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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.

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how does a plant find out about its surroundings and react to them
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.
  • 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.


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


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
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.

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?

  • Water can be absorbed by seeds and plants
  • 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
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.
  • Secreted in some area – caused by environmental or internal stimulation
  • Diffuses or transported to target cells
  • 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
  • Capture the substance secreted
  • Replacement – can you get the effect back with addition of substance.
  • Chemical analysis

Typical plant hormones, - water soluble, fairly small

May need transport system to cross membrane.


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 stimulates cell lengthening = elongation of stem.

NOTE: Pleiotropic effects = same hormone can affect different parts of the plant in different ways.


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?


Other hormones in plants:

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


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.

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.


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.