Plant Responses

Question 1

What is abiotic stress?

Answer 1

A non-living environmental factor that could harm a plant e.g. mineral deficiency, drought, depleted oxygen supply and pollution.

Question 2

How do plants respond to abiotic stress and herbivory?

Answer 2

• May produce antifreeze enzymes
• May contain bitter-tasting tannins
• May contain bitter-tasting nitrogen compounds called alkaloids
• Release cell-signalling pheromones to trigger defensive responses in other organisms.

Question 3

How does Mimosa pudica respond to being touched?

Answer 3

Semimonastic (touch sensitivity) causes the leaves to fold.

Question 4

What is a plant tropism?

Answer 4

The directional growth response of plants:

• Phototropism- response to light
• Geotropism- response to gravity
• Hydrotropism- response to water
• Thermotropism- response to temperature
• Thigmotropism- response to touching a surface or an object

Question 5

How is leaf loss (leaf abscission) in deciduous plants controlled?

Answer 5

1. As the leaf ages, cytokinin and auxin levels lower, the ethene level increases.
2. This triggers the production of cellulase enzymes, which weakened leaves by breaking down cell walls in abscission layer.
3. The leaves break from the branch. Below the abscission layer, suberin layer forms to prevent the entry of pathogens.

Question 6

What are the functions of gibberellins?

Answer 6

• They stimulate germination
• They stimulate elongation at the cell internodes
• They stimulate fruit growth
• They stimulate rapid growth and flowering

Question 7

How is germination stimulated?

Answer 7

1. Seed absorbs water, activating embryo to secret gibberellins.
2. Gibberellins diffuse to aleurone layer, which produces amylase.
3. Amylase diffuses to endosperm layer to hydrolyse starch.
4. Hexose sugars act as respiratory substrate to produce ATP as ‘energy currency’.

Question 8

What are the functions of auxins?

Answer 8

• Involved in trophic responses e.g. IAA.
• Control cell elongation
• Suppress lateral buds to maintain apical dominance.
• Promote root growth e.g. in rooting powders

Question 9

Why do shoots show positive phototropism?

Answer 9

1. Indoleacetic acid (IAA) diffuses to the shaded side of the shoot tip.
2. As IAA diffuses down the shaded side, it causes active transport of hydrogen ions into the cell walls.
3. Disruption to hydrogen bonds between cellulose molecules and action of expansins make the cell more permeable to water (known as the acid growth hypothesis).
4. Cells on the shaded side elongate faster due to higher turgor pressure.
5. The shot bends towards the light.

Question 10

Why do roots show positive gravitropism?

Answer 10

1. Gravity causes IAA to accumulate on the lower side of the root.
2. IAA inhibits elongation of the root cells.
3. Cells on the upper side of the root elongate faster, so the root tip bends downwards.

Question 11

How do hormones stimulate the stomata to close?

Answer 11

1. Abscisic acid binds to the complementary receptors on the guard cell membrane, causing calcium ion channels on the tonoplast to open. Calcium ions diffuse from the vacuole into cytosol.
2. Positive feedback triggers other ion channels to open. Other ions e.g. Potassium diffuse out of the guard cell.
3. Water potential of guard cell becomes more positive. Water diffuses out via osmosis.
4. Guard cells become flaccid so the stomata close.

Question 12

What is apical dominance?

Answer 12

Phenomenon where during the growth of the shoot, the growth side shoots does not take place. Maintained by the action of auxin, abscisic acid and cytokinins.

Question 13

Explain the experimental evidence that auxins maintain apical dominance?

Answer 13

Auxin production in apex maintains high levels of abscisic acid. Inhibits the growth of side shoots.
When the apex is removed:
1. Auxin levels drop, causing abscisic acid levels to drop.
2. Cytokinins (initially concentrated near auxin reserve in bud) diffuse evenly to promote bud growth in
other part of the plant = later buds.

Question 14

Explain the experimental evidence that gibberellins control stem elongation and germination?

Answer 14

Stem elongation- tall plants have higher gibberellin concentration than dwarf plants
Germination- mutant seeds with non-functional gibberellin gene do not germinate unless gibberellin is applied externally. Inhibitors of gibberellin production to prevent germination.

Question 15

How are auxins and cytokinins used commercially?

Answer 15

Auxins- rooting powder, growing seedless fruit, herbicides, low concentrations prevent leaf and fruit growth, high concentrations promote fruit drop.
Cytokinins- prevent yellowing of lettuce leaves, promotes shoot growth

Question 16

How are gibberellins and ethene used commercially?

Answer 16

Gibberellins- delay senescence in citrus, elongation of apples and grape stalks, brewing beer for malt production, increase sugar cane yield, sped up seed formation in conifers and to prevent lodging.
Ethene- speeds up ripening, promotes lateral growth, promotes fruit grop