE OF COMMON PLANT TROPISMS
TROPISM
STIMULUS
Positive
light
phototropism
Positive
gravitropism
gravity
BIOLOGICAL CHANGE
Cell elongation is stimulated on
the shaded side of a stem but
inhibited on the lighted side.
RESPONSE
Growing shoot bends
towards the light.
Growing root bends
Cell elongation is inhibited on
downwards towards
the lower portion of the root but
the centre of the
stimulated on the upper portion.
gravity.
touch or
Positive
thigmotropism pressure
Cell elongation is inhibited on
the stem side touching an object
but stimulated on the nontouching side
Growing shoot coils
around the object. For
example, in some
climbing vines.
touch or
Negative
thigmotropism pressure
Cell elongation is stimulated on
the stem side touching an object Shoot grows away
but inhibited on the non-touching from an obstacle.
side.
water
Positive
hydrotropism availability
Root tips closest to a source of
water and soluble minerals
simply grow faster than those
further away. (Hormones not
involved here.)
Growth of the root
towards a water
source.
OTHER PLANT HORMONES AND RESPONSES TO ENVIRONMENTAL
STIMULI
Cytokinins
Hormones which in combination with auxin stimulate cell division and influence
tissue differentiation.
Gibberellins
Hormones that control stem elongation in most plants. Many dwarf mutants of
plants will grow and develop normally if gibberellin is applied.
Ethylene
A simple hydrocarbon plant hormone. It controls abscission (=dropping) of
leaves, flowering and fruiting, and hastens fruit ripening. It can be used
commercially to cause fruit ripening or hasten fruit drop.
Abscisic acid
Abscisic acid suppresses cell growth. It also promotes leaf senescence (death)
which results in the colour changes of leaves in autumn before they are dropped
from deciduous plants. Abscissic acid also appears to be involved in stomatal
opening and closing. It may have a role in root gravitropism.
Phytochromes
These chemicals change in concentration in plants in response to changes in
length of dark (night)/light (day) periods. In turn, these changes in phytochromes
stimulate or repress the flowering of plants. There are two categories of plants
with respect to flowering. Short day plants flower in response to long nights.
Long day plants flower in response to short nights. (The names now seem
silly, as it is the night length rather than day length which is the stimulus for the
response. But this is an example of many scientific discoveries; the response
was observed, and named, long before the mechanism of its action was worked
out.)
Vernalization
This is the observation that some seeds and bulbs will not germinate or flower
unless they have had a suitable period of time at cold temperatures (usually -2oC
to +10oC). (This discovery has implications in areas where there are mild winters;
if a gardner in Melbourne wants to grow good tulips, she should put the bulbs in
the refrigerator for several months before planting them in autumn, in spring
beautiful tulips should flower!)
MOVEMENT OF SUBSTANCES IN/OUT OF LEAVES
During daylight, plants are performing photosynthesis at a greater rate than cellular
respiration if their stomata are open.
6CO2 + 6 H2O ----> C6H12O6 + 6O2
For this to occur, there must be a net input of carbon dioxide and water in the leaves.
Water enters the plant via osmosis in the roots. It travels up the xylem to the leaves. In
the leaf, water enters the air spaces as water vapour. As photosynthetic cells use water
more is drawn into them, again the process is osmosis.
Water can also leave the air spaces, where the atmosphere is at close to 100% humidity,
to the outside of the leaf, where the atmosphere may be well below 100% humidity. The
water passes out through the stomatal pores in this situation. In this case the process is
diffusion, as water is passing down its concentration gradient.
Diffusion is also the process by which gases move in and out of the leaf. In the case of
photosynthesis dominating, CO2 is used by the cells. The concentration gradient of CO2
is therefore high in the air spaces, lower in the cell, so the CO2 diffuses into the cell. This
means that the leaf air spaces become deficient in CO2 when compared with the external
environment. Thus CO2 diffuses into the leaf via the stomatal pores.
Using similar reasoning, it can be expalined why O2 diffuses out of the leaves of
photosynthesising plants.
Now write the equation for cellular respiration and think through the inputs and
outputs when that is the dominant process (ie at night)!
Remember that even when photosynthesis is occurring, so too is cellular respiration, so in
the day, some of the O2 produced in photosynthesis is immediately used for respiration.
Similarly, some of the CO2 produced in respiration is immediately used in
photosynthesis. There is, of course, a point where the oxygen output is just balanced by
its rate of use, and the same is true for carbon dioxide. This is the compensation point.
This is often illustrated graphically:
Graph of rates of cellular respiration and
photosynthesis at varying light intensities.
Note the following:
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At A. Cellular respiration continues at a
constant rate regardless of light intensity.
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At B. Photosynthesis is either unable to occur, or occurs at a rate limited by the
amount of light. This is true at low/moderate light intensities.
At C. The Compenstaion point. Here the rates of photosynthesis and respiration
are equal.
At D. Here the light intensity is high. The enzymes of photosynthesis are working
at their maximum level, so the rate of photosynthesis is maximum. The enzymes
(or perhaps the rate of supply of substrate) are controlling the rate of
photosynthesis.
