Chapter 16 Plant hormones

Question 1

What are the affects does the plant hormone gibberellins have on a plant?

Answer 1

Promotes seed germination.
Promotes stem elongation.
Side shoot formation.
Flowering.

Question 2

Describe the process in which gibberellins promotes seed germination.

Answer 2

The production of gibberellins stimulates the production of digestive enzymes (like amylase) to break down food stores (like starch) in the cotyledons of a seed.
The digestive enzymes hydrolyses food stores like starch to give glucose, which respires to produce ATP.
This ATP is used for building materials so it can grow and break out of the seed coat.

Question 3

Evidence suggests that ABA, abscisic acid, is an antagonist to gibberellins in seed germination. What does this mean?

Answer 3

This just means that ABA interferes or works against gibberellins, which is responsible for seed germination. Hence, the levels of these two hormones determines when the seed will germinate.

Question 4

What experimental evidence has been provided to show that gibberellins is responsible for seed germination?

Answer 4

One experimental evidence is where mutant varieties of seeds have been bred that specifically lack the gene to make gibberellins. For this reason, these seeds do not germinate in their natural state. However, when gibberellins is applied externally, they start to germinate.
Another experimental evidence is that gibberellins biosynthesis inhibitors stops a seed from germinating, as it prevents gibberellins from being synthesised.

Question 5

Gibberellins also promotes stem elongation. In some cases farmers may want to inhibit gibberellins with respect to this fact. Why may they do this?

Answer 5

For some farmers, the stem of the plant is not the selling point, but rather, the fruits. If more energy is used to grow the stem, not enough fruits can be produced, decreasing the yield, and hence decreasing the profits.
Another reason why farmers may want to inhibit gibberellins is because of bad weather. Taller plants may not be beneficial n bad weather conditions- taller plants are more exposed to strong winds and more.

Question 6

What are the affects does the plant hormone auxins have on a plant?

Answer 6

Promotes cell elongation.
Gives apical dominance- inhibits growth of side shoots.
Inhibits leaf abscission (leaf fall).

Question 7

Where are auxins found in a plant?

Answer 7

Auxins are found in the tips of roots and shoots and in meristem cells.

Question 8

During a tropism, how may auxins move through a plant?

Answer 8

Via diffusion.

Question 9

Suggest how apical dominance aids with the growth of a plant.

Answer 9

Apical dominance promotes the growth of the main apical shoot but inhibits the growth of the lateral shoots. Lateral shoots near the top of the plant is smaller because it is close to the apical bud which has a high auxin concentration. Lateral shoots near the bottom of the plant are longer because the bottom of the plant has low auxin concentrations. Hence, this gives a plant an overall conical shape. In this sense, the smaller leaves at the top allows sunlight to reach the bottom leaves (which are longer)- so photosynthesis can occur all around the plant.

Question 10

What happens to a plant if you remove the apical bud?

Answer 10

If you remove the apical bud, you remove the high concentration of auxin and this allows all the lateral buds to grow more evenly.

Question 11

During an experiment in investigating the phototropism in a plant, a student covers the apical bud of the plant in a light-proof material (the plant is in the presence of unilateral light). What do you predict to be the response of the plant?

Answer 11

Responses to light is brought about by receptors at the tip of the shoot which detects light. Covering the apical bud with a light-proof material prevents receptors from detecting light, and hence any responses the plant would bring about (the movement of auxins). Hence, covering the apical bud in a light proof material brings about no response in a plant. This also applies if the apical bud was cut off.

Question 12

Describe the process of cell elongation in a plant.

Answer 12

Auxins move into a pant cell.
Auxins activates a protein in the plant cell.
The auxin-protein complex activates a protein pump in the plant cell and guides it to fuse with the plasma membrane.
Using ATP, the protein pump moves hydrogen ions across the plasma membrane into the plant cell wall and acidifies the cell wall (or lowers pH).
The increased acidity activates an enzyme called expansins.
Expansins disrupts interactions between cellulose polymers.
This cause cellulose microfibrils to loosen and expand (with the help of turgor pressure).
(Auxins also promotes the nucleus to synthesise more protein pumps).

Question 13

Why is the movement of water into plant cells alone not enough to carry out cell elongation?

Answer 13

The movement of water into plant cells causes the large permanent vacuole to expand, which pushes the plasma membrane against the surface of the cell wall- the cell is turgid. Although the vacuole and plasma membrane expands, the cell as a whole does not expand. This is because of the strong plant cell. The bonding and arrangement of cellulose fibres in the cell wall, ensures the plant cell wall does not expand under turgor pressure; this is why the cell as a whole does not expand.

Question 14

What are the affects does the plant hormone ethene have on a plant?

Answer 14

Promotes fruit ripening.
Abscission of fruits and leaves.
Commercial use- ripening of tomatoes and citrus fruits.

Question 15

What are the affects does the plant hormone abscisic acid have on a plant?

Answer 15

Causes stomatal closure during abiotic stress (like low water availability).
Commercial use- controls fruits drop to allow fruit picking.

Question 16

What are the affects does the plant hormone cytokinin have on a plant?

Answer 16

Promotes cell division.

Question 17

How does high auxin concentrations vary the rate of growth in different parts of a plant?

Answer 17

High auxin concentrations promotes growth in the shoots of a plant, but inhibits growth in the roots of a plant.

Question 18

Define photoperiodism.

Answer 18

A period of time when plants are sensitive to a lack of light in the environment.

Question 19

What are phytochromes?

Answer 19

Phytochromes are light sensitive pigments that detects seasonal changes in light.

Question 20

Describe the process of abscission.

