MODULE 5 SECTION 4 - PLANT RESPONSES AND HORMONES

Question 1

Examples of ways plants respond to the environment (basic ideas of tropisms)?

Answer 1

• They sense the direction of light and grow towards it to maximise light absorption for photosynthesis.
• They sense gravity, so that shoots and roots grow in the right direction.
• Climbing plants have a sense of touch, so they can find things to climb and reach the sunlight.

Question 2

Plant responses to abiotic stress

Answer 2

• Abiotic stress is anything that is potentially harmful to a plant that is natural but non-living, like a drought (water stress).
• Some plants respond to extremely cold temperatures by producing their own form of antifreeze.
• Antifreeze proteins (e.g produced by carrots at low temperatures) bind to ice crystals and lower the temperature at which water freezes at, preventing more ice crystals from growing.

Question 3

Plant responses to herbivory - chemical defences

Answer 3

Plants can produce toxic chemicals in response to being eaten:

• Alkaloids: chemicals with bitter tastes, noxious smells or poisonous characteristics that deter or kill herbivores. E.g tobacco plants produce nicotine, which is poisonous to many insects.
• Tannins: chemicals with bitter tastes, and in some herbivores, can bind to proteins in the gut, making the plant hard to digest. These factors deter animals from eating the plant.

Plants can produce signalling chemicals in response to herbivory:

• Pheromones: Signalling chemicals that produce a response in other organisms.
• E.g Some plants release alarm pheromones into the air in response to herbivory. When the pheromones are detected by nearby plants, they start making chemical defences like tannins.
• E.g When corn plants are being eaten by caterpillars, they produce pheromones that attract parasitic wasps. They lay their eggs in the caterpillars, killing them.

Question 4

What is herbivory

Answer 4

When plants are eaten by animals, including insects.

Question 5

Plant responses to herbivory - response to touch

Answer 5

Mimosa pudica:

• If a single leaflet is touched, a signal spreads through the whole leaf, causing it to quickly fold up.
• ideas of protection from herbivory include: helps to knock of small insects feeding on plant, could scare animals trying to eat it.

Question 6

What is a tropism?

Answer 6

A tropism is a response of a plant to a directional stimulus. They respond to it by regulating their growth.

Positive tropism: growth towards the stimulus.
Negative tropism: growth away from the stimulus.

Question 7

Phototropism

Answer 7

• Growth of a plant in response to light.
• Plant shoots are positively phototrophic and grow towards light.
• Roots are negatively phototrophic and grow away from light.

Question 8

Geotropism

Answer 8

• Growth of a plant in response to gravity.
• Plant shoots are negatively geotrophic and grow upwards.
• Roots are positively geotrophic and grow downwards.

Question 9

Other tropisms?

Answer 9

• Hydrotropism: plant growth in response to water. Roots are positively hydrotrophic and grow towards a water source.
• Thermotropism: plant growth in response to temperature.
• Thigmotropism: plant growth in response to contact with an object.

Question 10

Where are growth hormones produced and how do they work?

Answer 10

• Produced in the growing regions of a plant such as shoot tips and root tips.
• They move to where they are need in other parts of the plant.

Question 11

Role of auxins in plants

Answer 11

• cell elongation
• prevents leaf fall
• maintain apical dominance
• involved in tropisms
• stimulates release of ethene
• involved in fruit ripening
• Auxins stimulate growth in shoots, but high concentrations inhibit growth in roots.

Question 12

Role of gibberellins in plants

Answer 12

• stimulates seed germination
• stem elongation
• side shoot formation
• flowering.

Question 13

IAA (indoleacetic acid)

Answer 13

• produced in tips of shoots and roots of flowering plants.
• moves by diffusion and active transport over short distances.
• moves via phloem over long distances.
• Uneven distribution of IAA in different parts of the plant means that there is uneven growth of the plant.

Question 14

IAA and phototropism

Answer 14

IAA move to MORE SHADED PARTS of the shoots and roots, leading to uneven growth (be careful whether the IAA is promoting or inhibiting growth - different for shoots and roots).

Shoots:

• IAA diffuses to more shaded parts of the shoot.
• Growth of cells on this side is promoted and cell elongation causes the shoot to bend and grow towards the light.

Root:

• IAA diffuses to more shaded parts of the root tip.
• Growth of the cells on this side are inhibited, causing the root to bend and grow away from the light.

REMEMBER:

• IAA have opposite effects on shoots and roots.
• High concentrations of IAA promote shoot growth.
• High concentrations of IAA inhibit root growth.

Question 15

IAA and geotropism

Answer 15

IAA move to the UNDERSIDE of shoots and roots, leading to uneven growth (be careful whether IAA is promoting or inhibiting growth - different for shoots and roots).

Shoots:

• IAA diffuses to underside of shoots.
• Growth of cells on this side are promoted and cell elongation causes the shoot to bend and grow upwards.

Roots:

• IAA diffuses to underside of roots.
• Growth of cells on this side are inhibited, causing the root to bend and grow downwards.

Question 16

Effects of IAA on shoots and roots?

Answer 16

• High concentrations of IAA promote shoot growth.

- High concentrations of IAA inhibit root growth.

