3.3 (Plant responses)

Tropism

The directional growth of a plant in response to an external stimulus. Growth can occur towards (+) or away (-) from the stimulus. It is caused by the unequal distribution of auxin which results in different rates of cell elongation. This response is slow and permanent.

Growth response

A permanent change in the growth or direction of growth of an organism in response to external or internal stimulus

Steps involved in a growth response

1. A receptor detects an environmental stimulus

2. The stimulus causes auxin to be redistributed within the plant

3. More auxin accumulates on one side of the plant.

4. Cells on one side elongate more (or less in roots) than cells on the other side

5. The plant bends towards or away from the stimulus, producing a permanent change in growth direction (tropism)

Phototropism

The directional growth of a plant in response to light. Growth can be towards (+) or away from (-) the light source.

Geotropism (gravitropism)

The directional growth of a plant in response to gravity. Growth can be towards (+) or away from (-) gravity.

Thigmotropism

The directional growth of a plant in response to touch or physical contact. Growth can be towards (+) or away from (-) the stimulus.

Hydrotropism

The directional growth of a plant in response to water. Growth can be towards (+) or away from (-) the stimulus.

Chemotropism

The directional growth of a plant in response to a chemical stimulus. Growth can be towards (+) or away from (-) the stimulus.

Nastic response

The non-directional movement of a plant in response to an external stimulus. The direction of the response is independent to the direction of the stimulus.

Turgor response

A plant response caused by changes in the turgor pressure of cells due to the movement of water into or out of the cells. This response is rapid and reversible.

Turgor pressure

The pressure of cell contents on the cell wall when water enters a plant cell via osmosis.