Answer 20

1. Phytochromes detects falling light levels.
2. The production of auxins reduces.
3. This causes the production of ethene to increase.
4. Ethene activates genes in cells in abscission zone to synthesis more digestive enzymes.
5. Enzymes (cellulase) digests the cell wall in the separation zone.
6. Vascular bundles just before the abscission zone are sealed off to prevent the transport of water and assimilates.

Question 21

What hormones controls abscission?

Answer 21

Ethene. (+ fall in auxins)

Question 22

What are the advantages of abscission in winter?

Answer 22

Less energy is needed to maintain leaves.
Fallen leaves insulates the roots of the plant.
Rotting leaves produces minerals to plants.

Question 23

In times of abiotic stress, what hormones controls the flow of the transpiration stream or loss of water?

Answer 23

ABA abscisic acid.

Question 24

Describe the process of stomatal closure with the help of ABA.

Answer 24

Roots in the soil detects the lack of water- the start to produce ABA.
ABA travels to leaves and binds to receptors on the surface of guard cells. (These receptors are directly attached to channel proteins).
The binding of ABA causes ions (e.g, K) to move out of the guard cells via the channel proteins.
The movement of these ions increases the water potential in guard cells, and decreases water potential outside guard cells.
Water leaves the guard cells down the water potential gradient- the lose of water causes the guard cell to become flaccid, and stomata closes.

Question 25

State some physical defences in plants.

Answer 25

Thorns, Spikes, Hairy leaves, (waxy cuticle).

Question 26

State some chemical defences of a plant.

Answer 26

Tannins.
Alkaloids.
Terpenoids.
Pheromones.
Volatile organic compounds (VoCs).

Question 27

What chemicals (defences) affects animals and insects because of their bitter taste?

Answer 27

Tannin and alkaloids.

Question 28

What chemical (defence) is toxic to insects and fungi?

Answer 28

Terpenoids.

Question 29

Pheromones is a chemical defence in plants. How does it help defend the plant?

Answer 29

It affects the social behaviour of members of the same species. (Warns other plants of the same species).

Question 30

Volatile Organic Compounds (VoCs) is a chemical defence in plants. How does it do this?

Answer 30

Affects the social behaviour of members of other species.

Question 31

How does the plant Mimosa pudica (latin name) defend itself?

Answer 31

The plant executes a folding response every time it is touched. This can trap any predators or preys.

Question 32

State what the stimulus is in each of these tropisms:
Phototropism.
Gravitropism.
Chemotropism.
Hydrotropism.
Aerotropism.

Answer 32

Phototropism- Light.
Gravitropism- Gravity.
Chemotropism- Chemicals.
Hydrotropism- Water.
Aerotropism- Oxygen.

Question 33

Where does positive phototropism occur in a plant?
Where does negative phototropism occur in a plant?

Answer 33

Positive phototropism in the shoots of a plant; shoot grows towards light; ensures max rate of photosynthesis.
Negative phototropism in the roots of a plant; roots grow away from the light into the soil; ensures max uptake on minerals and water.

Question 34

When light is hitting a plant from all directions, what happens to the distribution of auxins in the plant?

Answer 34

There is an even distribution of auxins through the plant- hence, cell elongation takes place at an equal pace all around the plant, so each part of the plant grows at the same rate.

Question 35

Describe the process of a plant’s positive response to unilateral light.

Answer 35

The unilateral light is detected by receptors in the shoot tip and causes auxins to move to the shaded side of the plant.
There is now a higher auxin concentration on the shaded side of the plant.
A higher concentration promotes cell elongation on the shaded side, and hence causes shaded side to grow at faster rate.
This causes the shoot to grow towards the light.

Question 36

Different parts of a plant demonstrates different geotropism. What part of the plant demonstrates positive geotropism and why?

Answer 36

Roots demonstrates positive geotropism.
In the stem of the roots, the auxins falls of the bottom (the stem is horizontal, because of the original orientation of seed) due to gravity.
In roots, high auxin concentration inhibits root growth.
Hence, the part of the stem at the top grows at a faster rate- less auxins, more cell elongation takes place.
The growth of the top bit of the stem causes the root to grow downwards.
As it is growing downwards with gravity, it is positive geotropism.

Question 37

Different parts of a plant demonstrates different geotropism. What part of the plant demonstrates negative geotropism and why?

Answer 37

The shoots demonstrate negative geotropism.
In the stem of the shoots, the auxins falls of the bottom (the stem is horizontal, because of the original orientation of seed) due to gravity.
In the shoots, high auxin concentrations promotes shoot growth.
This means the bottom part grows faster with more cell elongation.
For this reason, the shoots grow upwards for more light; for more photosynthesis to occur.
As the shoot is growing in the opposite direction to gravity, this is negative geotropism.

Question 38

In a seed, the auxins fall to the bottom of the stems of the roots and shoots due to gravity. Why is the direction of their growth in different directions?

Answer 38

A high concentration of auxins can promote growth in the shoots but inhibit growth in the roots. So in the roots, cell elongation won’t occur in the place where auxins are present. Instead, there will be a faster growth rate in where auxins are not- the top of the stem of the roots. This causes the roots to grow downwards.

Question 39

What are the commercial uses of auxins?

Answer 39

Encourages root growth (in cuttings).
Inhibits/ slow down leaf fall and fruit droppings.
Synthetic auxins acts as weed killers.

Question 40

What are the commercial uses of gibberellins?

Answer 40

Speeds up barley seed germination during alcohol brewing.
Delays senescence in citrus fruit.

Question 41

What are the commercial uses of ethene?

Answer 41

Speeds up ripening.
Promotes fruit dropping.

Question 42

What are the commercial uses of cytokinin?

Answer 42

Prevents senescence.