Module 5.4 - Plant responses and hormones

Question 1

What are two examples of toxic chemicals plants produce to defend themselves being eaten by animals (herbivory)?

Answer 1

> Alkaloids - Have bitter tastes, noxious smells or poisonous characteristics that deter/kill herbivores. (e.g. tobacco plants producing nicotine).
Tannins - taste bitter and in some herbivores can bind to proteins in their gut, making the plant hard to digest. Both which deter animals from eating the plant.

Question 2

What are pheromones?

Answer 2

Pheromones are signalling chemicals that produce a response in other organisms.

Question 3

Give two examples of plants using pheromones?

Answer 3

> Some plants release alarm pheromones into the air in response to herbivores grazing. This can cause nearby plants that detect these chemicals to start making chemical defences such as tannins.
When corn plants are being eaten by caterpillars, they can produce pheromones which attract parasitic wasps. These wasps lay their eggs in the caterpillars which eventually kills them.

Question 4

Describe the mechanism of how the Mimosa pudica plant folds up in response to being touched?

Answer 4

If a single leaflet is touched, a signal spreads through the whole leaf, causing it to quickly fold up. It’s thought that this could help protect Mimosa pudica against herbivory by knocking off small insects or scaring off animals for example.

Question 5

What is carrots having antifreeze properties an example of?

Answer 5

Abiotic stress which is anything harmful that’s natural but non-living like a drought. If plants are able to respond to abiotic stress they are more likely to survive.

Question 6

What is a tropism?

Answer 6

The response of a plant to a directional stimulus.

Question 7

How do plants respond to stimuli?

Answer 7

By regulating their growth.

Question 8

How does a positive tropism differ to a negative tropism?

Answer 8

A positive tropism is growth toward the stimulus whereas, a negative tropism is growth away from the stimulus.

Question 9

What is phototropism?

Answer 9

The growth of a plant in response to light.

Question 10

Are shoots and roots negatively or positively phototrophic?

Answer 10

Shoots - postive - grow towards light.

Roots - negative - grow away from light.

Question 11

What is geotropism?

Answer 11

The growth of a plant in response to gravity.

Question 12

Are shoots and roots negatively or positively geotrophic?

Answer 12

Shoots - negative - grow upwards.

Roots - positive - grows downwards.

Question 13

What is hydrotropism?

Answer 13

The growth of a plant in response to water.

Question 14

What is thermotropism?

Answer 14

The growth of a plant in response to temperature.

Question 15

What is thigmotropism?

Answer 15

The growth of a plant in response to contact with an object.

Question 16

Are roots positively or negatively hydrophobic?

Answer 16

Positively hydrotrophic.

Question 17

What are growth hormones?

Answer 17

Chemicals that speed up or slow down plant growth.

Question 18

Where are growth hormones located?

Answer 18

Produced in the growing regions of the plant (shoot tips, leaves) and they move to where they’re needed in the other parts of the plant.

Question 19

What does gibberellin do?

Answer 19

Stimulates seed germination, stem elongation, side shoot formation and flowering.

Question 20

What do auxins do?

Answer 20

Stimulate the growth of shoots by cell elongation - where cell walls become loose and stretchy, so the cells get longer.

Question 21

How can auxin act in high concentrations also?

Answer 21

High concentrations of auxins inhibit growth in roots.

Question 22

What is one important auxin?

Answer 22

Indoleacetic acid (IAA).

Question 23

How is IAA transported around a plant to control tropisms?

Answer 23

It moves by diffusion and active transport over short distances and via the phloem over long distances.

Question 24

Describe how the uneven distribution of auxin (IAA) leads to the uneven growth of a plant in phototropism in a shoot?

Answer 24

Auxin moves to the more shaded parts of the shoots and roots and so therefore, these cells elongate. The shoot bends towards the light

Question 25

Describe how geotropism works in a root?

Answer 25

Auxin moves to the lower side of the root and growth is inhibited. So the root grows downwards.

Question 26

Describe the practical investigation you could carry out into phototropism and how plant shoots respond to light?
Use steps

Answer 26

1) Take nine (equal in height) shoots and plant them in identical soil in individual pots which are equal in height.
2) Cover the tips of three shoots with a foil cap, wrap the bases of three shoots with foil leaving only the tip exposed and leave 3 shoots without any foil.
3) Set up the shoots in front of a light source and leave them for 2 days.
4) Record your results by recording the amount of growth (in mm), as well as the direction of growth to give you quantitative results.

