The Effects Of Plant Horomes

Question 1

Describe and explain what apical dominance is

Answer 1

The shoot tip at the top of a flowing plant is called the apical bud.
Auxins stimulate growth of the apical bud and inhibit the growth of side shoots from lateral buds.
This is called apical dominance - the apical bud is dominant over the lateral buds

• apical dominance prevents side shoots from growing - this saves energy and prevents side shoots from the same plant competing with the shoot tip for light
• apical dominance allows plants to grow tall very fast to reach sunlight
• if you remove the apical bud then the plant wont produce auxins, so the side shoots will starts growing by cell division and cell elongation
  However if you replace apical bud with source of auxin, side shoot development is inhibited. This shows apical dominance is controlled by auxin

Question 2

Describe and explain role of gibberellins

Answer 2

• growth hormones produces in young leaves and seeds
• they stimulate stem elongation, seed germination, side shoot formation and flowering
• unlike auxins, gibberellins don’t inhibit plant growth in anyway
• gibberellins stimulate seed germination by triggering the breakdown of starch into glucose in the seed. The plant embryo in the seed can then use the glucose to begin respiring and release the energy it needs to grow.

Gibberellins are inhibited (and so seed germination is prevented) by the hormone abscisic acid.

Question 3

Describe and explain how auxins and gibberellins work together

Answer 3

They are often synergistic - they work together to have big effect
E.g. auxins and gibberellins work together to help plants grow very tall

They are also sometimes antagonistic - this means they oppose each other’s actions
Gibberellins stimulate the growth of side shoots but auxins inhibit the growth of side shoots

Question 4

Describe and explain leaf loss in deciduous plants

Answer 4

• deciduous plants are plants that lose their leaves in winter - this helps conserve water (when cold it may be difficult to absorb water from soil)

Leaf loss is triggered by shortening day length in the autumn and is controlled by hormones:

-auxins inhibit leaf loss - auxins are produced by young leaves, as leaves get older less auxin is produced leading to leaf loss
- ethene stimulates leaf loss - ethene is produced by ageing leaves. A layer of cells (called the abscission layer) develops at the bottom of the leaf stalk. The abscission layer separates the leaf from the rest of the plant.
Ethene stimulates the cells in the abscission layer to expand, breaking the cell walls and causing the leaf to fall off

Auxins are antagonistic to ethene

Question 5

Describe and explain the role of abscisic acid (ABA) in stomata closure

Answer 5

The opening and the closing of the stomatal pore is controlled by guard cells. When guard cells are full of water, they are plump and turgid causing the pore to be open. When guard cells lose water, they become flaccid causing the pore to close.

ABA is able to trigger stomatal closure:

1) ABA binds to receptors on the guard cell membranes. This causes specific ion channels to open, which allows calcium ions to enter the cytosol from the vacuole
2) the increased conc of calcium ions in the cytosol causes other ion channels to open. These ion channels allow other ions (such as K+) to leave the guard cells raising the water potential of the cells
3) water then leaves the guard cells by osmosis
4) the guard cells become flaccid and the stomata close, resulting in reduced water loss by transpiration

Question 6

Commercial use of ethene?

Answer 6

• fruit ripening
  Ethene stimulates enzymes that break down cell walls, break down chlorophyll and convert starch into sugars. This makes fruit soft, ripe and ready to eat

Question 7

Commercial uses of auxins

Answer 7

• weed killer (make weeds grow too fast, so cant get enough water and they die)
• used as rooting powder for plant cuttings
  Production of seedless fruit

Question 8

Commercial uses of gibberellins

Answer 8

• delay ripening of fruit