Question 17

Shoot tip

Answer 17

• Shoot tip is where IAA is produced.
• Shoot tip is the most sensitive to light.
• Due to being the most sensitive to light, covering it will cause the shoot to continue to grow straight up (and not bend towards light), in the presence of a directional light source.

Question 18

Auxins and apical dominance

Answer 18

• Shoot tip of a flowering plant is called apical bud.
• Auxins stimulate growth of apical bud and inhibit growth of side shoots from lateral buds.
• This is apical dominance: apical bud is dominant over the lateral buds.
• apical dominance prevents side shoots from growing.
• saves energy and prevents side shoots from same plant competing with the shoot tip for light.
• Since energy is not used to grow side shoots, apical dominance allows plants in crowded areas to grow tall very fast, past the smaller plants, to reach the sunlight.

Question 19

Effect of removing apical bud and replacing apical bud with a source of auxin?

Answer 19

• Removing apical bud means that the plant stops producing auxin.
• Side shoots will start growing from lateral buds by cell division and elongation (since there is no auxin to inhibit side shoot development).
• If you replace the removed tip with a source of auxin, growth of side shoots from lateral buds (side shoot development) will be inhibited.
• This demonstrates that apical dominance is controlled by auxin.

Question 20

Why do side shoots grow at the bottom of tall plants?

Answer 20

Auxin concentration decreases as you move further away from the apical bud.

If plants grow very tall, the bottom of the plant has low auxin concentration so side shoots will start to grow near the bottom.

Question 21

More on gibberellins

Answer 21

• They are growth hormones produced in young leaves and seeds.
• Stimulates seed germination, stem elongation, side shoot formation, flowering.
• Gibberellins stimulate stem growth by stem elongation which helps plants to grow very tall. If a dwarf variety of a plant is treated with gibberellin, it will grow to the same height as the tall variety.
• Unlike auxins, gibberellins do not inhibit plant growth in any way.
• Gibberellins stimulate seed germination by breaking down starch into glucose in the seed.
• Plant embryo in the seed uses the glucose to begin respiring and release energy needed for it to grow.

Inhibition of gibberellins:

• Gibberellins are inhibited by abscisic acid.
• This means that seed germination is also inhibited by abscisic acid.
• abscisic acid is a hormone.

Question 22

Auxins and Gibberellins working together

Answer 22

Auxins and gibberellins are often synergistic. This means that they work together to have a big effect.

• E.g They work together to help plants grow very tall.

Auxins and gibberellins are sometimes antagonistic. this means that they oppose each other’s actions.

• E.g Auxins inhibit side shoot development / inhibit the growth of side shoots, gibberellins promote side shoot development / stimulate the growth of side shoots.

Question 23

Why do deciduous plants lose their leaves in winter?

Answer 23

• Helps to conserve water (water lost from leaves via transpiration) during cold parts of the year.
• During winter months, it may be difficult to absorb water from soil, as the water may be frozen.
• During winter months, there is less light available for photosynthesis as the days are shorter.
• leaf loss is called abscission.

Question 24

What triggers leaf loss and what controls it?

Answer 24

• Triggered by shortening day length in the autumn.

- Controlled by hormones auxin and ethene.

Question 25

Plant hormones and their roles in leaf loss?

Answer 25

Auxins:

• Auxins inhibit/prevent leaf loss.
• Auxin is produced by young leaves.
• As the leaves get older, less auxin is produced, leading to leaf loss.

Ethene:

• Ethene is produced by ageing leaves.
• As leaves get older, more ethene is produced.
• Ethene stimulates cells in the abscission layer to expand, breaking the cell walls, causing the leaf to fall off.

Question 26

Stomatal closure

Answer 26

• Stomatal closure needs to take place to reduce water loss through transpiration.
• This is done by guard cells.
• When guard cells are full of water, they are turgid and the pore is open.
• When guard cells lose water, they become flaccid and the pores close.

Question 27

What triggers stomatal closure and steps?

Answer 27

Abscisic acid (ABA) triggers stomatal closure.

• ABA binds to receptors on the guard cell membranes.
• This causes specific ion channels to open, allowing Ca2+ ions to enter the cytosol from the vacuole.
• Influx of Ca2+ ions in cytosol causes other ion channels to open, allowing ions, like K+, to leave the guard cells, increasing water potential of the cells.
• Water leaves the guard cells by osmosis.
• The guard cells become flaccid and the stomata close.

Question 28

Commercial uses of ethene?

Answer 28

• Causes fruit to ripen.
• This happens because ethene stimulates enzymes that break down cells walls, break down chlorophyll, and converts starch into sugars.

E.g

• Bananas are harvested and transported before they are ripe.
• They are less likely to be damaged this way.
• They are often exposed to ethene on arrival so that all the bananas ripen at the same time on the shelves and in people’s homes.

Question 29

Commercial uses of auxins?

Answer 29

Auxins are used commercially by farmers and gardeners.

Auxins are used as selective weed killers:

• They make weeds produce long stems instead of lots of leaves.
• Causes weeds to grow too fast, so they cannot get enough water or nutrients, so they die.

Auxins are used as rooting hormones, such as in rooting powders:

• Auxin makes cuttings grow roots.
• The cutting can then be planted and grown into a new plant.
• Many cuttings can be taken from a single plant and be treated with rooting hormones, so lots of the same original plant can be grown quickly and cheaply from just one plant.