Module 5: Section 4 Plant Responses and Hormones

Question 1

What is abiotic stress?

Answer 1

Anything that is potentially harmful to a plant that’s natural, but non living, like a drought.

Question 2

Example of a plant responding to abiotic stress

Answer 2

Carrots produce antifreeze proteins at low temperatures. The proteins bind to ice crystals and lower the temperature that water freezes at, stopping more ice crystals from growing.

Question 3

What is herbivory?

Answer 3

When plants are eaten by animals, including insects.

Question 4

Examples of chemicals produced by plants against herbivory?

Answer 4

1) alkaloids- these chemicals taste bitter, have noxious smells or have poisonous characteristics that deter/kill herbivores. For example tobacco plants produce alkaloid nicotine in response to tissue damage, which is poisonous to many insects.

2) Tannins- these chemicals taste bigger and in some herbivores eg sheep they can bind to the proteins in their gut making the plant hard to digest. Both these things deter animals eating the plant.

Question 5

What are pheromones?

Answer 5

Pheromones are signalling chemicals that produce a response in other organisms. Plants release pheromones in response to herbivory.

Question 6

Examples of plants releasing pheromones?

Answer 6

When corn plants are being eaten by caterpillars, they can produce pheromones which attract parasitic wasps. These wasps lay eggs in the caterpillars, which eventually kills them.

Question 7

Example of plant responding to being touched?

Answer 7

If a leaflet of Mimosa pudica is touched, a signal spreads through the whole leaf causing it to quickly fold up. It’s thought that this mechanism helps scare animals trying to eat it and knock off any small insects feeding on the plant.

Question 8

What is a tropism?

Answer 8

A tropism is the response of a plant to a directional stimulus. A positive tropism is growth towards the stimulus and negative is away from the stimulus.

Question 9

What is phototropism?

Answer 9

Phototropism is a plants response to light. Shoots are positively phototropic and grow towards the light and roots are negatively phototropic and grow away from the light.

Question 10

What is geotropism?

Answer 10

Geotropism is the growth of a plant in response to gravity. Shoots are negatively geotropic and grow upwards. Roots are positively geotropic and grow downwards.

Question 11

What are some examples of other tropisms?

Answer 11

1) hydrotropism - plant growth in response to water. Roots are positively hydrotrophic
2) thermotropism - plant growth in response to temperature
3) Thigmotropism- plant growth in response to contact with an object

Question 12

What are growth hormones?

Answer 12

Growth hormones are chemicals that speed up or slow down plant growth. They are produced in growing regions of the plant. Examples are auxins, gibberellins and Ethene.

Question 13

What are the effects of auxins?

Answer 13

Auxins stimulate the growth of shoots by cell elongation. Auxins are produced in the tips of shoots in flowering plants and if this tip is removed, no auxin will be available and the shoot stops growing. Auxins stimulate growth in shoots but high concentrations inhibit growth in roots.

Question 14

What is IAA?

Answer 14

Indoleactic acid (IAA) is an auxin that’s produced in the tips of shoots and roots in flowering plants. It’s moved around the plant by diffusion and active Transport over short distances and by the phloem in long distances. This results in different parts of the plant having different amounts of IAA. The uneven distribution of IAA leads to uneven growth of the plant.

Question 15

How does IAA affect phototropism and geotropism?

Answer 15

Phototropism- IAA moves to the more shaded regions of the shoots and roots so cells elongate. shoots bend towards the light and roots bend away.

Geotropism - IAA moves to the underside of shoots and roots. Shoots grow upwards and roots grow downwards.

Question 16

What is an experiment for phototropism?

Answer 16

1) take 9 shoots roughly equal in height and plant them in individual plant pots in the same type of soil

2) cover the tips of three shoots with a foil cap. Leave three shoots without foil. Wrap the bases of three shoots with foil leaving only the tip exposed.

3) set the shoots up infront of a light source. Make sure they are all equal distances from the light. Make sure other variables are controlled eg moisture levels.

4) leave shoots to grow for 2 days. After the 2 days, record the amount of growth in mm and the direction of the growth.

5) the tip with the foil cap will grow straight up. The other two with no foil cap and a foil cap at the base will grow towards the light source.

Question 17

What is an experiment for geotropism?

Answer 17

1) line 3 Petri dishes with moist cotton wool. Use the same volume of water/cotton for each.

2) space out 10 cress seeds on the surface of the cotton wool in each dish and lush each seed into the wool slightly.

3) tape a lid onto each dish and wrap one in foil and make sure there are no gaps. The dish with foil prevents any light reaching the seeds.

4) leave the dishes in a warmish area with consistent temp. Place them at different angles. One at 90 degrees, one at 45 and one completely flat.

5) leave seeds for 4 days. After 4 days unwrap each dish and note the direction of the shoot and root growth of the cress seedlings.

6) you will find that at whatever angle the dishes were placed at, all the shoots would’ve grown away from gravity and all the roots grew towards gravity.

Question 18

What is apical dominance?

