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

Video Transcript

In this video, we will learn what a tropism is. We will explore some typical examples of tropisms in plants and how they might be advantageous to these organisms. We will also take a look at some simple experiments that can be conducted to investigate some of these different plant tropisms.

Have you ever seen a young sunflower plant move its stem and flower to track the movement of the Sun across the sky from east to west during the day? This process is called heliotropism, as the prefix helio- comes from the Greek word for Sun and a tropism is a directional growth movement response. Heliotropism is proposed to increase the temperature of flowers in addition to the amount of light they’re exposed to. These changes can increase the young plants’ growth. The increased temperatures further help a mature plant to sexually reproduce with the help of pollinators like bees.

Bees and other insects like to warm themselves in the sun in the cold morning hours. They will therefore be happier to land on and therefore pollinate the flowers that face the morning sun instead of the ones that face away from it in the shade. As a result, plants that face the sun in this manner tend to produce more seeds than the poorly heliotropic plants of the same species. This is just one example of the incredible movements plants can exhibit in response to the world around them.

Tropisms like heliotropism describe a movement either towards or away from a stimulus. Many people might think of plants as static unmoving organisms, but they can actually respond to a huge range of stimuli. Remember, a stimulus is any detectable change in an organism’s internal or external environment that causes an effect in that organism, like a tropism. Some examples of tropic movements in plants can be in response to stimuli like water, light, temperature, and gravity. Some plants can even respond to touch, like this vine that’s wrapping around a stick to steady itself.

Plants move more slowly than we do, however, as their movements require them to grow, compared to the quick muscle contractions that are required for movement in animals like humans. The type of tropism shown by this vine is thigmotropism. The prefix thigmo- means touch, showing that this occurs when the plant grows in response to a touch stimulus, like touching the stick. The main tropisms that we’ll be focusing on in this video are phototropism, which is a response to a light stimulus; hydrotropism, which is a response to a water stimulus; and geotropism, which is sometimes known as gravitropism. And you might deduce that this movement is in response to gravity.

Let’s look at phototropism first. The prefix photo- means light. So phototropisms are growth movements of a plant or another organism towards or away from light. You probably heard the prefix photo- before in words like photosynthesis. Photosynthesis quite literally means making substances or synthesizing them using light. Many photosynthetic organisms like plants will also carry out phototropism. This is because photosynthesis is the process by which plants convert light energy usually from the sun into chemical energy in their cells, often in the form of glucose. This is effectively a photosynthetic organism’s self-made food source.

So, the more they can grow towards light through phototropism, the more photosynthesis they’re able to do and the more glucose they’re able to make to use as a food source. Plants can either store this glucose as starch or they can use it in cellular respiration, where it’s broken down to release energy.

As photosynthesis is the only way in which most plants are able to obtain food, it’s essential that plants are in the presence of light in order to survive and grow. This is why plant stems and leaves are often observed growing towards light via phototropism. This allows the plant’s photosynthesizing cells to absorb more light for photosynthesis.

Let’s look at a commonly accepted mechanism of how phototropism occurs in plant cells. Many plant cells produce substances that control their growth in response to certain stimuli, like light. These are often termed plant growth regulators or sometimes plant hormones. An example of a plant hormone is called auxin. Auxin is produced in the coleoptile. This is a sheath surrounding the shoot tip in the growing regions of plants. 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’s acting and its concentration.

If the light source is directly above the plant shoot, then auxin is produced in the tip of the plant and diffuses down each side of the plant stem equally. This would cause symmetrical elongation in the cells on either side of the stem and would cause the plant stem to grow directly upwards towards the light source. However, if the light source arrives from one side, auxin produced by the shoot tip accumulates in the cells on the shaded side of the shoot. This causes these cells that are not in direct sunlight to elongate comparatively more than the side that is in direct sunlight, which causes the stem to bend in the direction of light.

Different tropisms can either be positive, growing towards a stimulus like light, or they may be negative, growing away from a stimulus like light. The plant stem and leaves tend to be positively phototropic, which means that they grow towards light. But which regions in the plant do you think would be negatively phototropic? And what might be the benefits of this? Some plant roots, especially those of aerial plants, like this spider plant, have been shown to be negatively phototropic, in this case growing downwards away from the source of light that’s directly above the plant.

The directional growth movements that are observed in certain plant roots, like these ones, might result from the fact that plant roots tend to have no chloroplasts in their cells and, therefore, no reason to absorb light as they’re not carrying out photosynthesis. Therefore, it might make more sense for these plant roots to grow downwards away from a source of light. The deeper down that a plant root can grow, the more likely it is to find what it really needs, which is usually water and mineral ions. It’s important to note that phototropism is not the only factor that will affect plant root growth, however. And we will explore these other factors later on in the video.

Let’s look at how auxin influences growth in the roots, which happens in a slightly different way than in the shoots. This plant root is growing close to the soil surface, so it will be receiving some light. And we can see that auxin will sometimes accumulate in the cells in the shaded side of the root. While in the shoot an accumulation of auxin would cause cell elongation in those cells, in the plant root high concentrations of auxin actually inhibit cell elongation. So the cells on the top of the root, which receive comparatively more light and have less auxin, actually elongate far more than the cells in the bottom of the root, where less light is received and cells have accumulated high concentrations of auxin. This asymmetrical cell elongation causes the root to bend and grow downwards away from the direction of light, showing negative phototropism.

A simple experiment can be conducted to investigate phototropism in plant shoots. Two similar plants can be placed in a box each, with one source of light coming from different directions in each box. This describes our independent variable, which is the only factor that we intend to change. It’s important to keep all other factors apart from the direction of light that might affect plant growth the same. These are called control variables. Can you think of what other factors might affect plant growth?

