5.5.3 Plant Hormones

Question 1

What are deciduous plants?

Answer 1

Plants which lose their leaves in hot and dry environmental conditions in order to reduce water loss.

Also during winter when absorption of water is difficult due to frozen soil, and because photosynthesis is limited by low temps and reduced light.

Question 2

What is the abscission layer?

Answer 2

• Develops at base of leaf stalk
• Layer of parenchyma cells with thin walls, so weak and easy to break

Question 3

How do hormones make leaves fall off?

Answer 3

• Ethene stimulates breakdown of cell walls in abscission layer
• Auxin concentration decreases as the leaf ages (auxins inhibit leaf loss, produced in young leaves making them insensitive to ethene)

Question 4

How and why do hormones cause stomatal closure?

Answer 4

• When there is water stress (high temperatures, reduced water supply)
• ABA (abscisic acid) is produced
• Closes the stomata
• ABA binds to ABA receptors on guard cell surface membrane
• Proton pump is inhibited, so active transport of H+ out of guard cells is stopped
• ABA causes Ca2+ ions move into cytoplasm of guard cells through cell surface membrane
• Ca2+ act as second messengers
• Cause channel proteins to open so negatively charged ions leave guard cell
• This stimulates opening of further channel proteins allowing K+ to leave guard cell
• Calcium ions stimulate closing of channel proteins meaning K+ cannot enter
• Loss of ions increases water potential of guard cells
• So water leaves guard cells by osmosis
• Guard cell becomes flaccid and closes

Question 5

What are the roles of giberellin?

Answer 5

• Seed germination
• Stem elongation

Question 6

What do seeds contain?

Answer 6

• Embryo - plant grows from here
• Endosperm - starch containing energy store around embryo
• Aleurone layer - protein rich layer on outer edge of endosperm

Question 7

How does gibberellin work in seed germination?

Answer 7

• Seed starts to absorb water
• Stimulates embryo to produce gibberellins
• Diffuses into aleurone layer stimulating cells to synthesise amylase
• Amylase hydrolyses starch stores in endosperm
• Maltose is produced
• Maltose is converted to glucose
• Transported to embryo
• Glucose is respired by embryo, breaking dormancy and providing embryo with energy needed for it to grow

Question 8

What effect does abscisic acid on seed germination?

Answer 8

• Maintains dormancy by inhibiting amylase production
• Opposite effect to gibberellins
• Start of germination determined by balance of abscisic acid and gibberellins present in seed