Response and Coordination
Senior 3 · Biology
Response and Coordination
Lesson 1 of 4
25 min
Brush your finger along a leaf of the sensitive plant, Mimosa pudica, a common weed of warm tropical places, and within a second or two its leaflets fold together in pairs and the whole leaf droops. Wait ten or twenty minutes and it opens again. A bean shoot growing towards a window also responds to its surroundings, but it takes hours to bend, and it never bends back. In Senior 1 you met tropisms. This lesson asks how a plant controls a tropism, why the same chemical makes a shoot bend one way and a root the other, and why the sensitive plant's movement is a different kind of response altogether.
Definition
Stimulus (plural stimuli): a change in the surroundings, or inside the body, that an organism can detect, such as light, gravity, water, touch or a chemical.
Tropism: a growth movement of part of a plant in response to a stimulus that comes from one direction. The direction of the growth depends on the direction of the stimulus. Growth towards the stimulus is positive; growth away from it is negative. Example: a shoot growing towards light shows positive phototropism.
Nastic response (nastic movement): a response of a plant to a stimulus in which the direction of the movement does not depend on the direction of the stimulus. Example: the leaflets of Mimosa pudica fold the same way whichever side they are touched from.
Auxin: a plant hormone made in the tips of shoots and roots that controls how fast cells just behind the tip elongate (grow longer). The main natural auxin is indole-3-acetic acid, IAA.
A tropism is named after its stimulus. The table brings together what you learnt in Senior 1 and adds aerotropism.
| Tropism | Stimulus | Usual response of shoots | Usual response of roots | Why it helps the plant |
|---|---|---|---|---|
| Phototropism | Light from one side | Positive: grow towards light | Negative or none | Leaves get light for photosynthesis |
| Geotropism (gravitropism) | Gravity | Negative: grow upwards | Positive: grow downwards | A buried seedling's shoot reaches the air; its root reaches water and anchors it |
| Hydrotropism | Water (moisture gradient) | None | Positive: grow towards damper soil | Roots find water |
| Thigmotropism | Touch, contact with a solid | Tendrils and twining stems coil round a support | Roots grow round stones | Climbers such as beans and passion fruit climb to the light |
| Aerotropism | Oxygen (air) | Little studied | Reported to grow towards better-aerated soil | Root cells need oxygen to respire |
Aerotropism is the least studied of the five, and you will not be asked for more than its meaning and the example in the table.
Auxin is made in the tip and moves back, by diffusion and by active transport from cell to cell, into the region just behind the tip, the zone of elongation. There it makes the cell walls stretch more easily, so the cells take in water and grow longer.
Look at Figure 1 again. In both organs auxin collects on the lower side, yet the shoot bends up and the root bends down. The reason is that root cells are far more sensitive to auxin than stem cells. A concentration that speeds up the elongation of stem cells is high enough to slow down the elongation of root cells. So the lower side of the shoot grows faster (the shoot turns up) while the lower side of the root grows more slowly than its upper side (the root turns down). Figure 2 shows this as two curves.
Figure 2: typical curves. The exact concentrations differ between species, but the pattern does not: roots are far more sensitive to auxin than stems. The stem curve is dashed so the two can be told apart without colour.
Chemists have made substances that act like auxin. Gardeners dip the cut ends of stem cuttings in rooting powder, which contains a synthetic auxin and makes roots form sooner. Some synthetic auxins, such as 2,4-D, are used as selective weedkillers: sprayed at the right dose on a cereal field, they make broad-leaved weeds grow so fast and so abnormally that they die, while narrow-leaved cereals such as maize, rice and wheat are much less affected. Like every pesticide, they must be used only as the label directs (Unit 4).
Warning
Safety in the practical below. Seeds bought for planting are often coated with a fungicide, which is poisonous: never eat seeds used in experiments, handle them with clean hands and wash your hands afterwards. Use a waterproof pen or a thread dipped in ink, not a sharp point, to mark the roots, and tell your teacher straight away if anything is spilled or broken.
Worked example 1: where does a root grow?
A student germinates a bean seed on damp paper towel in a jar. When the radicle (young root) is about 20 mm long, she marks it with ink lines every 2 mm from the tip upwards. After 24 hours the gaps between the lines are as follows (illustrative data):
| Gap (counting from the tip) | 1st | 2nd | 3rd | 4th | 5th | 6th |
|---|---|---|---|---|---|---|
| Length after 24 h (mm) | 3 | 7 | 5 | 2 | 2 | 2 |
Worked example 2: auxin in agar blocks
In the 1920s the Dutch scientist Frits Went cut the tips off oat coleoptiles (the sheath round a young grass shoot) and stood them on small blocks of agar jelly, so that auxin diffused out of the tips into the agar. He then placed each block on one side of a decapitated coleoptile, in the dark, and measured how far it curved. A class repeats the idea (illustrative data):
| Number of tips that stood on the block | 0 (plain agar) | 1 | 2 | 4 | 8 |
|---|---|---|---|---|---|
| Angle of curvature after 2 h (°) | 0 | 6 | 11 | 20 | 23 |
A nastic response is usually much faster than a tropism, is not a growth response in the commonest examples, and its direction is fixed by the structure of the plant, not by where the stimulus comes from. In Mimosa pudica there is a swelling, the pulvinus, at the base of each leaf and leaflet. When the leaf is touched, cells on one side of the pulvinus quickly lose water and become flaccid, so the leaf folds or droops. When they take water back in, the leaf reopens. A response to touch of this kind is called thigmonasty (or seismonasty when caused by shaking).
| Feature | Tropism | Nastic response |
|---|---|---|
| Direction of the response | Depends on the direction of the stimulus | Does not depend on the direction of the stimulus |
| Usual mechanism | Unequal growth, controlled by auxin | Often changes in turgor of special cells; some (e.g. flower opening) are growth |
| Speed | Slow: hours to days | Often fast: seconds to minutes (Mimosa) |
| Reversible? | No: growth is permanent | Turgor movements reverse when the cells refill with water |
| Example | Shoot bending towards a window | Mimosa leaflets folding when touched |
Warning
Common mistakes. (1) Writing that light "destroys" auxin on the lit side: in the accepted explanation auxin is moved to the shaded side. (2) Writing that the shoot bends because the lit side grows faster: it is the shaded side that grows faster. (3) Saying a root bends down because auxin "pulls" it: the lower side is inhibited, so the upper side, growing faster, curves the tip down. (4) Calling the folding of Mimosa a tropism: it is a nastic response, because its direction does not depend on where it is touched.
PLANT GROWTH FACTORS: IAA and tropisms, Miss Estruch Biology. Watch for how IAA makes shoot cells elongate but inhibits root cells, and compare it with Figures 1 and 2. The video is pitched slightly above O-level; the core idea is the same.
Tip
Memory hook: "stems are slow to be stopped, roots are quick to be stopped". Roots respond to tiny amounts of auxin and are inhibited by levels that still speed up stems. So in a horizontal plant, the extra auxin on the lower side helps the stem and holds back the root.
Practice: plant responses
Key points
Sources and credits
Written by Ganzaa: the text, Figures 1 and 2 (typical curves), the tables, the worked examples (illustrative data) and the practice questions. This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
