6.1.2 Growth Factors in Flowering Plants

Types of Tropisms

1. Phototropism:

• Growth in response to light.
• Shoots: Positively phototropic (grow towards light).
• Roots: Negatively phototropic (grow away from light).

2. Gravitropism (Geotropism):

• Growth in response to gravity.
• Shoots: Negatively gravitropic (grow upwards, against gravity).
• Roots: Positively gravitropic (grow downwards, with gravity).

Role of Growth Factors

• Growth in plants is regulated by growth factors such as indoleacetic acid (IAA), an auxin produced in the shoot and root tips of flowering plants.
• IAA distribution controls tropisms by influencing cell elongation.

Mechanism of Phototropism

3. When a shoot is illuminated equally from all sides:

• IAA is evenly distributed.
• Cells in the zone of elongation grow at the same rate, causing straight growth upwards.

4. When a shoot is illuminated from one side:

• IAA moves to the shaded side of the shoot.
• Cells on the shaded side elongate faster, causing the shoot to bend towards the light.

Mechanism of Gravitropism

5. In roots:

• IAA accumulates on the lower side due to gravity.
• In roots, IAA inhibits growth, so cells on the upper side elongate faster.
• This causes the root to bend downwards, into the soil.

6. In shoots:

• IAA also accumulates on the lower side, but in shoots, IAA promotes growth.
• Cells on the lower side elongate, causing the shoot to bend upwards, against gravity.