Plant Responses Flashcards
(20 cards)
What are auxins and what do they do?
- Growth stimulants.
- Auxin affects the plasticity of plant cell walls (how flexible they are). The presence of of auxin allows cell wall to stretch more easily- so cell can enlarge/grow.
How do auxins work?
1- It binds to specific receptors on plant cell walls.
2- Causes the release of H+ ions into plant cell walls.
3- This lowers the pH of plant cell wall (to around 5.0).
4- This makes cellulose more flexible increasing plasticity of wall.
5- activates “expansin” enzymes.
6- These allow cells to elongate.
What is the distribution of auxin in a shoot affected by?
Light intensity.
Where do shoots grow towards?
Shoots are positively phototropic growing towards the light.
Shoots are negatively geotropic growing away from gravity.
Describe experiment to investigate effects of auxin on phototropic responses.
- control plant is illuminated from all sides, while experimental plant is illuminated from one side (10 reps each).
- in each plant, roots and shoots are marked every 2mm at the start.
Results:
Control plant: - uniform light shone onto plant, so uniform distribution of auxin down whole shoot tip
- all of the cells underneath shoot tip elongate at an equal rate, which causes shoot to grow straight upwards.
Experimental plant: - unilateral light, shone from one side
- causes more auxin to pass from tip to shaded side
- higher auxin conc in these cells causes them to elongate more than cells on illuminated side, which causes shoot to bend towards the light.
What are the effects of auxin on roots and shoots?
- Generally with shoots, the higher the auxin conc, the faster they grow.
- With shoots, very low conc of auxin stimulate growth, but then as the conc increases, growth is inhibited.
What is apical dominance?
When high concentrations of auxin suppress the growth of the side shoot (lateral shoots).
What happens if apical shoot is removed?
- If apical shoot is removed from a plant, the auxin-producing cells are removed, so there is no auxin present. In this case, lateral side shoots grow freely and quickly.
- If auxin is then applied artificially to the cut stem, apical dominance is restored and side shoots are restored again.
What is an advantage of apical dominance?
- Apical dominance allows plants to grow tall quickly. This helps them absorb more sunlight for photosynthesis, so they can out-compete their neighbours.
Plant hormones to do with stem elongation.
Gibberellins -> affect lengths of internodes - regions between leaves on a stem.
Antagonism
What happens to growth of plant in the dark?
- If a plant is in the dark, it needs to grow rapidly upwards to try and reach a light source in order to photosynthesise.
- Experimental evidence has shown that gibberelins are responsible for extreme elongation of the internodes when a plant is grown in the dark.
What are the abiotic and biotic stresses plants react to?
Abiotic = non-living component of the environment, e,g. Drought, extreme temperatures.
Biotic = living components of the environment e.g. pests, diseases, herbivores.
Where in a plant does growth occur?
In meristems.
What are tropisms?
A directional growth response to an external environmental stimulus.
Either towards (positive) or away from (negative) the stimulus.
Why do much of the work on tropisms involve using seeds and very young seedlings?
- Easy to work with and manipulate.
- They grow and respond quickly - any responses to the environment show up quickly.
- Responses are easy to see in measure (in adult plants, different parts of the plant respond differently).
Examples of tropisms.
Phototropism -> response to light.
Geotropism -> response to gravity. Shoots show negative geotropism, roots show positive geotropism.
Hydrotropism -> response to moisture. Root tips tend to grow towards damper area of soil, increasing access to water.
Chemotropism -> response to chemicals.
Chemical defences which reduce effects of herbivores
- alkaloids : chemicals with better tastes or poisonous characteristics which deter or kill herbivores. E.g. tobacco plants produce alkaloid nicotine in response to tissues damage. Nicotine highly poisonous to many insects.
- tannins : these taste bitter and in some herbivores they can bind to proteins in gut, making plant hard to digest.
- Pheromones : signalling chemicals that produce a response in other organism.
How do plants respond to drought?
By closing their stomata (restricts water loss) or by losing leaves altogether.
What are the main plant hormones and their roles?
Auxins ->
- promotes cell elongation (more on shaded side, more produced in dark), maintain apical dominance and inhibit side shoots.
Gibberellins ->
- promote seed germination, cause stem elongation and delay fruit ripening.
Cytokinins ->
- promote cell division. Inhibit leaf ageing.
Ethane ->
- promotes leaf fall, fruit ripening.
For a plant to start growing it needs to germinate. How does it happen?
1- seed absorbs water
2- this activates the embryo plant inside
3- this produces gibberellins which activate genes coding for amylase and proteases.
4- The proteases are used to break down proteins in the seed, the amino acids are used to make new enzymes.
5- Amylase used to break down starch stored in seed to produce glucose.
6- Glucose released is then respired to produce ATP.
7- ATP used for protein synthesis/ cell division/ growth.
8- Embryo plant breaks through the seed coat: a shoot tip forms at one end and appears through the soil. A shoot tip forms at the other end and grows down into the soil.
