14.2 Plant Growth Factors

Question 1

In order to survive what must plants be able to respond to

Answer 1

Changes in both their internal and external environments

Question 2

What do plants respond to

Answer 2

1. Light - Shoots grow towards due to photosynthesis
2. Gravity - Plants must be firmly anchored into soil. Roots grow in the direction of gravity
3. Water - Almost all plant roots grow towards water because it is needed for photosynthesis

Question 3

What do plants responses to external factors involve

Answer 3

Hormone-like substances called Plant Growth Factors

Question 4

Why is the term ‘Plant growth factors’ appropriate

Answer 4

1. They exert their influence by affecting growth and, they may be made by cells throughout the plant
2. Unlike animal hormones, some PGF affect the tissues that release them

Question 5

What kind of quantities are PGF produced in

Answer 5

Small quantities.

Question 6

What is an example of a Plant Growth Factor (PGF)

Answer 6

IAA, Indoleacetic acid

Question 7

What is a tropism

Answer 7

The directional growth of a plant in response to a directional stimulus

Question 8

Describe posititve phototropism in flowering plants in response to unilateral light

Answer 8

1. Cells in the tip of the shoot produce IAA which is transported down the shoot
2. IAA is initially transported evenly throughout all regions as it begins to move down
3. Light causes the transport 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 rather than the light side
5. As IAA causes elongation of shoot cells, the cells on the shaded side elongate more
6. Shaded side elongates more than the light side, causing growth towards the light

Question 9

What is the difference between the roots and shoots in regards to IAA and cell elongation

Answer 9

High IAA concentration PROMOTES cell elongation in the shoots

High IAA concentration INHIBITS cell elongation in the roots

Question 10

Describe positive grapitropism in flowering plants

Answer 10

1. Cells in the tip of the root produce IAA, which is transported along the root
2. The 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 of the root rather than the upper side
5. As IAA inhibits the elongation of root cells and there is a greater concentration on the lower side, the upper side elongates more than the lower
6. The relatively greater elongation of the upper side causes the root to grow downwards by the force of gravity