C16 - Plant Responses Flashcards
Define Tropism
A growth response by a plant in response to a unidirectional stimulus
Define phototropism
A growth response by a plant in response to a unidirectional light
Define geotropism
A growth response by a plant in response to gravity
Define hydrotropism
A growth response by a plant in response to water
Define node
Point at which a petiole/leaf attaches to stem
Define internodal length
Length between two adjacent nodes
Define lateral shoots
Side shoots, those below the apical shoot/bud
Define apical shoot
The topmost shoot
Define apical dominance
When the main central stem grows more strongly than the side stems
Define abscission
Leaf drop
Define synergism
When several hormones work together to give a greater response
Define antagonism
When hormones have opposite effects (one promotes, one inhibits)
Define meristem
tissue of undifferentiated stem cells
Define deciduous
A plant that looses its leaves in autumn/winter
Why do plants need hormones
Movement (tropism) Seed germination Fruit ripening Abscission Growth Protection (herbivores and abiotic stress)
What makes a tropism positive or negative
Positive - towards stimulus
Negative - away from stimulus
Name the 4 plant growth hormones we study
Gibberellins
Ethene
Auxins
ABA (abscisic acid)
What are the roles of gibberellins?
- cause stem elongation
- trigger metabolism of food stores at germination
- stimulate pollen tube growth in fertilisation
Describe how gibberellins are used in seed germination
Seed absorbs water, embryo activated and gibberellins produced
Gibberellins switch on genes that code for amylases and proteases
Stimulates enzymes to break down food stores in seed
ATP produced, grows and breaks out of the seed coat
What is the experimental evidence for the role of gibberellins
- seeds bred to not produce gibberellins do not germinate
(if applied to seeds externally they will then germinate)
-inhibitors of gibberellins applied they do not germinate
Name an application of gibberellins in industry
Affect length of internode (few gibberellins = short stems)
Dwarf plants made
-reduces waste
- reduced vulnerability (eg. wind)
What is the role of auxins?
- responsible for geotropism and phototropism
- Stimulates elongation in shoots and inhibits elongation in roots
Describe how auxins cause cell elongation
1) auxins bind to receptors on cell surface membrane
2) Stimulates H+ ions to be actively pumped across cell membrane into cell wall
3) H+ ions lower pH to 5
4) Expansins activated
5) Loosen cellulose cell wall - more plastic
6) water diffuses into vaculole, causing it to expand, along with the rest of the cell
Describe how the concentrations of auxins change ina shoot in response to unilateral light
Auxin produced in the tip
Diffuses down stem
Diffuses to shady side
Causes shaded side cells to elongate thus bending it towards the light
Evidence for phototropism:
How would the growth a plant shoot change if…
exposed to unilateral light
Grow towards the light
Positive phototropism
Evidence for phototropism:
How would the growth a plant shoot change if…
Shoot tip removed and exposed to unilateral light
no response as auxins produced in the tip
Evidence for phototropism:
How would the growth a plant shoot change if…
Lightproof cover placed over shoot tip and exposed to unilateral light
No response
Ausins act on tip in response to light
Evidence for phototropism:
How would the growth a plant shoot change if…
Thin layer of mica placed on sunny side and exposed to unilateral light
Bends towards light
Hormone still able to diffuse along shaded side and act on those cells causing elongation
Evidence for phototropism:
How would the growth a plant shoot change if…
Thin later of mica place on shaded side and exposed to unilateral light
No response
Hormone not able to diffuse down shadede side and have its effect
Evidence for phototropism:
How would the growth a plant shoot change if…
tip removed, gelatin inserted and tip replaced and exposed to unilateral light
Movement of the chemical still possible, gelatin would stop electrical messages thus it must be a hormone
Evidence for geotropism
Plant placed on a clinostat that rotates slowly
Evens out gravitational force and grows straight out
Plant replaced sideways not on clinostat - Shoot grows up and roots grown down
Descibe the process of geotropism
Starch statoliths in root tip
Sink to the bottom of the cells in the roots in response to gravity
Causes downwards growth
Describe what a plant with apical dominance will look like as opposed to a plant without apical dominance
Apical domincace - tall, thin, apical shoot leads the way
Without - bushy, fat, lateral shoots grow strongly
What are the 3 wasy plants can respond to abiotic stress
Leaf loss
Preventing freezing
Stomatal control
Why is leaf loss bebficial to a plant
In winter, less daylight = less photosynthesis
- glucose required to prevent leaves from freezing becomes greater than the glucose able to be produced
- greater surface area, more likely to be affected by high winds etc.
What is photoperiodism?
Reponse to lack fo light caused by phytochrome - light sensitive pigment
Descibe the process of abscission
- falling auxin levels
- ethene produced
- triggers gene activation (enzymes produced)
- enzymes digest cell wall in separation zone
- vascular bundles sealed off
- Cells below swell and push leaf off
Fat deposited in cells acting as a scar
Why is freezing dangerous for plant cells?
Water expand
Destroy membrane
Die
How do plants respond to the risk of freezing?
produce solutes that dissolve in the vacuole, this lowers the freezing point
How does stomatal control help a plant respond to it environment
Closed stomata - conserve water
Open stomata - allows CO2 in an helps coll the plant by evaporation of the water
What hormone controls stomatal opening and closing
ABA (abscisic acid)
How does ABA control the stomata
Increased ABA dereases the water potential of the stomata
Water moves out of the cells
Cells become flaccid and close (turgid means they open)
What are the 3 types of plant responses
Chemical
Physics
Mechanical
Give an example of a mechanical repsonse in plants
Thorns and hairs to deter herbinvores
Leaves that drop when touched
Mimicry
Give a physical response in plants to stress
Cellulose cell walls
Tough waxy cuticle
Layers of dead cells around the stem
What are tannins
Chemical response in plants
toxic to insects
What are alkaloids?
Chemical repsonse in plants
nitrogenous
act as drugs
What are volatile organic compounds (VOCs)
Chemical reposnse in plants
smelly repellents
Names 4 types of chemical response in plants
pheromones
tannins
alkaloids
VOCs
Name 3 commercial uses of plant hormones
ripening
hormone rooting powder
weed killers
Describe how plant hormones can be used in artificial ripening
Ethene can be sprayed on to unripe plants to ripen them
State an advantage of artificial ripening
Unripe plants can be transported more easily and produced less waste
Outline the reaction that causes ripening fruits to become sweeter
Starch –> maltose (hydrolysed by amylase)
maltose –> glucose
As ethene concentration increases what happens to CO2 prodcution
Increases as respiration increases, then decreases as starch hydrolyses
What does hormone rooting powder do
Dipping cuttings into auxins
stimulate root growth from cut stem
How do weed killers use plant hormones
Dicot auxins applied to area
Absorbed by broad leafed dicot weeds but not moncot crops
causes rapid and unsustainable growth —> death