GAS EXCHANGE OR WATER CONSERVATION? A delicate balance
during hot, dry Australian summers.
If stomata were constantly open, gasses could diffuse in/out as concentration gradients
dictated. Respiration could go on at a maximum rate constantly, and photosynthesis
would only be regulated by light intensity. But what about the water?
In a well watered habitat this would not be a problem for the plant. Soil water would be
freely available and the atmosphere would be relatively humid from evaporation of soil
water. The need to conserve water would not be great in these conditions. In fact, plants
adapted for life in or near permanent water have either unusual positioning of stomata, or
very little change in stomatal aperture each day.
However, in a hot, dry habitat, water constantly evaporates from the leaves (good for
cooling) and a plant is in danger of dehydration if this water is not replaced sufficiently
quickly. If the soil around the roots is dry, failure to replace water lost is inevitable.
Water diffuses out of the plant cell vacuoles, the cell become flaccid, and the plant wilts.
(Remember the plants in your garden during the long hot summer in January and
February? Did you take pity on them and provide their roots with some water? Why was
it the non-Australian plants which wilted first?)
Many Australian plants have adaptations for hot dry summers, such as that of 1996-97.
These plants are able to survive because of the compromise their guard cells achieve.
Frequently other adaptations such as thick waxy cuticles and leaves which hang down, or
are narrow and short, and have reduced exposure to the direst sun are also seen in
Australian plants from arid regions.
Guard cells
Guard cells are specialised cells in the leaf epidermis (usually mostly on the lower leaf
surface). There is a space of variable size between pairs of guard cells, the stomatal pore
or stoma (pl. stomata). Guard cells, unlike other epidermal cells, possess chloroplasts
and are therefore able to perform photosynthesis. (This is the 'secret weapon' against
dehydration!) Here's how one of nature's cleverest survival tricks works ...
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In the presence of light, photosynthesis begins in the guard cells and the other
photosynthetic cells of the leaf.
Glucose accumulates.
Water is drawn into the guard cells by osmosis because of the osmotic gradient
which develops as the glucose increases.
Guard cells become fully turgid, and because of their specialised structure, they
'bend' apart.
The stomata are open - gas diffusion is possible.
That's all very well, if it's early morning and the air is still relatively humid. But
what about later in the day, when the temperature rises, humidity falls, and
water vapour would be pouring out of the plant? Here's the clever bit ...
Water diffuses out of the guard cells in response to the decreasing water content
of the air adjacent to them.
Guard cells become flaccid.
Stomatal pores close.
Gas exchange, including water loss, through stomata is shut down.
Photosynthesis can still provide the inputs for cellular respiration to continue
in the cells inside the sealed leaf whilst the stomata are closed.
At dusk and again in the early morning, when temperatures are cooler, humidity
higher and stomata are open again photosynthesis can again accumulate stores of
glucose for the heat of the day and at night.
Now isn't that clever? It's actually a bit more complicated than this according to the
plant biochemists There are also some plant hormones and movement of K+ ions
involved, and of course wind makes the water loss even worse, but the basics here are
correct.
From this it can be seen that plants have specialised adaptations, either structural
(such as cacti' or ferns' specialised leaves), or functional (hormones or stomatal
rhythms) which enable them to survive in their own particular environments.
A SUMMARY OF COMMON ADAPTATIONS OF PLANTS IN PARTICULAR
ENVIRONMENTS
ENVIRONMENT (and
plant type)
HOT AND DRY
(xerophytes)
HOT AND HUMID
(tropical)
ADAPTATIONS
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Guttation (drops of water released from leaf surface) removes
excess water which enters plant due to root pressure.
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VERY COLD WINTERS temps below 0oC
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FRESH WATER (aquatic
plants = hydrophytes)
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MARINE (algae) seaweed
HIGH SOIL [SALT]
(halophytes) eg mangroves
LOW SOIL [MINERAL]
(carnivorous plants)
Stomata closed during heat of day
Reduced or absent leaves (cacti)
Leaves aligned away from direct sunlight
Stoma almost all on lower leaf surface
Stoma protected in pits or surrounded by hairs
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Deciduous - plants lose leaves and reduce metabolism to
ensure survival.
Leaf oils - act as 'anti freeze' to protect, as in conifers.
Vernalization ensures that reproduction occurs during
warm weather.
Large vacuoles collect and expel excess water.
Large air spaces in leaves allow leaves to float on or near
surface and obtain light.
Reduced stomata - gas exchange is by diffusion in/out of
water.
Non chlorophyll pigments (so they are not always green)
since penetration of light is different under water.
Flotation bladders, full of air, hold fronds near surface
where light and gas levels are highest.
Holdfasts anchor plants in presence of strong tides and
currents (not true roots, since water is plentiful and
minerals diffuse from environment).
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Salt excreted through leaves
Very thick leaf epidermis protects from salt.
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Special structures attract and trap insects.
Enzymes digest the insects, releasing minerals to diffuse
into plant.