16.1 - Plant Hormones And Growth In Plants

Question 1

Tropism

Answer 1

Directional growth in response to environmental cues, eg light and gravity

Question 2

Function of auxins

Answer 2

Control cell elongation
Prevent leaf fall
Maintain apical dominance
Cause tropisms
Stimulate release of ethene
Involved in fruit ripening

Question 3

Function of gibberellins

Answer 3

Cause stem elongation
Trigger mobilisation of food stores in a seed at germination
Stimulate pollen tube growth in fertilisation

Question 4

Function of ethene

Answer 4

Causes fruit ripening
Promotes abscission in deciduous trees

Question 5

Function of ABA

Answer 5

Maintains dormancy of seeds and buds
Stimulates cold protective responses (antifreeze)
Stimulates stomatal closing

Question 6

Seed germination

Answer 6

When the seed absorbs water, embryo is activated and gibberellin production begins
Gibberellins stimulate production of enzymes to break down food stores in the seed
Food stores are used to produce ATP
Gibberellins switch on genes to code for amylases and proteases
ABA is an antagonist to gibberellins - balance of both triggers germination

Question 7

Where are food stores found in dicots?

Answer 7

Cotyledons

Question 8

Where are food stores found in monocots?

Answer 8

Endosperm

Question 9

Experimental evidence for gibberellins

Answer 9

Mutant seeds without gene coding for gibberellins do not germinate until it is added externally

Question 10

Experimental evidence for gibberellins

Answer 10

Mutant seeds without gene coding for gibberellins do not germinate until it is added externally
If gibberellin biosynthesis inhibitors are applied to the seed, they do not germinate

Question 11

Auxins and plant growth

Answer 11

Stimulate growth of main apical shoot
Auxins affect plasticity of the cell wall
Auxins bind to specific receptor sites in the plant cell membrane, causing pH to fall to 5
This is optimum pH for enzymes that keep the cell wall flexible and plastic
As cells mature, auxins are destroyed, pH rises and enzymes for plasticity become inactive
Therefore cell wall becomes more rigid and cells can no longer grow or expand

Question 12

Apical dominance

Answer 12

High concentrations of auxin causes main shoot to grow quickly and the lateral shoots are inhibited

Question 13

Root growth

Answer 13

Low concentrations of auxin promote root growth
Auxins are produced by the root tips and also reaches the roots in low concentrations from growing shoots
If apical shoot is removed, auxins reaching roots is greatly reduced

Question 14

Synergism

Answer 14

Different hormones working together, complementing each other and giving a greater response together