RESPONDING TO THE ENVIRONMENT (PLANTS)

Question 1

FUNCTIONS OF THE AUXINS

Answer 1

• stimulate growth
• cause apical dominance
• cause tropism in stems and roots
• stimulate the development of adventitious roots in stem cuttings
• stimulate the development of fruit
• control the abscission of leaves and roots

Question 2

FUNCTIONS OF THE GIBBERELLINS

Answer 2

• stem elongation
• root growth
• the germination of seeds
• promotes lowering

Question 3

FUNCTIONS OF THE ABSCISIC ACID

Answer 3

• is an inhibitor of growth
• causes plants to become dormant in winter
• abscission in leaves and fruit
• lack of water stimulates the production of abscisic acid
• causes the closing of stomata when the plant wilts

Question 4

THE ROLE OF AUXINS IN GEOTROPISM (RESPONSE TO GRAVITATIONAL STIMULUS)

Answer 4

PLANT UPRIGHT POSITION:
* auxin produced at the tip of the root
* this even distribution brings about equal growth
on all sides of the root
* root grows downward and the shoot grows
upwards

PLANT HORIZONTAL POSITION:
* the auxin concentration will be high on the
underside of the root
* more growth occurs on the upper side of the
root because auxins on the lower side inhibit
growth
* root bends downwards and the shoot bends
upwards

Question 5

THE ROLE OF AUXINS IN PHOTOTROPISM (response to light stimulus)

Answer 5

multi-directional light (from all sides):
* auxins move downward evenly
* this even distribution brings about equal growth
on all sides of the stem
* as a result, the stem grows upward

unilateral light (light from one side only):
* the auxin concentration will be high on the
dark side – light destroys auxins
* more growth occurs on the dark side because
auxins stimulate growth on the dark side
* stem bends upwards towards the light

Question 6

THE CONTROL OF WEEDS USING PLANT HORMONES

Answer 6

• some synthetic herbicides contain high concentrations of auxins that accelerate the metabolism of broad-leaved dicotyledonous weeds and therefore stimulate growth
• these weeds grow so fast that their water absorption and food production are insufficient
• the plants weaken and die
• higher concentration of auxins are needed to kill monocotyledons

Question 7

CHEMICAL AND MECHANICAL DEFENCE MECHANISMS IN PLANTS

Answer 7

MECHANICAL:
* thorns discourage herbivores from feeding on plants
* presence of thorns slows the feeding rate of herbivores and wear down their molars over time

CHEMICAL:
* acts as repellents or influence the digestibility of plant material