Types of Responses in Plants

Tropisms

A tropism is a growth response in which the direction of growth is determined by the direction of the stimulus. Growth towards a stimulus is called a positive tropism, and growth away from it is a negative tropism.

Because tropisms involve growth, they are usually slow and permanent.

• Phototropism, growth in response to light; a shoot is positively phototropic, growing towards light, while a root is usually negatively phototropic.
• Geotropism (gravitropism), growth in response to gravity; a root is positively geotropic, growing downward, while a shoot is negatively geotropic, growing upward.
• Hydrotropism, growth of roots towards a source of water.
• Thigmotropism, growth in response to touch, such as a climbing plant's tendril coiling around a support.

Nastic movements

A nastic movement is a response in which the direction of movement does not depend on the direction of the stimulus; instead it is determined by the plant's own structure. Nastic movements are usually rapid and often reversible, caused by changes in turgor pressure in specific cells rather than by growth.

• The rapid folding of Mimosa pudica leaves when touched (a response to touch, but the folding direction is fixed, not directional).
• The opening and closing of stomata in response to light and water status.
• The closing of a Venus flytrap's leaves when trigger hairs are touched.

Comparing tropisms and nastic movements

How it is examined

You may be asked to classify a described plant response as a tropism or a nastic movement and justify your answer, to name the specific type of tropism shown in an experiment, or to explain the difference in cause between growth-based tropisms and turgor-based nastic movements.

Worked exam-style question

Question. A germinating seedling was placed horizontally in a dark box with a single small hole on one side, and light was allowed to enter only through this hole. After three days, the shoot had curved and grown out through the hole, while the root had curved and grown downward into the soil.

(a) Name the type of response shown by the shoot, stating whether it is positive or negative. (b) Name the type of response shown by the root, stating whether it is positive or negative.

(c) Explain, in terms of a plant hormone, why the shoot curved towards the hole. (d) Suggest one advantage to the seedling of the root growing downward regardless of where light enters the box.

Model answer. (a) The shoot shows positive phototropism, growth towards the direction of light. (b) The root shows positive geotropism (gravitropism), growth in the direction of gravity, downward.

(c) Auxin accumulates on the shaded side of the shoot, and because auxin stimulates cell elongation in shoot cells, the shaded side elongates faster than the illuminated side, causing the shoot to bend towards the light. (d) Growing downward regardless of light direction anchors the seedling in the soil and directs the root towards a reliable supply of water and mineral salts, which do not depend on where light happens to enter.

Practice question

Try this. A gardener notices that the tendrils of a pea plant coil tightly around a stake within a few hours of touching it, while the plant's shoot takes several days to bend visibly towards a nearby window. Suggest why the tendril's response is so much faster than the shoot's response to light.

Exam tip

Key terms

These Chapter 20 terms name the responses and the hormone examined in this topic:

• Tropism, a plant growth response in which the direction of growth is determined by the direction of the stimulus.
• Phototropism, growth of a plant part towards or away from light.
• Geotropism, growth of a plant part towards or away from the direction of gravity.
• Auxin, a plant hormone that stimulates cell elongation and is redistributed unevenly to produce tropic curvature.

Source:SRC-DSKP-EN

Frequently asked questions

More for Responses in Plants