Plant Growth (9.3)

Question 1

List the five plant hormones.

Answer 1

1) Auxins
2) Cytokinins
3) Gibberellins
4) Ethylene
5) Abscisic acid (ABA)

Question 2

Give the functions of the five plant hormones (phytohormones).

Answer 2

1) Auxins
Promotes cell elongation and the differentiation of shoots and roots.

2) Cytokinins
Promotes cell division, the opening of stomata and delaying the ageing in leaves.

3) Gibberellins
Triggers the germination of seeds (initiates plant growth).

4) Ethylene
Stimulates maturation and the ripening of fruit. Contributes to the loss of leaves and the death of flowers.

5) Abscisic acid
Functions to inhibit plant growth and development. Controls the closing of stomata and hence regulates water loss (transpiration) in plants.

Question 3

Define tropism.

Answer 3

The growth or turning movement of a plant in response to a directional external stimulus. If it is towards the stimulus, it is said to be positive. If it is away from the stimulus, it is said to be negative.

Question 4

What is auxin?

Answer 4

Auxin is a hormone produced by the tip of a shoot or root (i.e. apical meristems) that regulates plant growth.

Question 5

Describe the mechanism of action of auxin in shoots and roots.

Answer 5

In the shoots, auxin stimulates cell elongation and thus high concentrations of auxin promote growth (cells become larger).

In the roots, auxin inhibits cell elongation and thus high concentrations of auxin limit growth (cells become relatively smaller).

Question 6

How does auxin control the tropic response? Use light as an example.

Answer 6

If the light is shone on a seedling from the side, the auxin will migrate to the dark side, causing the cells there to elongate, therefore the shoot will bend towards the light.

Question 7

What are meristems?

Answer 7

Tissues in a plant consisting of undifferentiated cells capable of indeterminate growth.

Question 8

Discuss the differences between apical meristems and lateral meristems.

Answer 8

Apical meristems occur at shoot and root tips and are responsible for primary growth (i.e. lengthening of the plant). They have high concentrations of auxin so the shoot is stimulated to grow.

Lateral meristems occur at the cambium and are responsible for secondary growth (i.e. widening of the plant). They have a lower concentration of auxin.

Question 9

Describe apical dominance.

Answer 9

The production of auxins prevents growth in lateral buds, a condition known as apical dominance. Apical dominance ensures that a plant will use its energy to grow up towards the light in order to outcompete other plants. However, as the distance between the terminal bud and the lateral bud increases, the inhibition of the lateral bud by auxin diminishes, which produces the triangular shape of trees such as conifers.

Question 10

Define micro-propagation.

Answer 10

A technique used to produce large numbers of identical plants (clones) from a selected stock plant.

Question 11

Describe the process of micro-propagation.

Answer 11

A specific plant tissue (typically the meristem) is selected from a stock plant and sterilised. The explant is grown on a sterile nutrient agar gel. It is then treated with growth hormones (e.g. auxins) to stimulate development. The growing shoots can be continuously divided and separated to form new samples (multiplication phase). Once the root and shoot are developed, the cloned plants are transferred to soil.

Question 12

What is micro-propagation used for?

Answer 12

• Rapid bulking.
• Virus-free strains of plants.
• Propagation of rare or endangered species.