Lesson Explainer: Plant Tropisms Biology • Second Year of Secondary School

In this explainer, we will learn how to define the term tropism and describe examples of common tropisms found in plants.

Have you ever seen a sunflower track the movement of the sun across the sky? This is just one example of the incredible movements plants can exhibit in response to the world around them. Many people may think of plants as static, unmoving organisms, but they can actually respond to a huge range of stimuli. A stimulus (plural stimuli) is any detectable change in an organism’s internal or external environment that causes an effect in that organism.

Plants move in response to water, to light, to temperature, and to gravity. Some plants even respond to being touched, like the stem of the vine in Figure 1.

Plants move more slowly than we do, however, as their movements require them to grow, while ours simply require a muscle contraction. The movement response of plants to grow toward or away from a stimulus is called a tropism. There are different types of tropism that we will examine in this explainer. The example shown in Figure 1 is called thigmotropism, and it is when a plant grows in response to a touch stimulus, like touching a pole.

The main tropisms that we will be investigating in this explainer are phototropism, hydrotropism, and geotropism, which is sometimes called gravitropism. These are directional plant responses to light, water, and gravity respectively.

Let’s look at phototropism first. “Photo” means light, and phototropisms are growth responses of a plant toward or away from light. You probably heard the word “photo” in several words referring to plants before, such as photosynthesis. The word photosynthesis literally means making substances using light. It is the process by which plants convert light energy, usually from the sun, into chemical energy within their cells. This chemical energy is in the form of glucose, which is effectively the plant’s self-made food source.

Plants can then use this glucose in cellular respiration where it is broken down to release energy. Though they can store energy in the form of starch for short periods of time, photosynthesis is the only way in which most plants are able to obtain food. Therefore, it is essential that plants are in the presence of light in order to survive and grow. This is why plant stems, leaves, and even flowers, are often seen growing toward light, as you can see in Figure 2. This process is called phototropism, and it means that the plant’s photosynthesizing cells can access more light to absorb and increase the rate of photosynthesis.

It is interesting to note that plants can also track the path of the sun as it moves across the sky from East to West during the day. This process is called heliotropism, as the prefix “helio” comes from the Greek word “helios” for “sun.” Heliotropism differs from phototropism where plants move to face a fixed light source, as in heliotropism, the plant moves throughout the day!

A clear example of heliotropism can be observed in young sunflower plants, which move their stem and flowers to face the sun over the course of a single day. This has been proposed to increase the temperature of the flowers and the light they are exposed to. These changes can increase the young plants’ growth and may even make them more attractive to pollinators. In fact, it has been shown that foraging pollinators spend more time visiting heliotropic plants that are better at tracking sunlight, likely as they are able to bask in the warm sunlight while they pollinate the plants. As a result, the poorly heliotropic plants of the same species tend to produce fewer seeds!

Let’s look at a commonly accepted mechanism of how phototropism occurs.

Plants produce substances that control their growth in response to stimuli, often termed plant-growth regulators or plant hormones. An example is a plant hormone called auxin. Auxin is produced in the coleoptile in the growing regions of plants, which is a sheath surrounding the shoot tip. Some evidence also suggests that auxins can be produced from the root tips of plants. The presence of auxin can either stimulate or inhibit cell elongation depending on where in the plant it is acting and its concentration.

In Figure 3, you can see a diagram of a plant stem bending in response to light. You can also see auxin molecules being produced by the tip of the stem and diffusing down the plant from cell to cell. Auxin accumulates on the side of the plant that is not in direct sunlight and causes these cells to grow longer than the cells that are absorbing light. This is called cell elongation, and it means that the shaded side of the plant grows more than the nonshaded side, causing the stem to bend in the direction of light. This is especially interesting to observe when plants are placed in darkness, as they will grow long and spindly in efforts to reach light that, from the point of view of plants, is likely somewhere up above the dark “soil.”

Tropisms can either be negative or positive, growing away from or toward a stimulus. Let’s look at our phototropism example to understand this better. The plant stem is positively phototropic. This means that it grows toward light, as we have seen in Figures 2 and 3. Some plant roots, however, such as the aerial roots of Chlorophytum comosum, which is sometimes called a spider plant, have been shown to be negatively phototropic in certain conditions. You can see the roots of a spider plant growing away from a source of light (above the plant) in the image in Figure 4 below.

This directional root growth that is observed in certain plants may result from the fact that most plant root cells have no chloroplasts and no need to absorb light for photosynthesis. Plant roots, therefore, will sometimes grow downward away from any source of light. The deeper a plant root grows, the more likely it is to encounter what the root needs: water and mineral ions. It is important to note that phototropism is not the only factor affecting plant root growth, however, and we will explore these other factors later on in this explainer.

It is interesting to observe that auxin also works differently in the roots but is still often influencing the direction of root growth. Let’s have a look at an example of how this might occur. In Figure 5, we can see that auxin sometimes accumulates in cells at the bottom of the plant root, away from light. In some plant roots, low concentrations of auxin can actually encourage a little root growth, but in higher concentrations, auxin inhibits cell elongation in the roots. This latter example is visible in Figure 5, as the cells at the bottom with high concentrations of auxin do not elongate. The cells at the top of the root do elongate, however, and this asymmetrical growth causes the root to bend downward, showing it is negatively phototropic.

