Gravitropism

Plants can change the direction in which they grow in response to gravity. This directional growth response is called gravitropism.

Growth towards gravity is called positive gravitropism. Roots generally show this response, growing downwards. Growth away from gravity is called negative gravitropism. Shoots generally show this response, growing upwards.

Investigating gravitropism in seedlings

Seedlings can be placed horizontally under uniform lighting and observed as they continue to grow. Uniform lighting helps prevent a difference in light direction from influencing the direction of growth.

Over time, new growth reveals the responses: roots generally bend downwards, towards gravity, while shoots generally bend upwards, away from gravity. The bending shows a change in the direction of growth, rather than movement of the whole seedling.

Phototropism

Plants can change the direction in which they grow in response to light. This directional growth is called phototropism.

Growth towards a light source is positive phototropism. Shoots generally show positive phototropism, growing towards light. Growth away from a light source is negative phototropism; roots can show this response.

Investigating phototropism

Seedlings can be illuminated from one side and their new shoot and root growth observed over time. The direction of new growth can then be compared with the position of the light source: growth towards it is positive phototropism, while growth away from it is negative phototropism.

The seedlings’ orientation relative to gravity should be kept unchanged during the investigation. This helps distinguish growth in response to light from growth in response to gravity.

Auxin control of shoot tropisms

A shoot changes its direction of growth when cells on one side elongate more than cells on the other. This unequal growth is controlled by auxin, a plant growth chemical made in the shoot tip. Auxin diffuses through the plant from the tip and stimulates shoot cells to elongate.

Light and gravity can cause auxin to become unevenly distributed across a shoot. The side with more auxin elongates more, making the shoot bend towards the side that elongates less.

Shoot growth towards light

When light reaches a shoot from one direction, more auxin accumulates on the shaded side. Cells on that side elongate more than cells on the illuminated side, so the shoot bends towards the light. This growth response is called positive phototropism.

Shoot growth away from gravity

When a shoot is horizontal, more auxin accumulates on its lower side. Cells on the lower side elongate more than those on the upper side, causing the shoot to bend upwards, away from the direction of gravity. This growth response is called negative gravitropism.