16.1 Plant Hormones & Growth in Plants

Question 1

What roles do auxins have in plants?

Answer 1

• cell elongation
• prevent abscission
• maintain apical dominance
• stimulate ethene release

Question 2

What roles do gibberellins have in plants?

Answer 2

• stem elongation
• trigger hydrolysis of food stores during germination
• stimulate pollen tube growth at fertilisation

Question 3

What roles does ethene have in plants?

Answer 3

• ripens fruit
• promotes abscission

Question 4

What roles does abscisic acid have in plants?

Answer 4

• maintains seed & bud dormancy
• stimulates cold protective responses
• stimulates stomatal closure

Question 5

What is synergism? (of hormones)

Answer 5

Interaction between 2 hormones

which have similar effects

to give a greater response.

Question 6

What is antagonism? (of hormones)

Answer 6

Interaction between 2 hormones

which have opposite effects.

Overall effect depends on balance.

Question 7

What is a tropism?

Answer 7

Directional growth response by a plant

in response to

a unidirectional stimulus

Question 8

Outline the process of germination:

Answer 8

1. Seed absorbs water, produces gibberellins.
2. Gibberellins stimulate production of enzymes, hydrolysing food stores.
3. Glucose from food stores used to produce ATP by aerobic respiration.
4. ATP used to produce building materials for growth.

Question 9

What plant hormone is antagonistic to gibberellins?

Answer 9

abscisic acid

(gibberellins promote germination, abscisic acid maintains dormancy)

Question 10

How can dwarf species of plants be made?

Answer 10

by interrupting the pathway of gibberellin production

Question 11

What can grafting 2 dwarf plants together do?

Answer 11

(if the 2 plants lack a different step in the gibberellin production pathway…)

Grafting will complete the pathway,

gibberellins can be produced,

leading to growth of internodes/stem.

Question 12

What experimental evidence is there for the role of gibberellins in germination?

Answer 12

• Mutated varieties of seeds, which do not contain the gibberellin gene, do not germinate.
• Applying gibberellin biosynthesis inhibitors prevents germination.

(in both experiments, adding gibberellins externally leads to germination)

Question 13

In what 3 ways does auxin influence plant growth?

Answer 13

• stimulates growth of apical shoot
• inhibits growth of lateral buds
• stimulate root growth @ low concentrations

Question 14

What is apical dominance?

Answer 14

phenomenon where

growth of central stem of plant

is dominant over lateral stems

Question 15

Outline the mechanism by which auxins cause growth of the apical shoot:

Answer 15

1. Auxin molecules bind to specific, complementary receptors on plasma membrane
2. Decreases pH, which increases cell wall flexibility. (optimum pH for enzymes responsible for keeping cell walls flexible)
3. Allows cell to absorb more water by osmosis & expand
4. As cells mature, auxin destroyed, pH rises, cell wall becomes rigid.
   - No further growth

Question 16

What is the effect of changing auxin concentration on different tissues?

Answer 16

a. High concentrations = stimulate apical shoot growth (apical dominance)
b. High concentrations = inhibit lateral shoot growth
c. Low concentration = stimulate root growth

Question 17

Why does lateral shoot growth increase further down the stem?

Answer 17

• Tips of shoot = high auxin concentration
  
  • Inhibits lateral shoot growth
• lower down stem = lower auxin concentration
  
  • more lateral growth, ∵ less inhibition.