5.1.5 Plant Responses COMPLETE Flashcards
Directional plant growth
Known as tropisms and are controlled by plant hormones
Non Directional plant growth
Known as Nastic Responses and are not controlled by hormones. They are fast and controlled by temporary changes in the cells such as turgidity.
Examples: Venus Fly Trap
Responses to Herbivory
- Physical Defences
- Chemical Defences
- Pheromones
- Thigmonasty
Chemical Defences
ALKALOIDS- Bitter toxic chemicals
TANNINS- Binds to digestive enzymes
TERPENOIDS- Smell may deter insects
Pheromones
Chemicals produced by an individual to affect the behaviour of another, damaged plants create them so that other plants build their defences.
Thigmonasty
Move quickly in response to touch, sudden folding of leaves may dislodge small insects and scare others.
Responses to Abiotic Stress
- Leaves fall in autumn
- Antifreeze production, genes are switched on to produce antifreeze proteins which stop crystallisation
- Stomatal Closure
Tropisms
Geotropism
Phototropism
Chemotropism
Thigmotropism
Geotropism
Roots show positive and shoots grow away from gravity so is negative
Phototropism
Shoots grow towards the light, positive
Chemotropism
Pollen tubes grow towards chemicals directing them to the ovary.
Thigmotropism
Winding around a structure as seen in climbing plants.
Features of plant hormones
- Produced in a variety of tissues
- Move by active transport, diffusion, mass flow
- Bind to complimentary receptors
- Slow
- Permanent changes
- Present in small quantities
- Work antagonistically and synergistically
Auxins
Promote cell elongation
Inhibits the growth of side shoots
Inhibits leaf abscission
Cytokinins
Promote cell division
Gibberellins
Causes stem elongation
Promotes seed germination
Abscisic Acid
Maintains seed and bud dormancy
Stimulates stomatal closure
Stimulates antifreeze
Ethene
Promotes fruit ripening
Promotes leaf abscission
Role of the plant tip
Required for growth
Where light is sensed for phototropism
When tip is removed or shaded it doesn’t grow towards light
Effect of the addition of a mica sheet
Found that auxin does down shaded side of the plant
When the sheet blocks the shaded side the curvature stops but continues if the illuminated side is blocked.
Effect of gelatine when inserted in a cut tip
Jelly allows normal growth and a repose to light as the auxin can diffuse through it still.
Effect of soaking agar in auxin
The coleoptile bends even in the dark
The angle increases alongside the concentration of auxin however then begins to decrease again
Auxins
Made in the meristem near tips
Causes plant elongation
How auxins work
Bind to receptors on the cell surface membrane and promotes the active transport of H+ ions into the cell walls
Causes a drop in pH making it more acidic
This weakens cellulose bonds and activates enzymes that breaks them down more
As water is taken in by osmosis the cell permanently elongates as cell wall is weakened
As cell matures auxins are destroyed and cell wall becomes rigid again.
Auxins and Phototropism
Cells in the shoot and tip contain PHOTOTROPINS
When hit by blue light they become phosphorylated causing auxin produced in the shoot to be moved into the shade
So the cell therefore bends/ elongates
Different effects of auxins
In the stem auxins gather at the bottom and stimulates growth so it bends up
In the root auxin gathers at the bottom and inhibits growth so it bends down
Apical Dominance
A region near the top of the terminal bud contains the apical meristem
This shows dominance over the lateral buds
The plant therefore grows straight up and can compete for light
If the tip is cut off then the lateral buds start growing and it bushes out
Control of apical dominance
Auxin is made in the apical bud where it causes cell elongation
It diffuses down to the lateral buds where it inhibits them
Its thought the auxins keeps levels of abscisic acid high in the lateral buds which inhibits growth, when cytokinins applied it’ll override the inhibition
Gibberellins Details
Produced in young leaves and stems and cause growth of the stem in the internodes
Works by affecting gene expression
Causes cell division and cell elongation
SYNERGISTIC- Plant grows tall
ANTAGONISTIC- Auxin inhibits side shoots
Gibberellins and Germination
When water is absorbed the embryo releases gibberellin
Travels to the aleurone layer in the endosperm of the seed
This switches on genes which codes for amylases and proteases
Stored starch is hydrolysed into glucose providing a substrate for respiration so the embryo can grow
Leaf abscission in Deciduous Plants
In winter theres less light and lower temp so less photosynthesis
Water is lost through leaves, exposure to fungal infection and frost damage means its best to lose leaves
Abscission explained
Reduced light means less auxin and balance shifts to there being more ethene which causes ageing.
This stimulates enzymes to weaken thin walls and the cellulose bonds are broken
A layer of Suberin develops underneath for protection
Vascular bundles are sealed off and the leaf is blown away by the wind
Role of Abscisic acid
-Plants produce ABA under abiotic stress to close the stomata which helps reduce transpiration
Effect of Abscisic acid
ABA binds to receptors on the cell surface membrane on the guard cells
Ca2+ enters the cell causing K+ to leave
Water potential in the cell increases
Water leaves by osmosis
Guard cells become flaccid and close the stomata
Commercial use of Auxin
- Selective weed killer
- Used in cuttings to grow roots
- Seedless fruits
Commercial use of Gibberellins
- Improve plant shape
- Delay fruit ripening
- Larger grapes
- Longer sugar canes
Commercial use of Cytokinins
- Stops lettuces going yellow once packaged
- Causes shoot growth in tissue culture
Commercial use of Ethene
- Causes fruit to drop at a certain time
- Speeds up the ripening process
- Promotes female flowers
