0610 · 14.5Tropic responses

Tropisms in shoots and roots

0:00Leave a plant on a windowsill for a week, and it starts to lean towards the window. Plant a bean seed upside down, and its root still turns and grows down into the soil, while its shoot turns and grows up. A plant has no nerves, no muscles and no brain. So how does it know which way the light is, or which way is down, and how does it bend towards them?

0:20By the end of this video you will be able to describe gravitropism and phototropism, describe how to investigate them in shoots and roots, and explain how a plant hormone called auxin makes a shoot bend. Plants cannot move from place to place. But they can control the direction they grow in. A response in which part of a plant grows towards or away from a stimulus is called a tropic response, or a tropism.

0:43The stimulus has a direction, like light coming from one side, or gravity pulling down, and the plant grows in a direction that depends on it. Notice that a tropism is a growth response. The plant is not moving like an animal does. It bends because one side of it grows faster than the other. That idea will be the key to the whole video. There are two tropisms you need to know.

1:05The syllabus describes phototropism as a response in which parts of a plant grow towards or away from the direction of the light source. Photo means light. And it describes gravitropism as a response in which parts of a plant grow towards or away from gravity. Growing towards the stimulus is called a positive response. Growing away from it is called a negative response. So let's see how shoots and roots respond.

1:28Shoots grow towards light. So shoots are positively phototropic. That makes sense: the leaves on the shoot need light for photosynthesis, so growing towards the light helps the plant make more food. Shoots also grow away from gravity, upwards. So shoots are negatively gravitropic. Even a seed buried in the dark soil, with no light at all, sends its shoot upwards, towards where the light will be. Roots grow towards gravity, downwards.

1:54So roots are positively gravitropic. Growing down takes the root deeper into the soil, where it can anchor the plant and absorb water and mineral ions. And roots usually grow away from light, so they are negatively phototropic.

Investigating plant tropisms

2:07The syllabus asks you to investigate and describe gravitropism and phototropism in shoots and roots. So let's look at how the experiments are set up. Seedlings are ideal, because they grow quickly, and you can see a response in a few days. Start with phototropism. Take two trays of young seedlings, the same type and the same age, grown in the same soil. Put one tray inside a box with a slit cut in one side, so that light only reaches the seedlings from one direction.

2:34This tray gets one-sided light. The other tray is the control. It gets light evenly from all sides, or from directly above. Keep everything else the same for both trays: the temperature, the amount of water, and the time they are left for. After a few days, the seedlings with one-sided light have shoots that have bent over towards the slit. The control seedlings have grown straight up. The only difference was the direction of the light, so the light from one side must have caused the bending.

3:01Shoots are positively phototropic. Now gravitropism. Take some bean seedlings that have just started to grow a short root and shoot. Pin them to damp cotton wool inside a dish, so they stay moist, and stand the dish on its edge with the seedlings lying sideways, with their roots and shoots pointing horizontally. Keep the dish in the dark. That is important, because it removes light as a stimulus, so any bending must be caused by gravity.

3:26After a day or two, the roots have curved downwards, and the shoots have curved upwards. So roots are positively gravitropic, and shoots are negatively gravitropic. But how do you know it was really gravity that caused this? You need a control, and the usual one uses a clinostat. A clinostat is a device that slowly and steadily rotates the seedlings, round and round. Because the seedlings are always turning, gravity acts on every side of the root and shoot equally.

3:53So gravity has no one direction for the plant to respond to. Seedlings on a clinostat, lying sideways in the dark, keep growing straight out sideways. Their roots do not curve down, and their shoots do not curve up. So the curving in the first dish must have been a response to gravity. A clinostat can be used as a control for phototropism in the same way. If a seedling is rotated while light shines from one side, every side of the shoot gets the light in turn, and the shoot grows straight.

Shoot tips and auxin

4:20One more experiment gives a clue about how this works. Take three seedlings, and shine light on them from one side. Leave the first one as it is. Cover the tip of the second one with a small cap of foil, so that no light reaches its tip. And cut the tip off the third one. Only the first seedling bends towards the light. The seedling with the covered tip, and the one with no tip, both grow straight up.

