5.4 - Plant Hormones

What is a tropism?

The response of a plant to a specific stimulus.

What is phototropism?

The response of a plant’s shoot or root to light.

What is gravitropism?

The response of a plant’s shoot or root to gravity.

What is auxin?

A hormone which controls the growth of a plant’s shoots and roots. When it’s unevenly distributed in a growing plant, the rate of growth will also be unequal.

How do plant shoots show positive phototropism?

• One side of the shoot is in the light, causing auxin to move to the shaded side.

• At the shaded side, the cells are stimulated to grow - the shoot bends towards the light.

• As a result, photosynthesis can occur at a faster rate.

How do plant shoots show negative gravitropism?

• In a horizontal shoot, auxin accumulates in the lower side due to gravity.

• Cells on the lower side of the shoot grow more, causing the shoot to bend away from the direction of gravity.

• As a result, more light is usually available for photosynthesis.

How do plant roots show positive gravitropism?

• In a horizontal root, auxin accumulates in the lower side, causing cells to grow less.

• The root bends in the direction of gravity.

• As a result, more water and nutrients are available to the plant roots.

What are gibberellins?

A type of plant hormone involved in seed germination.

What is ethene?

A type of plant hormone involved in cell division and ripening.

What are the uses of auxins?

• Weedkillers - auxin causes cells to grow at a rapid rate, causing plant death.

• Rooting powder - auxin causes new plants to grow very quickly.

• Tissue culture - auxin promotes growth of roots and shoots.

What are the uses of ethene?

Control of food ripening in the food industry - allows fruit to be ripened just before they are sold.

What are the uses of gibberellins?

• Termination of seed dormancy.

• Promotion of flowering.

• Increase of fruit size.