14.2 Plant growth factors

Question 1

What are plant growth factors

Answer 1

• They exert their influence by affecting growth and may be made by cells located throughout the plant rather than in particular organs
• Unlike animal hormones, plant growth factors affect all tissues that release them rather than distant organs

Question 2

What is IAA

Answer 2

Stands for indoleacetic acid which is a type of auxin.

Question 3

What is unilateral light

Answer 3

Light coming from one direction, eg we can look at the effect of it coming from a lamp on the growth of a plant.

Shoot grows towards it so it is positive phototropism

Question 4

Describe process of phototropism in flowering plants

Answer 4

. Cells in tip of shoot produce IAA which is transported down the shoot

. The IAA is initially transported throughout all regions as it begins to move down the shoot

. Light causes IAA to move to the shaded side from the light side so theres a high concentration of it that builds up.

. IAA causes elongation of shoot cells so cells on shaded side elongate more.

. This causes the shoot tip to bend towards light

Question 5

How does IAA affect the roots

Answer 5

It inhibits cell elongation in the roots

This means in roots, the cells on the lighter side elongate more than those on the shaded side (where IAA builds up). So they bend down and away from light.

Question 6

Process of gravitropism

Answer 6

. Cells in the tip of root produce IAA which is transported along the root

. It is initially transported to all sides of the root, but then gravity causes it to build up on lower side of root

. This means the cells without IAA will elongate faster than those with IAA as elongation is inhibited there.

. This causes root to bend downwards towards the force of gravity

Question 7

What effects does IAA have on plant cells

Difference between young and old plant cells

Answer 7

• Increases the plasticity (ability to stretch) of young plant cell walls where they can elongate.
• In old plant cells they have greater maturity and rigidity so older parts of plant can’t respond

Question 8

Acid growth hypothesis

Answer 8

Hydrogen ions are actively transported out of cytoplasm to places in cell walls which causes it to become more plastic so allows cell to elongate by expansion.