Topic 9.3 Chemical control in plants

Question 1

Name three groups of plant growth factors

Answer 1

• Auxins
• Cytokinins
• Gibberellins

Question 2

List the functions of auxins

Answer 2

• Involved in trophic responses e.g. IAA
• Control cell elongation
• Suppress lateral buds to maintain apical dominance
• Promote root growth e.g. in rooting powders

Question 3

How do auxins cause cell elongation?

Answer 3

Acid growth hypothesis

1. IAA causes transport of H⁺ ions into cell wall
2. Disruption to hydrogen bonds between cellulose molecules and action of expansins make cell more permeable to water
3. Cells with higher turgor preasure elongate faster

Question 4

How does light effect auxin, how does this effect plant growth?

Answer 4

There will be more auxin on the side with less light, therefore the side with less light will undergo more cell elongation, so the plant will bend towards the light

Question 5

List functions of gibberellins

Answer 5

Stimulate:
* Germination
* Elongation at cell internodes
* Fruit growth
* Rapid growth/ flowering

Question 6

How is germination stimulated?

Answer 6

1. Seed absorbs water, activating embryo to secrete gibberellins
2. Gibberellins diffuse to aleurone layer, which produces amylase
3. Amylase diffuses to endosperm layer to hydrolyse starch
4. Hexose sugars act as respiritory substrate to produce ATP as ‘energy currency’

Question 7

List the functions of cytokinins

Answer 7

• Stimulate development of lateral buds by promoting cell division at apical meristems
• Promote leaf abscission synergyistically with ethene

Question 8

Name the 2 ways plants growth hormones interact

Answer 8

• Synergistically to achieve the same effect e.g. auxins and fiberellins
• Antagonistically with inverse effects e.g. auxins (suppress lateral buds) and cytokinins (stimulate lateral buds)

Question 9

What is phytochrome?

Answer 9

Plant photoreceptor with bilin chromophore group. Converts between 2 forms:
* Biologically inactive Pr absorbs red light
* Biologically active Pfr absorbs far-red light

Question 10

State the time of day when each form of phytochrome is most abundent

Answer 10

In darkness: Pr abundent
In sunlight: Pfr abundent

Ratio of Pr:Pfr enables plant to detect how long days are.

Question 11

How does phytochrome control flowering?

Answer 11

Pr absorbs red light and converts to Pfr , which stimulates flowering. (signifies that light intensity is high enough for photosynthesis).

Question 12

What are long day short day plants?

Answer 12

Long day: flower when sunlight hours exceed a critical value

Short day: flower when darkness hours exceed a critical value

Question 13

What is photomorphogenesis?

Answer 13

Pattern of plant growth and development determined by light intensity.

Question 14

How does phytochrome control photomorphogenesis?

Answer 14

Transition from Pr to Pfr controls: localisation of proteins within cells, transcription of certain genes, phosphorylation of proteins.

Therefore effects: germination, circadian rhythm, flowering.