Transcript Plant interaction with environment

Plant interaction with
environment
How does a plant find out about
its surroundings and react to
them?
Needs to know
• When is water present
• When is sun (good wavelength)present
• Season of the year.
Mechanisms
• Environmental sensors – without a
nervous system = chemical reactions
sensitive to environment
• Hormones – to regulate growth.
How does a plant (organism) tell time?, like when it is Spring?
Animal
pattern of
activity.
Why is
inaccurate?
Circadian
clock = 24hr
cycle.
Entrainment
= resetting.
Plants; length of day vs night to tell seasons (temperate climate)
Plant measures length of night – animals do also – so can change
flowering time with a flash of light (change night length) cat
breeding.
Basis of system:
Chemical change based on light vs dark periods – accumulation of
something to a certain level stimulates plant
How does a tropical plant tell time?? If day length always the same?
In tropics, time of sunrise varies, even though day length constant
Also, position of sunrise varies, north to south.
How does a plant know the ‘right
kind” of light is present
• Right kind = proper wavelength for
photosysthesis
• If in shade of a tree, this light is absorbed
by the tree – does not reach the plant
below.
Phytochromes (bluish pigment – absorbs red – converted by light wavelength
Different wavelengths of light affect growth.
Far red light stops growth - far red light absorbed by green leaves
So no growth if in shade
Far red light does not penetrate ground very far, so no growth if buried deep
So what does a seed to if it senses proper wavelength not present?
• wait; many seeds can wait a long time
• grow but put all energy into elongation, not leaves – try to reach the sun.
Plant growth
regulated by
hormones.
Gibberellin favors
stem growth vs
leaf growth = all
effort put into
getting tall, vs
building
photosynthetic
surface area –
good if you are
under someone –
need to reach sun
– but how do you
know? = light
quality.
How does a plant sense water?? How does it tell if there is enough water?
1. Water can be absorbed by seeds and plants
2. How to tell amount?
Two possibilities?
- if seed coat scratched (washed down a stream
bed- there is lots of water
- Only germinate 10% of seeds in response to
water. If not enough, other seeds wait till next
time. A major feature of agriculture; selection of
synchronous germination.
Other Issues
• How to tell which way is up, down?
response to gravity, sunlight.
• Can you be sensitive to temperature?
do you need to be?
• Can you respond to being eaten?
•
• Mechanism: nearly all plant growth mediated by
hormones.
Mechanism of hormone action.
0. Small molecules, soluble in water, move through vascular system and
between cells.
1. Secreted in some area – caused by environmental or internal
stimulation
2. Diffuses or transported to target cells
3. Either enters these cells or interacts with cell membranes to cause a
response.
How to study hormones
1. remove hypothesized secretory area – see if you can eliminate effect
2. Capture the substance secreted
3. Replacement – can you get the effect back with addition of substance.
4. Chemical analysis
Typical plant hormones, - water soluble, fairly small
May need transport system to cross membrane.
Darwin and Darwin 1880: Classic experiment to discover auxin;
comes from tip of plant
Fritz Went: substance isolated; replacement therapy
restores action.
How does stem
know to go up,
root to go down??
Low level of auxin
– root grows down
High level of auxin
Step grows up.
Auxin moves in response to light (negative – away from)
And in response to gravity (positive – towards)
Note: pleiotropy – one hormone can do many different
things in different areas of a plant.
Mechanisms = complicated biochemical
Auxin is actively transported cell to cell
Auxin stimulates cell lengthening = elongation of stem.
NOTE: Pleiotropic effects = same hormone can affect
different parts of the plant in different ways.
Sensitivity to water-
When should a seed germinate??
•In presence of water
in deserts – how do you know there is enough??
In presence of light? – how do you sense it??
When there is no one above you – shade
After winter – in Spring – how do you know it is spring??
Plus – how does a seedling know which way is up?
o
t
Other
hormones in plants:
h
e
r
defense – secreted in response to injury – stimulate toxin secretion
or wall off invader = plant galls,
leaf drop – deciduous trees – stimulated by cold, lack of water
ripening of fruit – ethylene – artificially stimulated by farmers
Most systems have an opposing effect – can stimulate or inhibit fruiting, leaf
growth, stem elongation, leaf abcission
Either absence of the positive effect hormone or presence of an opposing
hormone.
Plant options:
a. be toxic: wasteful if no predator
b. secrete toxin in response to injury
which is best??
predator strategies:
eat only young leaves
eat very few leaves and then move
on
be able to detoxify.
When to germinate or flower?
New England = cold winter, plenty of water; based on cold shock to avoid fall
development
Claremont = not very cold winter, very seasonal water; based on water presence
We have some trees, ornamentals from the east that get ‘fooled’ every year as
they flower in the winter after a cold shock.
What should a desert plant do? – very rare water.
need abrasion – flood water = lots of water
or only 10% of seeds germinate – avoid not having enough water.
(farmers hate this – select for 100% germination = early
domestication of plants = selecting for synchronous germination)
Major issue for plants: be an annual or be a perennial
Annual: advantages: don’t have to worry about bad conditions, live only
when things are good. Need to sense this.
tend to behave like r species; short life, small size, lots of offspring.
disadvantages: need to grow from seed = takes time. Must
reproduce very quickly, when there is a lot of competition.
Perennial: advantages; already big when conditions become good – head
start at competition for light, resources
disadvantages: must survive harsh conditions; cold, dry, etc.
Like K species ; large, long lives, fewer offspring
So: question.
Where would you expect to the flora with a lot of annuals?
Where with very few?
Annuals are small – can’t grow well where shade.
So: deserts, after fire =good place for annuals.
tropics – lots of big trees, bad place, except on flooded river banks.
What does annual need to sense??
Presence of sunlight, also need to survive as a
seed a long time.
Another major choice for perennial plants; keep leaves year round or be annually
deciduous
• Year round – leaves present when conditions
are good, but thin leaves cannot survive frost
and loose too much moisture in dry conditions.
• Deciduous – maximum surface area for
photosynthesis if conditions are good,
•
but takes time to grow leaves after
dry or cold period.
• Note: all trees are deciduous, even pines.
Leaves wear out (get eaten, etc) and need to be
replaced. However, seasonal deciduous = drop
all leaves at once, in response to dryness or
cold.
Palo Verde;
How to be perennial without the
issue of leaves.
to avoid dessication, do away
with leaves – photosynthesis in
bark.
So: in terms of deciduous or evergreen
What would you expect to find in tropics?? (always moist and warm)
What would you expect to find in moderate conditions (short winter)
What would you expect to find in severe conditions? (longer winter)
Based on graph of competition in terms of amount of photosynthesis possible
Why are there tropical pine trees?? – where are there tropical pine trees.
High altitude, bristlecone pine
Coast redwoods
How come, in a wet, not too
cold climate, an evergreen
prevails
If you can outgrow your
opponent, even if slow growing
you can win.