Question 27

What are the steps taken in order to ensure the phototropism experiment is a fair test?

Answer 27

> The shoots are all roughly the same height.
All the shoots are planted in the same type of soil.
Each shoot is in an individual pot (to prevent them growing into each other, disturbing experiment)
The shoots all being the same distance from the light source and experiencing equal light intensity.
All other variables in the 2 days of growth should be kept the same including temperature and exposure to moisture should be controlled.

Question 28

What results do you think you should observe of the plants in the phototropism experiment?

Answer 28

> Covering the tip with a foil cap prevents growth towards light for it is the tip where auxin is produced and is the most sensitive to light. Therefore, it would be expected to grow straight up.
The shoots with exposed tips should have grown towards the light source (positive phototropism).
The shoots with no foil would have grown towards the light and can be used as a constant.

Question 29

Describe the investigation you could perform into geotropism?
Use steps

Answer 29

1) Line 3 petri dishes with moist cotton wool.
2) Space out 10 cress seeds on the surface of the cotton wool in each dish and push them into the wool.
3) Tape a lid onto each dish and wrap them with foil.
4) Choose somewhere you can leave the dishes (e.g. cupboard).
5) Prop one dish upright at a 90 degree angle, the next at a 45 degree angle and the third on a flat horizontal surface. Make sure to label carefully.
6) Leave the seeds for 4 days, then take a look at the shoot and root growth.
7) To get quantitative results, measure the amount of growth of the shoots and roots and the angle of the growth.

Question 30

Why do we tape the lid onto each dish and wrap it with foil?

Answer 30

Will prevent any light reaching the seeds and affecting your results.

Question 31

What are the steps taken in order to ensure the geotropism experiment is a fair test?

Answer 31

> You should use the same volume of water and the same amount of cotton wool in each dish.
Choose a place to store the petri dishes for 4 days where the temperature is likely to be warmish and pretty constant.

Question 32

What results would you expect from the geotropism experiment?

Answer 32

You should find that whatever the angle of the dishes were placed at, the shoots have all grown away from gravity (negative geotropism) and the roots have grown towards gravity (negative geotropism).

Question 33

What is an apical bud?

Answer 33

The shoot tip at the top of a flowering plant.

Question 34

What is apical dominance?

Answer 34

Where auxins stimulate the growth of the apical bud and inhibits the growth of side shoots from the lateral buds. Prevents side shoots from growing which saves energy and prevents side shoots from the same plant competing with the shoot tip for light.

Question 35

What is the benefit of apical dominance to a plant surrounded by loads of other plants?

Answer 35

Because energy isn’t being expended in growing side shots it allows a plant to grow tall very fast, past the smaller plants, to reach the sunlight.

Question 36

What happens if you remove the apical bud?

Answer 36

The plant won’t produce auxins, so the side shoots will start growing by cell division and cell elongation.

Question 37

What happens if you remove the apical bud and replace it with a source of auxin?

Answer 37

Side shoot development is inhibited, demonstrating that apical dominance is controlled by auxin.

Question 38

How does the concentration of auxin distribute throughout the plant?

Answer 38

Auxins become less concentrated as they move away from the apical bud to the rest of the plant. If a plant grows very tall, the bottom of the plant will have a low auxin concentration so side shoots will start to grow near the bottom.

Question 39

Describe the experiment you could do to investigate apical dominance?
Use steps

Answer 39

1) Plant 30 (pea) plants of a similar age/height/weight in pots.
2) Count and record the number of side shoots growing from the main stem of each plant.
3) For 10 plants, remove the tip of the shoot and apply a paste containing auxins to the top of the stem.
4) For another 10 plants, remove the tip of the shoot and apply a paste without auxins.
5) Leave 10 plants as they are - these are untreated controls.
6) Let each group grow for 6 days in the same conditions (light intensity, water etc).
7) After 6 days, count the number of side shoots growing from the main stem of each plant.

Question 40

What is the importance of having untreated controls in the apical dominance investigation?

