Plant Hormones Flashcards
Define ‘plant hormones’
Chemicals that are produced in one region of the plant and transported both through transport tissues and from one cell to another
What are the 6 roles of auxins?
Control cell elongation Abscission Maintain apical dominance Involved in tropisms Stimulate release of ethene Involved in fruit ripening
What are the 3 roles of gibberellins?
Causes stem elongation
Trigger the mobilisation of food stores in a seed at germination
Stimulate pollen tube growth in fertilisation
What are the 2 roles of ethene?
Causes fruit ripening
Promotes abscission in deciduous trees
What are the 3 roles of ABA?
Maintains dormancy of seeds and buds
Stimulates cold protective responses
Stimulates stomatal closing
Why are scientists still unsure about the details of many plant responses?
Plant hormones work at low concentrations so isolating them and measuring changes is difficult
Multiple interactions between chemical systems so difficult to isolate one chemical system
Explain the process of how germination is triggered
Seed absorbs water and the embryo is activated
Gibberellins start to be produced
Stimulates production of enzymes that breakdown the food stores of the seed
Embryo plant uses the food stores to produce ATP for building materials so they can grow and break out seed coat
Gibberellins switch on genes which code for amylases and proteases which are key for germination
Give 2 pieces of evidence that support the role of gibberellins in the germination of seeds
Mutant varieties of genes have been produced without gibberellins and they do not germinate. When fertilised with gibberellins externally they germinate.
Gibberellin biosynthesis inhibitors are applied to seeds, they don’t germinate as they cannot break dormancy
Give an example of an auxin
IAA- indoleacetic acid
Where are auxins produced?
Tips of roots and shoots and in the meristems
How does lateral growth in a plant occur?
Apical bud removes
Decrease in the concentration of auxins leads to the growth of lateral shoots
How does apical dominance occur?
High concentrations of auxins inhibit the growth of lateral shoots and apical shoots is stimulated
Explain the mechanism of action of auxin
Auxin binds to receptors on the cell membrane
H+ ions are actively pumped across the membrane into the cell wall
Build up of H+ ions lowers the pH to 5
Decrease in pH activates proteins called expansins
Cellulose cell wall becomes more plastic and there is cell expansion
What are the benefits of having low levels of gibberellins?
The plants are less vulnerable to damage by weather and harvesting
Define synergism
Different hormones working together, complimenting each other and giving a greater response than they would have individually
Define antagonism
Substances have opposite effects, the balance between them determining the response
Give 5 abiotic stresses
Seasonal changes in day length Temperature changes Lack of water Too much water High wind
Why is not efficient for plants to continue photosynthesizing through winter?
As plants need to produce enough glucose to support respiration in leaves
Glucose required for chlorophyll based process that protect the plant against freezing
How do deciduous trees combat seasonal changes and the inefficiency of photosynthesis?
Lose all of their leaves in the winter and remain dormant until the days lengthen and temperatures rise
Explain the phenomenon that is ‘wind rock’
Trees in full leaf are top heavy
During winter months there are often heavy winds
Heavy winds can cause uproot which damages the roots.
Stems may break which damages the xylem and phloem tissue in the stem.
How do trees respond to ‘wind rock’?
Losing all their leaves in winter months to make them less vulnerable as less heavy
Explain how ethene triggers abscission
Ethene is produced when the days get shorter
Ethene triggers the genes to switch on
Enzymes produced
Enzymes digest and weaken the cell walls of the cells in the separation zone
Vascular bundles supplying the leaf are sealed off and the cells bellow the abscission zone swell
Leaves are pushed off
How does the plant respond to leaf loss via abscission?
Fatty material deposited in the cells on the stem side of the separation layer
What is the abscission zone?
Two layers of cells that are sensitive to ethene
Define photoperiodism
Physiological response to the length of the night/dark period
What process are affected by the photoperiod?
Breaking of dormancy
Time of flowering
What causes the sensitivity of plants to dark length?
A light sensitive pigment called phytochrome
How do plants prevent freezing?
Plants produce chemicals which activate the genes needed to produce sugars and amino acids
Substances dissolve in the cell sap of the vacuole
The sap lowers the freezing point preventing the sap from freezing
What would occur if the sap of a cell froze?
The cell would expand and rupture the cell membrane
What hormone causes stomatal closing?
ABA
How does ABA cause stomatal closing?
ABA binds to receptors on the guard cell membranes.
Specific ion channels open causing calcium ions to enter the cytosol from the vacuole
Increased calcium ions cause other ion channels to open, K+ leaves the cell
Cells water potential increases so water leaves the cell by osmosis
Cell becomes flaccid ands stomata closes
When is ABA produced and where?
Released in times of water shortage
Produced in roots and transported to the leaves
What is herbivory?
The process by which herbivores eat plants
Give 7 physical defences that plants tend to have
Thorns Barbs Spikes Spiny leaves Fibrous and inedible tissue Hairy leaves Stings
What are the 4 main types of chemical defence?
Tannins
Alkaloids
Terpenoids
Pheromones
How do tannins act as chemical defences?
Bind to digestive enzymes produced in the saliva and inactivate them
Give 2 examples of things rich in tannins
Tea
Red wine
Give 4 examples of alkaloids
Caffeine
Nicotine
Morphine
Cocaine
How do alkaloids work?
Act as drugs that affect the metabolism of the animals that take them in
Give 2 examples of terpenoids
Citronella
Pyrethrin
What a is a pheromone?
A chemical made by an organism which affects the social behaviour of other members of the same species
What is a phototropism?
A plant response to light
What is a geotropism?
A plant response to gravity
What is a thigmotropism?
A plant response to touch
What is chemotropism?
A plant response to chemicals
What are phototropism’s the result of?
The movement of auxins across the shoot or root if exposed to light that is stronger on one side than the other
What happens to the direction of plant growth if a plant is exposed to more light on one side than the other?
The shoots of the plant will grow towards the light
Roots of the plant will grow away from the light
What happens to plant growth if a plant is exposed to light but the shoot tip is removed? Why?
No response
The tip must either detect the stimulus or produce the messenger as its removal prevents any response
Compare the growth of plants in the dark with those in the light
The growth will be greater in the dark because plants are searching for light, and seedlings want to reach the light to prevent competition
In normal conditions what type of gravitational stimulus do plants have?
A unilateral gravitational stimulus
What are the typical geotropisms seen by roots and shoots?
Shoots- negatively geotrophic (grow away from gravitational pull)
Roots- positively geotrophic (grow towards gravitational pull)
What are the two ways of investigating geotropisms?
Using a clinostat
Seeds in petri dishes stuck to the wall rotating every two hours
What is a climacteric fruit?
A fruit that continues to ripen after it has been harvested
How do plants continue to ripen after they’ve been picked?
Produce ethene which causes a series of chemical reactions
This can increase respiration rate
How are fruits chemically ripened?
Sprayed with ethene
Explain how auxins are used as hormone rooting powder
Propagates new plants
Small piece of stem cut from the plants
Dip end of cut stem into hormone rooting powder
Contains auxins
Which causes cell division and elongation
Explain how auxins are used as weedkillers
Auxins are sprayed onto fields where they acts as selective weed killers
Dicots absorb
Auxin stimulates rapid apical growth in dicots so plants outgrow themselves
What are gibberellins used for commercially?
Delaying ripening and ageing in fruit
Improve size and shape of fruits
What are cytokinins used for commercially?
Prevent ageing of ripened fruit and micropropagation
