Plant responses Flashcards
What are Auxins (IAAs)?
Growth stimulants produced in plants.
Made in tip of the roots + shoots
move down stem + up root both in the transport tissue + cell to cell.
Effect of auxin due to concentration.
What do auxins promote?
Control cell elongation
Maintain apical dominance
Involved in tropisms
What do auxins inhibit?
Fruit ripening
Abscission (leaves fall off)
Evidence for auxins?
Cell elongation
Affect the plasticity of the cell wall = stretches more easily
Auxin from meristem diffuse to zone of elongation down conc gradient.
Bind to specific receptor sites in cell membrane –>
open protein channel
H+ enters
= decrease pH (5 = optimum pH for enzyme to keep wall flexible + plastic)
How does cell wall become less rigid?
Low pH = cellulose cell wall more flexible
H2O moving into cell would expand cell wall w/o bursting it
~~>elongates cell
Smaller vacuole to stores water
What happens when cell matures?
Cells mature = auxin destroyed.
Auxin leaves receptor + broken down
protein channel closes —> prevent further entry of hydrogen ions
Hormone levels fall = pH rises so the enzymes maintaining plasticity become inactive.
Wall becomes rigid + no longer expand/grow.
What is apical dominance?
Apical shoot = main vertical stem
Lateral shoot = side branches
Auxins favour growth of apical shoot but inhibit growth of lateral shoot.
.:Lateral shoots near top where auxin produced = shorter
Further down with less auxin = longer
Role of ethene:
Promotes abscission
Promote fruit ripening
What does the term ‘
antagonistic’ mean?
Two hormones which oppose each other.
Auxin + Ethene
ABA + Gibberellin
What does the term ‘synergism’ mean?
Two hormones working together
Role of gibberellins:
Stem elongation
Increase length of internodes
(distance between lateral shoots)
Short stem = little gibberellin
Promote germination
Stimulate pollen tube growth in fertilisation.
Role of gibberellin in growth + development?
Stimulate production of digestive enzymes —> release glucose from food stores.
Switch on genes which code for amylases and proteases - the digestive enzymes required for germination.
Energy released= seedlings grow
Role of ABA:
(abscisic acid)
Maintains dormancy of seeds and buds.
Stimulates protective measures:
~ antifreeze chemicals
~stomatal closure
Evidence for plant hormone function?
Mutate genes that make hormone
Disrupt production pathway
Cut shoot tip (remove auxin)
~see changes
~apply hormones externally
~any further changes?
How does apical dominance and stem elongation maximise photosynthesis?
+ apical shoots
- lateral shoot
Allows all shoots to better compete for light
Plant hormone balance
Taller plants due to longer internodes = better compete for light