Answer 40

These are needed for comparison so that you know the effect you observe is likely to be due to the hormone and not any other factor.

Question 41

What are the results you should gather from the apical dominance investigation?

Answer 41

That removing the tips of shoots causes extra side shoots to grow, but removing tips and applying auxins prevented extra side shoots from growing. This suggests auxins inhibit growth of side shoots and are involved in apical dominance.

Question 42

Where are gibberellins produced and what do they stimulate?

Answer 42

Produced in young leaves and in seeds. They stimulate seed germination, stem elongation, side shoot formation and flowering.

Question 43

How do gibberellins stimulate the stems of plants to grow?

Answer 43

By stem elongation which helps plants to grow very tall. If a dwarf variety of a plant is treated with gibberellin, it ill grow to the same height as the tall variety. Gibberellins don’t inhibit plant growth in any way.

Question 44

How do gibberellins stimulate seed germination?

Answer 44

By triggering the breakdown of starch into glucose in the seed. The plant embryo in the seed can then use the glucose to begin respiring and release the energy it needs to grow.

Question 45

How are gibberellins inhibited and what does it prevent?

Answer 45

Inhibited by the hormone abscisic acid so prevents seed germination.

Question 46

Describe the investigation of the role of gibberellins in stem elongation?
Use steps

Answer 46

1) Plant 40 (dwarf pea) plants that are a similar age/height/mass in pots.
2) Leave 20 plants as they are to grow, watering them all in the same way and keeping them all in the same conditions - these are your controls.
3) Leave the other 20 plants to grow in the same conditions except water them with a dilute solution of gibberellin.
4) Let the plants grow for about 28 days and measure the lengths of all the stems once each week.

Question 47

What would you expect the results of the gibberellin experiment to be?

Answer 47

Stems grow more when watered with a dilute solution of gibberellin and you can calculate the rate of growth by dividing the amount the plant grew in the set amount of time.

Question 48

What does auxins and gibberellins being synergistic mean?

Answer 48

They work together to have a really big effect.

Question 49

Give an example of when gibberellins and auxins are antagonistic?

Answer 49

Gibberellins stimulate the growth of side shoots but auxins inhibit the growth of side shoots.

Question 50

What is a deciduous plant?

Answer 50

A plant that loses their leaves in winter.

Question 51

Why do deciduous plants lose their leaves during the autumn/winter?

Answer 51

Helps plants to conserve water during the cold part of the year, when it might be difficult to absorb water from the soil (the soil may be frozen), and when there’s less light for photosynthesis.

Question 52

How is leaf loss controlled by hormones?

Answer 52

> Auxins inhibit leaf loss and are produced by young leaves, as the leaf gets older, less auxin is produced, leading to leaf loss.
Ethene stimulates leaf loss - ethene is produced by ageing leaves. As the leaves get older, more ethene is produced.

Question 53

What hormone is involved in stomatal closure?

Answer 53

The plant hormone abscisic acid (ABA) is able to trigger stomatal closure.

Question 54

Describe how ABA is able to trigger stomatal closure?

Answer 54

ABA binds to receptors on the guard cell membranes causing specific ion channels to open, allowing calcium ions to enter the cytosol from the vacuole. The increased conc. of calcium ions in the cytosol causes other ion channels to open which allows ions to leave the guard cell, raising the water potential of the cells. Water then leaves the guard cells by osmosis, becoming flaccid and the stomata close.

Question 55

What does ethene do?

Answer 55

Breaks down cell walls, break down chlorophyll and converts starch into sugars.

Question 56

And how is ethene used commercially?

Answer 56

Makes the fruit soft, ripe and ready to eat. Such as harvesting and transporting bananas before they are ripe for they’re less likely to be damaged this way. Then exposed to ethene on arrival so they all ripen at the same time.

Question 57

How are auxins used commercially by gardeners/farmers?

Answer 57

> Selective weedkillers (herbicides) - make weeks produce long stems so they grow too fast, can’t get enough water or nutrients, so they die.
Rooting hormones - take a cutting and grow roots, can be planted and grown into a new plant. Many cuttings can be taken from one original plant and treated with rooting hormones, so lots of the same plant can be grown quickly and cheaply from just one plant.