Plant Hormones 5.5.3

Question 1

Where do deciduous plants lose their leaves and why?

Answer 1

In very hot and arid environmental conditions, to reduce water loss
They also do this during winter in temperate climates when absorption of water is difficult due to frozen soils

Question 2

Why do plants shed leaves in winter?

Answer 2

Due to photosynthesis being limited by low temperatures and reduced light. Also due to a lack of water from frozen soils

Question 3

What is responsible for the leaf loss above?

Answer 3

Hormones, in response to shortening day length in the autumn

Question 4

What is in the abscission layer?

Answer 4

Parenchyma cells with thin walls, making them weak and easy to break

Question 5

Where is the abscission layer found in plants?

Answer 5

At the base of the leaf stalk

Question 6

What does ethene stimulate in plants?

Answer 6

The breakdown of cell walls in the abscission layer, causing the leaf to drop off.
Also ripening of fruits

Question 7

How to auxins play a role in leaf loss

Answer 7

• Usually auxins inhibit leaf loss and are produced in young leaves, making the stalks insensitive to ethene
• The concentration of auxins decreases as leaves age until leaf loss can once again occur in response to ethene

Question 8

What is produced during times of water stress?

Answer 8

Abscisic acid is produced by plants to stimulate the closing of their stomata

Question 9

What environmental conditions cause water stress?

Answer 9

Very high temperatures or reduced water supplies

Question 10

What do guard cells have on their cell surface membranes?

Answer 10

ABA receptors

Question 11

How does ABA initiate the closing of the stomata

Answer 11

Binds with ABA receptors on guard cells, inhibiting the proton pumps and therefore stopping active transport of hydrogen ions out of the guard cells.

Also causes calcium ions to move into the cytoplasm through the cell surface membranes. These act as second messengers as they cause channel proteins to open that allow negative ions to leave guard cells , this stimulus the opening of further channel proteins to allow K ions to leave the guard cells, the calcium ions also stimulate the closing of channel proteins to allow K ions to enter.

This loss of ions increases the water potential of the guard cells and water leaves the guard cells by osmosis. The guard cells become flaccid, causing the stomata to close

Question 12

What are gibberellins involved in?

Answer 12

Controlling seed germination and stem elongation

Question 13

What state is a seed in when shed from the parent plant

Answer 13

Dormancy (contains very little water and is metabolically inactive)

Question 14

Why are dormant seeds good during germination?

Answer 14

Allows the seed to survive harsh conditions until the conditions are right for successful germination

Question 15

What does a seed contain?

Answer 15

Embyro -> will grow into the new plant when the seed germinates
Endosperm -> a starch containing energy store surrounding the embyro
Aleruone layer -> a protein rich layer on the outer edge of the endosperm

Question 16

What happens when conditions are right for a seed?

Answer 16

Seed starts to absorb water to begin germination
Stimulates the embryo to produce gibberellins
Gibberellin molecules diffuse into the aleurone layer and stimulate cells there to synthesise amylase.
The amylase hydrolyses starch molecules in endosperm, producing soluble maltose molecules
Maltose is converted to glucose and transported to the embryo
This glucose can be respired by the embryo, breaking dormancy and providing the embryo with the energy needed to grow

Question 17

What has the opposite effect to gibberellins?

Answer 17

Abscisic acid, as it maintains dormancy and inhibits amylase production

Question 18

What is the start of germination determined by?

Answer 18

The balance of abscisic acid and gibberellins present in the seed