An example could be keeping the temperature the same in the two boxes, or the humidity or water content, and even the light intensity. So we have to be careful that the two bulbs are of the same strength and that they’re the same distance from the box. Observations can then be made regarding the direction of each plant’s growth, which is our dependent variable. Having left our plants for three weeks, let’s make those observations now.

In this plant on the left, where the light was coming from directly above the plant, the plant grows directly upwards, towards the light source. This can be explained by the fact that auxin will have diffused equally down both sides of the plant stem, causing the cells on each side to elongate by the same length. In the plant on the right, the light was approaching from the right-hand side and shining on the cells on the right of the stem. We can observe that the plant stem bends in this case and grows towards the light on the right-hand side. This can be explained as the auxin will accumulate on the left shaded side of the plant, more so than those on the lit right side, causing the shaded cells to elongate more than those on the right and the stem to bend.

Let’s look at hydrotropism next. Hydro- means water, and hydrotropisms are growth movements of an organism like a plant towards or away from water. Water is a reactant in photosynthesis. So, like light, it is vital that a plant can access water in order to make its own food. Water is also a useful medium for transport around a plant, such as of mineral ions. Water is also useful for filling up plant vacuoles to maintain cell shape in a method of physiological plant support. Water is often obtained by plants from soil, absorbed by a process called osmosis into their root cells. Therefore, plant roots need to be positively hydrotropic so that they can grow towards moisture in the soil, where water molecules are in a higher concentration.

Hydrotropism is more challenging to investigate than the other tropisms. Water molecules are often found in higher concentrations deeper in the soil. So hydrotropism is closely tied to and often confused for gravitropism, which is the plant’s response to gravity. The gravitropic response often 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 we 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. The water molecules will leak out of the pores in the pot, making the water potential in the soil to the right of the plant higher. In this case, the plant roots are likely to be observed to grow towards the right, towards the soil with a higher water potential, thereby exhibiting positive hydrotropism.

Let’s look at gravitropism next. Gravi- is a shortened form of gravity. And gravitropism, which is sometimes called geotropism, is a growth response of an organism like a plant towards or away from gravity. Gravitropism is useful for a plant for two main reasons. The pull of gravity, on Earth at least, is always in a downwards direction. The plant shoot is negatively gravitropic, as it grows upwards against the pull of gravity. This means that light, which is usually in an upwards direction, is more accessible for the photosynthesizing parts of the plant, like the shoots and leaves.

The plant roots, however, are positively gravitropic, growing downwards with the pull of gravity. This response means that the plant roots are more likely to come into contact with water and mineral ions, which are generally found deeper in the soil. It might even be helpful with rooting the plant steady in its place. In fact, the gravitropic response is so strong that even when a plant is turned on its side, the shoot bends and grows upwards, showing negative gravitropism, while the roots bend and grow downwards, showing positive gravitropism.

It’s hard to disentangle the effects of the different tropisms. Gravitropisms can also be seen to be influenced by auxin, as when high concentrations of auxin accumulate in the bottom of the root, it inhibits cells’ elongation, while the cells at the top of the root do elongate, causing it to bend downwards. This is the same response that we saw in the phototropism example.

We’ve also already seen this response in the shoot, as auxin accumulates in the shaded bottom side of the shoot, causing these shaded cells to elongate comparatively more than the cells in the lit side of the shoot, causing the shoot to bend upwards. The effect of auxin is the same as is the response. It’s simply the terms that we use to describe the movements in response to light and gravity that are different. So, while the shoot is negatively gravitropic, it’s positively phototropic. And while the root is negatively phototropic, it’s positively gravitropic and often positively hydrotropic too.

Let’s see how much we can remember about plant tropisms by applying our knowledge to a practice question.

Plant shoots are positively phototropic. What does this mean? (A) They grow toward a light stimulus. (B) They grow away from bright sunlight. (C) They reflect the majority of the wavelengths of light. Or (D) they grow toward other brightly colored plants.

When a plant is described as positively phototropic, it means that it’s displaying a type of tropism called phototropism, where the prefix photo- means light. A tropism is a growth movement response. This means that we’re looking for an answer option that’s describing a plant moving by growing towards or away from something. So we can eliminate option (C), as this does not describe a growth movement response.

As the movement response is in response to a light stimulus, we can also eliminate option (D), as this is not talking about the plants growing towards or away from light, rather towards or away from brightly colored plants, which is not a thing that plants usually do. Different parts of a plant can either be positively phototropic, which means that they grow towards a light stimulus, or they can be negatively phototropic, which is growth away from a light stimulus. Plant shoots and leaves contain the majority of the photosynthesizing cells in a plant, which allows plants to make its own food, essential for its survival.

Photosynthesis requires light. So it’s beneficial for the shoots and leaves to grow towards light to access more of it for photosynthesis. Therefore, the shoots could be described as positively phototropic, as they’re growing towards light. The plant’s roots, however, do not require light as they don’t photosynthesize. So they’re negatively phototropic. Therefore, the roots grow downwards away from the source of light and towards the water and minerals in the soil. We’ve deduced that the shoots are positively phototropic, as they grow towards a light stimulus.

Let’s wrap up the video by reviewing the key points that we’ve covered about tropisms in plants. Tropisms are directional growth responses to a stimulus. In plants, these responses might be phototropism, which is a plant’s growth response to light; hydrotropism, which is a growth response to water; or gravitropism, which is a growth response to gravity. The shoots and roots of a plant can respond differently to the same stimuli, influenced by the plant growth hormone called auxin.