Plant Responses Flashcards
Ethene role in plants
Fruit ripening
Promotes abscission in deciduous trees
Auxins
Cell elongation
Prevent abscission
Stimulate release of ethene
Abscisic acid (ABA)
Maintains dormancy of seeds and buds
Stimulate cold protective responses
Gibberellin
Stem elongation
Trigger mobilisation of food stores in a seed at fertilisation
Seed germination
1 A seed absorbs water
2 Embryo is activated and produces gibberellins
3 They stimulate the produces tion of enzymes that break down the food stores found in the seed
4 The embryo uses these food stores to produce ATP for the building of materials so it grow and break out of the seed coat
5 There is evidence that suggests gibberellins switch on genes that code for atlases and proteases
6 Also evidence that ABA interferes with gibberellins
Experimental evidence for the role of gibberellins in seed germination
1 Mutant varieties of seeds have been bred which lack the gene that enables them to make gibberellins. These seeds don’t germinate. If gibberellins are applied to the seeds externally, the seeds germinate normally.
2 If gibberellin biosynthesis inhibitors are applied to seeds , they cannot germinate as they cannot produce gibberellins needed to break dormancy. If inhibition is removed, the seeds germinate.
How were gibberellins discovered?
Because they are produced by a fungus from the genus Gibberella that affects rice. The infected seedlings grew extremely tall and thin. Scientists investigated the rice and isolated the gibberellins which produce the same spindly growth in plants. It was then worked out that the plants themselves produce the same compounds. Plants with short stems produce very little gibberellins.
What is an example of an auxin?
IAA
High concentrations of auxins…
Reduce growth in lateral shoots which results in apical dominance.
Growth in main shoot is stimulated by auxin produced at the tips, so grow quickly.
Lateral shoots are inhibited by the hormone that moves back down the stem, so they don’t grow very well.
Further down the stem, the concentration of auxin decreases and so the literal shoots grow more.
Low concentrations of auxins…
Promote root growth
The more auxin in the roots, the more they grow
If the apical shoot is removed, the amount of auxin reaching the roots is reduced and root growth is slowed and stops.
What is the role of auxins?
Stimulate growth of main apical shoot.
How does auxins affect the plasticity of cell walls?
Auxin means the cell walls stretch more easily.
Auxin bind to specific receptor sites in the plant cell membranes.
Causes a fall in pH.
As cells mature, auxin is destroyed. So pH rises and enzymes maintaining plasticity become inactive.
So walls become rigid, cells can no longer expand and grow.
Physical defences of plants to herbivory
Thorns
Hairy leaves
Inedible tissue
Chemical defences of plants to herbivory
Tannins
Alkaloids
Terpenoids
Tannins
Make up 50% of dry weight of leaves
Bitter taste
Toxic to insects
Alkaloids
Bitter tasting nitrogeneous compounds
Acts as drugs, affecting the metabolism of animals that eat them
Terpenoids
Act as toxins to insects and fungi
What is a pheromone?
Chemical made by an organism which affects the social behaviour of other members of the same species
Give evidence that plants communicate by pheromones
Communicate by chemicals produced in the roots systems to tell neighbouring plants if it is under water stress
Why don’t plants rely a lot on pheronomes?
Because they don’t behave socially
What chemicals do plants produce?
VOCs (Volatile organic compounds)
Give an example of when VOCs are used
Wheat seedlings produce VOCs when they have been attacked by aphids which repel other aphids
How do VOCs work?
Diffuse through the air in and around the plant
What do plants use VOCs for?
To defend themselves
Which hormone can be used in the production of seedless fruit?
Auxin
Which hormone is used to promote fruit dropping?
Ethene
Which hormone is used to prevent ageing of ripened fruit?
Cytokinins
Which hormone is used to delay ripening and ageing in fruit?
Gibberellins
What are weeds?
Plants grown where they are not wanted
How do synthetic auxins acts as weedkillers?
- Staple foods around the world are narrow leaved monocot plants
- Most weeds are broad leaved dicots
- If synthetic dicot auxins are applied as weedkillers, they are absorbed by broad leaved plants and affect their metabolism
- Their growth rate increases and they die
- Narrow leaved plants are unaffected
What are the advantages of ripening by ethane?
- Prevents wastage
- Increases time available for selling of fruit
How does ethene ripen climatic fruit?
Ethene production causes chemical reactions and increases respiration
Geotropisms (Response to gravity)
Shoots are usually negatively geotropic and roots are positively geotropic.
What happens if plants are exposed to light which is brighter on one side than another?
Shoots will grow towards the light as they are positively phototropic and roots will grow away from the light as the are negatively phototropic.
Why are shoots positively phototropic?
For maximum photosynthesis.
Do plants grow more rapidly in the dark or when illuminated?
In the dark
Why do plants grow more rapidly when in the dark?
In the dark, the shoots are trying to reach the light so they can photosynthesise.
The seedlings which break through the soil first will not have compete with the other seedlings for light.
Why is it important that plant growth slows when exposed to light?
Resources can be used for synthesising leaves, strengthening stems and overall growth.
Example of growing plants in the dark.
In Yorkshire rhubarb was grown in dark sheds.
What is synergism?
When things work together and complement each other.
What is antagonism?
When substances have opposite effects.
How does the cytoplasm of cells and sap of vaucoles prevent freezing?
They contain solutes which lower the freezing point.
Why is it important to prevent freezing in plants?
The cells freeze and their membranes are disrupted. So they die.
What controls the opening and closing of the stomata in response to abiotic stresses?
ABA
What is abscission?
Leaf fall
Abscission due to falling levels of light…
1) Falling levels of light result in falling concentrations of auxin.
2) As a result, ethene is prodcued.
3) The abscission zone is made up of two layers of cells sensitive to ethene.
4) Ethene initiates gene switching in these cells, resulting in the production of new enzymes.
5) These enzymes digest and weaken the cell walls in the outer layer of the abscission zone.
6) Vascular bundles which carry materials into and out of the leaf are sealed off.
7) Also, fatty materials are deposited in the cells on the stem side of the separation layer.
8) This layer forms a protective scar when the leaf falls off. This prevents the entry of pathogens.
What is the outer layer of the abscission zone called?
Separation layer
What is photoperiodism?
Sensitivity of plants to lack of light.
What is phytochrome?
A light sensitive pigment.
