Plant responses Flashcards
Why do plants need to respond to the external environment ?
helps plant survive long enough to reproduce
What chemical defences are used in response to threat of herbivores?
- tannins
- alkaloids
- pheromones
Function of tannins
- toxic
- make leaf taste bitter
- prevent infiltration by pathogenic microorganisms
Function of alkaloids
feeding deterrent as they’re bitter
Function of pheromones
affect behaviour or physiology of other plants.
What are tropisms?
directional responses of plants
physical plant defenses
- folding of leaves in response to touch
- thorns
- spines
What are the types of tropisms?
- phototropism
- geotropism
- chemotropism
- thigmotropism
- hydrotropism
What’s phototropism?
directional growth response to light. shoots growing towards light (+vely phototropic), enables photosynthesis.
what’s geotropism?
directional growth response to gravity. Roots grow towards the pull of gravity, helps anchor them in soil.
What is chemotropism?
directional growth response to chemicals. On flowers, pollen tubes grow down the style attracted by chemicals, towards the ovary where fertilisation occurs.
What’s thigmotropism?
directional growth response to mechanical stimulation. shoots of climbing plants (ivy), wind around other plants or solid structures for support.
What are positive tropic responses?
plant responds towards a stimulus
What are negative tropic responses?
plant responds away from a stimulus
What are nastic responses?
non-directional responses to external stimuli - sensitive plants folding leaves when touched (thigmonasty) in Mimosa Pudica
What’s the role of plant hormones?
coordinate plant responses to environmental stimuli, transported from site of manufacture to target cells/tissues.
How are plant hormones different from animal ones?
not produced in endocrine glands, produced in a variety of tissues in plants.
What processes can plant hormones influence?
- cell division, elongation, differentiation.
Examples of plant hormones
- cytokinins
- abscisic acid
- auxins
- gibberellins
- ethene
effect of cytokinins?
- promote cell division and expansion
- delay leaf senescence
- OVERCOME APICAL DOMINANCE
effect of abscisic acid
- inhibits seed germination and growth
- stomatal closure when the plant is stressed by low water availability
effect of gibberellins
promote seed germination and elongation of stems by stimulating cell division + elongation
effect of ethene
- promotes fruit ripening
- stimulates the breakdown of cell walls in this abscission layer, causing the leaf to drop off
effect of auxin
- regulates plant growth
- promotes cell elongation and division
- inhibits growth of side shoots (causes apical dominance)
- inhibits leaf fall as it prevents production of ethene
- inhibits root growth at high conc
how do hormones travel around plant?
- diffusion
- active transport
- mass flow by diffusion in the phloem sap or in xylem vessels
what’s apical dominance
inhibition of the lateral bud growth further down the shoot by chemicals produced by the apical bud at the tip of a plant shoot.
Experimental evidence for the role of auxins in the control of apical dominance
- when shoot tip (apex) is removed, auxin levels in shoot drop and lateral buds grow.
- when apex removed and paste containing auxin applied to the cut end, buds didn’t grow
- upon cutting, a hormone could have been produced at cut end due to exposure to oxygen that promotes growth.
- Due to this, auxin transport inhibitors applied below apex and buds still grew.
- Based on results - high auxin levels inhibit lateral bud growth whereas low levels promote it.
what 2 hormones are now thought to be involved in apical dominance?
- abscisic acid (high levels of auxin maintain its levels high)
- cytokinins (high auxin levels make shoot apex a sink for cytokinins, when apex is cut, it spreads evenly around plant)
experimental evidence for the role of gibberellin in the control of stem elongation
1) If you graft a plant with a dominant Le gene onto the homozygous le plant, the plant grows tall.
2) tall pea plants had higher conc of gibberellin
3) adding gibberellins to short plants made them grow tall
4) Le codes for an enzyme involved in synthesising gibberellins (converts GA20 to GA1)
5) Scientists found that short plants were homozygous for recessive allele and big plants were homozygous for a dominant allele (Le).
6) Scientists compared gibberellin conc in dwarf pea plants and tall pea plants.
Explain how gibberellins promote germination
enables production of amylase by activating genes that code for it, amylase can break down starch to glucose which provides a substrate for RESPIRATION, causing growth.
What’s geotropism?
directional growth response to gravity
What’s phototropism?
directional growth in response to light
How is auxin distributed?
- If a plant is laid on its side, auxin gathers in the lower half of the stem and root.
- Auxin slows growth in the root, so the root curves downwards.
- roots are negatively phototrophic.
- Auxin stimulates growth (cell elongation) in the shaded side of shoot, so the stem curves upwards
- shoots are positively phototropic
What’s hydrotropism
- direstional growth in response to water.
- roots tips growing towards damper areas, increasing their access to water.
what stimulates release of gibberellins (GA)?
when seed absorbs water, embryo releases gibberellins into the aleurone layer in the endosperm region of the seed
where does growth occur in a plant?
meristem
How can geotropic responses be investigated?
- control plant is spun very slowly by klinostat so gravity is applied equally to all sides of the plant. Roots and shoots grow horizontally.
- other plants are on klinostat (switched off) so shoots grow upwards and roots grow downwards.
mechanism of auxins effect
- promotes active transport of H+ into cell wall.
- low pH provides optimum conditions for expansins to work which break bonds within cellulose (H+ also disrupts these bonds).
- cell wall becomes less rigid and can expand as cell takes in water
what’s the commercial use of ethene?
- stimulates enzymes that break down cell walls making fruit soft and ripe.
what’s the commercial use of auxin?
- used as herbicides, makes plants grow too fast so they can’t get enough water or nutrients so they die
- used a rooting hormone, lots of the same plant can be grown quickly and cheaply from one plant via cuttings.
what did Boysen-Jensen’s work confirm and what were his tests
- that water and/or solutes are need to be able to move backwards from the shoot tip for phototropism to happen
- when a permeable gelatin block inserted behind shoot tip, the shoot still showed +ve phototropism
- when an impermeable mica block was inserted, there was no phototropic response.
- tests:
- tip separated by gelatin block
- tip separated by mica (impermeable)
what did Darwin’s work confirm and what were his tests
- confirmed that the shoot tip was responsible for phototropic responses
- tests:
control
tip removed
tip covered by opaque cap
tip covered by transparent cap
base covered by opaque shield
