14.2: Plant Growth Factors

Question 1

What external factors do plants respond to

Answer 1

Light - shoots grow towards

Gravity - Plants need to be firmly anchored to the soil and grow in the direction of its pull

Water - All plant roots grow towards water. For use during photosynthesis

Question 2

What do plant responses involve

Answer 2

Plant Growth Factors

Question 3

Why is the term ‘Growth factors’ used

Answer 3

1. They exert their influence by affecting growth and, they may be made by cells located throughout the plant rather than in particular organs
2. Unlike animal hormones, some plant growth factors affect the tissues that release them

Question 4

What is an example of a plant growth factor

Answer 4

Indoleacetic acid (IAA)

Question 5

Phototropism in Flowering plants

Answer 5

1. Cells in the tip of the shoot produce IAA and this is transported down the shoot
2. The IAA is initially transported evenly throughout all regions as it moves down the shoot
3. Light causes the movement of IAA from the light side to the shaded side
4. A greater concentration of IAA builds up on the shaded side of the shoot
5. IAA causes the elongation of shoot cells, there is a greater concentration of IAA on the shaded side of the shoot, therefore the shaded side elongates more
6. The shaded side elongates more than the light causing the shoot tip to bend towards the light

Question 6

Gravitropism in flowering plants

Answer 6

1. Cells in the tip of the root produce IAA, which is then transported along the root
2. IAA is initially transported to all sides of the root
3. Gravity influences the movement of IAA from the upper side to the lower side of the root
4. A greater concentration of IAA builds up on the lower side than the upper
5. IAA inhibits the elongation of root cells, therefore the cells on the upper side elongate more than the lower side
6. The relatively greater elongation of cells on the upper side compared to the lower side causes the root to bend downwards towards the force of gravity.