3- chemical control in plants

Name 3 groups of plant growth factors (types of).

• Auxins

• Cytokinins

• Gibberellins

What are the functions of auxins?

• 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

How do auxins cause cell elongation?

Acid growth hypothesis

1. IAA causes active transport of H⁺ ions into cell wall.

2. Disruption to H-bonds between cellulose molecules and action of expansins make cell more permeable to water.

3. Cells with higher turgor pressure elongate faster.

What are the functions of gibberellins?

Stimulates:

• Germination

• Elongation at cell internodes

• Fruit growth

• Rapid growth/ flowering

How is germination stimulated?

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 respiratory substrate to produce ATP as ‘energy currency’.

What are the functions of cytokinins?

• Stimulate the development of lateral buds by promoting cell division at apical meristems.

• Promote leaf abscission synergistically with ethene.

What are the two ways plant growth hormones interact?

• Synergistically to achieve the same effect e.g. auxins and gibberellins.

• Antagonistically with inverse effects e.g. auxins (suppress lateral buds) and cytokinins (stimulate lateral buds).

What is phytochrome?

Plant photoreceptor with bilin chromophore group. Converts between 2 forms:

• Biologically inactive Pr absorbs red light

• Biologically active Pfr absorbs far-red light

When is each form of phytochrome most abundant?

In darkness: Pr is abundant

In sunlight: Pfr is abundant

The ration of Pr: Pfr enables plants to detect how long days are, so they know when to start flowering.

How does phytochrome control flowering?

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

What are long-day plants?

Plants which flower when the days are long and nights are short (when sunlight hours exceed a critical value).

What are short-day plants?

Plants which flower when the days are short and the nights are long (when darkness hours exceed a critical value).

What is photomorphogenesis?

Pattern of plant growth and development determined by light intensity.

How does phytochrome control photomorphogenesis?

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

Therefore affects: germination, circadian rhythm, flowering.