Example 2: Describing Phototropism in Plant Shoots

Plant shoots are positively phototropic; what does this mean?

1. They grow toward a light stimulus.
2. They grow away from bright sunlight.
3. They reflect the majority of the wavelengths of light.
4. They grow toward other brightly colored plants.

Answer

Plant shoots and leaves contain the majority of the photosynthesizing cells in a plant. Photosynthesis allows a plant to make its own food and is essential for survival. It is a process that requires light, so plant shoots need to grow toward an area where there is a lot of light to carry out photosynthesis efficiently. To do this, a plant shoot tip produces auxin. When light is only coming from one direction, auxin accumulates on the shaded side of the stem, causing these cells to elongate more than the nonshaded side and the shoot to bend toward the light. A shoot is positively phototropic, as it grows toward the light stimulus, as you can see in the figure below.

The roots are negatively phototropic, as they grow away from the light stimulus. High concentrations of auxin inhibit elongation of the cells at the bottom of the root, the opposite of its action in the shoot. This causes the cells at the top of the root to grow comparatively more, and for the root to bend downward away from light.

Our correct answer is therefore option A, they grow toward a light stimulus.

A simple experiment can investigate phototropism in plants. Two plants are placed in a box each, with one source of light coming from different directions, such as in the diagram in Figure 6.

Observations can then be made regarding the direction, if any, that the two plants bend.

On the left in Figure 6, the light is coming from directly above the plant, and we can observe that the plant has grown directly upward. This can be explained by the fact that auxin will have diffused equally down both sides of the plant, causing cells on each side to elongate by the same length.

On the right in Figure 6, the light is coming from the right-hand side, shining on the cells on the right of the stem. We can observe that the plant stem has bent toward the light on the right. This can be explained by the auxin accumulating on the left shaded side, causing those cells to elongate more than those on the right and the stem to bend.

Example 3: Identifying Plant Tropisms in Investigations

The diagram provided demonstrates a basic investigation into plant tropisms. What plant tropism is being displayed positively by the shoots?

1. Hydrotropism
2. Gravitropism
3. Phototropism
4. Thigmotropism

Answer

Phototropism is the response of a plant to grow toward or away from light. Positive phototropism means growing toward light, and negative phototropism means growing away from light. Plant shoots are positively phototropic as you can see from the diagram in this experiment. The experiment shows that, when light is approaching the plant from directly above, auxin produced at the tip of the shoot will diffuse down the plant evenly and cause cell elongation equally on both sides of the shoot. When light approaches just from the left, auxin accumulates on the shaded right side of the shoot. This causes these cells to elongate comparatively more than those on the lit left side. This asymmetrical cell elongation causes the plant to bend toward the light stimulus on the left so that its photosynthesizing cells access a higher light intensity. The opposite can be seen to occur when light approaches from the right as you can see in the diagram below.

Hydrotropism is the response of a plant to grow toward or away from water. Gravitropism is the response of a plant to grow toward or away from gravity. Thigmotropism is the response of a plant to grow toward or away from touch.

Our correct answer is therefore option C, phototropism.

Let’s look at hydrotropism next. “Hydro” means water, and hydrotropisms are growth responses of a plant toward or away from water. Plants require water for photosynthesis and as a medium for transporting mineral ions and for filling vacuoles to maintain cell shape. Water is obtained from the soil, absorbed via osmosis into root hair cells. Therefore, the roots will need to be positively hydrotropic so that they can grow toward moisture in the soil where water molecules are in a higher concentration.

Hydrotropism is more challenging to investigate than the other tropisms, as often a gravitropic response to gravity overpowers the hydrotropic response. There are, however, ways that a plant can reduce its gravitropic responses when its roots are hydrotropically stimulated by a steep water potential gradient in soil.

An example of an experiment you can carry out to investigate hydrotropism in plant roots is to place a plant in soil with a low water potential but with a porous pot filled with water nearby, as you can see in Figure 7. The water will leak out of the pores in the pot, making the water potential in the soil to the right of the plant higher. Figure 7 shows how the plant roots will then be observed to grow to the right, toward the soil with a higher water potential.

Let’s look at gravitropism next. “Gravi” is a shortened version of gravity, and gravitropism, sometimes called geotropism, is a growth response of a plant toward or away from gravity. Gravitropism is useful for two main reasons. The plant shoot is negatively gravitropic, growing upward against the pull of gravity. This means that light, which is typically in an upward direction, is more accessible in the photosynthesizing parts of the plant.

The plant roots are positively gravitropic, growing downward with the pull of gravity. This means they are more likely to come into contact with water and mineral ions deeper in the soil. The gravitropic response is so strong that this will even happen when the plant is turned on its side as you can see in Figure 8. This is an easy experiment you can carry out to observe gravitropism in plants.

Gravitropisms are also influenced by auxin, as the accumulation on the bottom of the root inhibits its growth, causing it to bend downward as we saw with the phototropism example. We have also already observed the response of the shoot by auxin accumulating on the bottom of the shoot, causing these cells to elongate comparatively more and the shoot to bend upward. The effect of auxin is the same, as is the response. It is just the terms we give to movements in response to light or gravity that are different. So, our shoot is positively phototropic and negatively gravitropic, while the root is negatively phototropic and positively hydrotropic and gravitropic.

Let’s recap some of the key points we have covered in this explainer.