Gravitropism

Gravitropism is a directional growth response to gravity. Roots and shoots respond in opposite directions: roots grow towards gravity, while shoots grow away from it.

When a root or shoot is placed horizontally, the plant hormone IAA becomes more concentrated on its lower side. This creates unequal growth between the lower and upper sides. The difference in elongation bends the root or shoot, but the direction of the bend depends on how its cells respond to IAA.

Gravitropism in shoots

In a shoot, the higher IAA concentration promotes greater cell elongation on the lower side. The lower side grows longer than the upper side, bending the shoot upwards, away from gravity. Growth away from gravity is called negative gravitropism.

Gravitropism in roots

In a root, the higher IAA concentration inhibits cell elongation on the lower side. The upper side elongates more than the lower side, bending the root downwards, towards gravity. Growth towards gravity is called positive gravitropism.

IAA and plant cell elongation

Indoleacetic acid (IAA) is a plant growth factor produced in growing regions of a plant. It is transported to other tissues, where it can affect how much cells elongate. As cells elongate, they become longer, contributing to the growth of the tissue.

IAA concentration and tissue response

The effect of IAA depends on both its concentration and the tissue responding. Roots are more sensitive to IAA than shoots: a concentration that promotes cell elongation in a shoot can inhibit cell elongation in a root.

IAA and bending

If IAA is distributed unevenly across a growing organ, cells on its two sides may elongate at different rates. The side that elongates more grows faster, causing the organ to bend.

For example, where a higher IAA concentration promotes elongation on one side of a shoot, that side grows faster and the shoot bends towards the other side. In a root, a higher concentration can inhibit elongation on one side. The opposite side then grows faster, bending the root towards the side with more IAA.

Phototropism

Plants can change their direction of growth in response to light. This response is called phototropism. Growth towards light is positive phototropism; growth away from light is negative phototropism.

Phototropism in shoots

When light reaches a shoot from one side, the plant growth substance IAA becomes unevenly distributed, with more IAA on the shaded side. In a shoot, a higher IAA concentration promotes greater cell elongation. Cells on the shaded side therefore elongate more than those on the illuminated side.

As the shaded side grows faster, the shoot bends towards the light. This is positive phototropism.

Phototropism in roots

Roots can show negative phototropism, growing away from light. In a simple IAA-based explanation, more IAA on the shaded side inhibits cell elongation there relative to the illuminated side. The illuminated side therefore elongates more, causing the root to bend away from the light.

The same uneven distribution of IAA can thus produce opposite bending responses because the higher IAA concentration promotes elongation in shoots but inhibits it in roots.