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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Needs to know
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.
Chemical change based on light vs dark periods – accumulation of something to a certain level stimulates plant
Also, position of sunrise varies, north to south.
Phytochromes (bluish pigment – absorbs red – converted by light wavelength
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
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.
- 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.
response to gravity, sunlight.
do you need to be?
May need transport system to cross membrane.
Darwin and Darwin 1880: Classic experiment to discover auxin; comes from tip of plant
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
NOTE: Pleiotropic effects = same hormone can affect different parts of the plant in different ways.
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.
a. be toxic: wasteful if no predator
b. secrete toxin in response to injury
which is best??
eat only young leaves
eat very few leaves and then move on
be able to detoxify.
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)
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
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
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.
How to be perennial without the issue of leaves.
to avoid dessication, do away with leaves – photosynthesis in bark.
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.
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.