Auxins

Question 1

Where are auxins produced?

Answer 1

• tip of the roots and shoots
• and in the meristems
• they are transported to surrounding cells and through transport syystems

Question 2

WHat does the effect of auxins depend on?

Answer 2

• conc
• interactions with other hormones

Question 3

What do auxins control the growth of?

Answer 3

the main, apical shoot

Question 4

How do auxins cause cell elongation

Answer 4

• they affect the plasticity of the cell wall
  
  • stretches more easily
• they bind to membrane receptors and cause the pH to fall to about 5
  
  • this is the optimum pH for the enzymes needed to keep the walls very flexible

Question 5

What happens as cells mature? Why does thius occur?

Answer 5

• auxin destroyed
• pH rises
  
  • enzymes maintaining plasticity become inactive
• wall becomes rigid and more fixed in shape and size

Question 6

How does elongation occur>

Answer 6

• zone of cell division (very tip)
  
  • auxins synthesised in meristem cells

zone of elongation:

• auxins diffuse from tip
  
  • bind to receptor sites
• vacuoles form and low pH develops
  
  • cells become flexible
• absorb more water due to expansion
  
  • cell absorb water
  • vacuoles get bigger
  • walls stretch
• large vacuole forms
  
  • auxins destroyed by enzyme
  • walls become rigid
  • no further elongation is possible

Question 7

What is the other effect of high conc of asuxins?

Answer 7

• prevent growth of lateral shoots
• results in apical domibnance

Question 8

How do lateral shoots grow?

Answer 8

• further down the plant the conc of auxins is lower
  
  • the lateral shoots grow better

Question 9

What is the evidence for apical dominance?

Answer 9

• apical shoot removed
  
  • auxin producing cells removed
  • no auxin
• lateral shoots, freed from dominance of apical shoot, grow faster
• if auxin is applied artificially to the cut apical shoot, apical dominance is reasserted and lateral shoot growth is supressed.

Question 10

What do low concentration of auxins promote?

Answer 10

• root growth
• up to a given conc:
  
  • more auxin that reaches root, the more auxin is produced

Question 11

What is the evidence for the auxin conc and root growth?

Answer 11

• auxin produced by root tip also some diffuses from shoot tip
• if shoot is cut the amount of root growth is reduced
• artificial auxin restores the growth of the roots
• high auxin conc inhibit root growth

Question 12

Label the diagram

Answer 12

1. root
2. lateral bud
3. apical bud