4:43So the tip of the shoot must be the part that detects the light. But the bending happens further down the shoot, below the tip. So something must travel from the tip down to the part that bends. That something is a chemical. For the extended syllabus, you need to explain phototropism and gravitropism of a shoot as examples of the chemical control of plant growth. The chemical is a plant hormone called auxin.

5:06The syllabus lists four points about how auxin controls shoot growth, so let's go through them in order. One. Auxin is made in the shoot tip. Two. Auxin diffuses through the plant from the shoot tip. It moves down from the tip, into the region just below it, where the cells are growing. Three. Auxin is unequally distributed in response to light and gravity. In other words, light and gravity make auxin collect more on one side of the shoot than the other.

5:34And four. Auxin stimulates cell elongation. Elongation means getting longer. The more auxin the cells receive, the more they elongate.

Auxin and shoot bending

5:44Now let's use those four points to explain phototropism. Light shines on a shoot from one side. Auxin is made in the shoot tip, and it diffuses down the shoot. But because of the light, the auxin is unequally distributed. More auxin collects on the shaded side, the side away from the light. Auxin stimulates cell elongation. So the cells on the shaded side, with more auxin, elongate more than the cells on the light side.

6:09The shaded side grows longer than the light side. And when one side of a shoot grows longer than the other, the shoot bends towards the shorter side, which is the side facing the light. So the shoot bends towards the light. Think of a pair of train tracks going round a bend. The outer rail is longer than the inner rail. In the same way, the longer, shaded side of the shoot is on the outside of the curve, and the shoot curves towards the light.

6:33Gravitropism in a shoot works in exactly the same way, with gravity as the stimulus. Lay a shoot on its side, in the dark. Auxin is made in the tip and diffuses back along the shoot. But gravity makes the auxin unequally distributed, so more auxin collects on the lower side of the shoot. So the cells on the lower side elongate more than the cells on the upper side.

6:54The lower side grows longer, and the shoot curves upwards, away from gravity. That is why a shoot is negatively gravitropic. Notice the pattern in both examples. The stimulus causes an unequal distribution of auxin. More auxin means more cell elongation on that side. And the shoot bends away from the side with more auxin. Let's see how this can be tested. A common question says: a plant on a windowsill bends towards the window.

7:21Explain why. Use the four auxin points, in order. Auxin is made in the shoot tip, and it diffuses down the shoot. Light from one side causes the auxin to be unequally distributed, with more auxin on the shaded side. Auxin stimulates cell elongation, so the cells on the shaded side elongate more. The shaded side grows longer, and the shoot bends towards the light. And to finish, say why this helps: the leaves receive more light for photosynthesis.

7:49Another question might ask you to describe how to show that roots respond to gravity. Describe the seedlings lying sideways on damp cotton wool, kept in the dark to remove light as a variable, with a clinostat as the control. And describe the result: the roots curve downwards, but the roots on the clinostat grow straight.

Common mistakes and recap

8:07Now some common mistakes. First, saying the plant moves towards the light. It does not move. It grows unevenly, and that makes it bend. Second, saying that the cells on the light side grow more. It is the other way round. More auxin collects on the shaded side, so the shaded side elongates more, and the shoot bends towards the light. Third, saying that auxin makes cells divide. For this syllabus, auxin stimulates cell elongation.

8:35Use the word elongation. Fourth, forgetting the dark in a gravitropism experiment. If light is not removed, you cannot be sure whether the plant responded to gravity or to light. And fifth, forgetting the control. Without a clinostat, or seedlings with even light, you cannot show that the direction of the stimulus caused the bending. Let's recap. Phototropism is a response in which parts of a plant grow towards or away from the direction of the light source.

9:04Gravitropism is a response in which parts of a plant grow towards or away from gravity. Shoots grow towards light and away from gravity. Roots grow towards gravity and away from light. You can investigate phototropism with seedlings given one-sided light, and gravitropism with seedlings lying sideways in the dark, using a clinostat, or even light, as a control. In a shoot, both responses are controlled by a chemical, auxin.

9:29Auxin is made in the shoot tip. It diffuses through the plant from the tip. It is unequally distributed in response to light and gravity. And it stimulates cell elongation. So the side of the shoot with more auxin grows longer, and the shoot bends away from that side: towards the light, and away from gravity. So the next time you see a plant leaning towards a window, you will know that it is not moving at all.

9:52It is simply growing a little faster on the side that is in the shade.