Answer 18

This is when auxins stimulate the growth of the apical bud (shoot tip) and inhibit the growth of side shoots from lateral buds.

Question 19

What is the purpose of apical dominance?

Answer 19

Apical dominance prevents side shoots from growing with saves energy and prevents side shoots from plant competing with the short tip for light. This allows a plant to grow faster to reach the sunlight past the smaller plants.

Question 20

What happens when you remove the apical bud?

Answer 20

If you remove the apical bud then the plant won’t produce auxins, so the side shoots will start growing by cell division/cell elongation. However if you replace the tip with a source of auxin, side shoot development is inhibited, demonstrating apical dominance is controlled by auxin.

If a plant grows really tall, the bottom of the plant will start growing side shoots as it is away from the apical bud, so there is a low concentration of auxin at the bottom of the plant.

Question 21

Where are gibberellins produced?

Answer 21

Gibberellins are growth hormones produced in young leaves and in seeds.

Question 22

What are the effects of gibberellins?

Answer 22

Gibberellins stimulate seed germination, stem elongation, side shoot formation and flowering. Unlike auxins, gibberellins don’t inhibit plant growth in any way.

Question 23

How do Gibberellins stimulate seed germination?

Answer 23

Gibberellins stimulate seed germination 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 24

What inhibits gibberellins?

Answer 24

Gibberellins are inhibited (and so seed germination is prevented) by the hormone abscisic acid (ABA)

Question 25

What is the name given to plant hormones working together + example?

Answer 25

Synergistic - auxins and gibberellins work together to help plants grow very tall.

Question 26

What is the name given to plant hormones opposing each other + example?

Answer 26

Antagonistic- gibberellins stimulate the growth of side shoots but auxins inhibit the growth of side shoots.

Question 27

Why do deciduous plants lose their leaves in winter?

Answer 27

Losing their leaves helps plants to conserve water, as during winter it is more difficult to absorb water from the soil (water may be frozen) and there is less light for photosynthesis.

Question 28

How is leaf loss triggered in plants?

Answer 28

1) auxins are produced in young leaves, so as the leaf gets older, less auxin is produced leading to leaf loss.

2) Ethene is produced by ageing leaves. As the leaf gets older, more Ethene is produced.

3) a layer of cells called the abscission layer develops at the bottom of the leaf stalk. The abscission layer seperates the leaf from the rest of the plant.

4) Ethene stimulates the cells in the abscission layer to expand, breaking the cell walls and causing the leaf to fall off.

5) auxins are antagonistic to ethene

Question 29

What plant hormone is responsible for stomatal closure?

Answer 29

Abscisic acid (ABA)

Question 30

How does ABA trigger stomatal closure?

Answer 30

1) ABA binds to receptors on the guard cell membranes. This causes specific ion channels to open, which allows calcium ions to enter the cytosol from the vacuole.

2) the increased concentration of calcium ions in the cytosol causes other ion channels to open. These ion channels allows ions eg potassium to leave the guard cells, raising the water potential of the cells.

3) water then leaves the guard cells by osmosis.

4) the guard cells become flaccid and the stomata close.

Question 31

What are the commercial uses of Ethene?

Answer 31

Ethene stimulates enzymes that freak down cell walls, break down chlorophyll and convert starch to sugars. This makes fruit ripe and ready to eat.

For example, bananas are harvested and transported before they are ripe because they are less likely to damaged. They’re then exposed to Ethene on arrival so they all ripen at the same time in the supermarket.

Question 32

What are the commercial uses of auxins?

Answer 32

1) auxins are used in selective weed killers (herbicides). These make weeds produce long stems instead of lots of leaves. This makes weeds grow too fast, so they don’t get enough water or nutrient so they die.

2) Auxins are used as rooting hormones (eg rooting powder). They make a cutting grow roots. The cutting can then be planted and grown into a new plant. Many cuttings can be taken from one original plant so lots of the same plant can be grown quickly and cheaply from just one plant.

Question 33

Experiment to show the role of auxins in apical dominance?

Answer 33

1) plant 30 plants of the same type and similar age, height and weight in pots containing the same type of soil.

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 with no auxins on top.

5) leave the last 10 plants as they are. These are controls.

6) leave the plant to grow for 6 days. Keep conditions eg light intensity and water the same. After 6 days count the number of side shoots growing from the main stem.

7) likely results are that the control and the plants with the paste containg the auxin have similar results with barely any new side shoots. The one with no auxin would have more side shoots.

Question 34

Experiment for the role of gibberellins in stem elongation?

Answer 34

1) plant 40 plants that are the same age, height and mass in pots with the same type of soil.

2) leave 20 to grow, watering them in the same way and keep other conditions the same. These are negative controls.

3) grow the other 20 in the same conditions but water them with a dilute solution of gibberellins. Let all plants grow for 28 days.

4) every 7 days measure the length of the stem of each plant. Calculate a mean for both conditions. The ones watered with gibberellins likely to have more growth.