Tropism

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An introduction to Tropism

Most of the time when we think about a living organism the first thought our mind gives birth to is the idea of movement. If something (the biotic object) is performing an act of movement it is pronounced living. All living beings exhibit some sort of motion. This motion or movement is studied under the label of tropism. It is very easy to observe movements in animals as they are physically active. But in the case of plants, it becomes harder to study the movement as there is no such locomotion expressed by them. Plants simply grow in a certain pattern or a specific direction. This helps to study their growth and behavioral patterns. Thus, we can say tropism is a biological process that is observed in all living organisms. Tropism is a term that is extracted from the Greek language. In the Greek language, this means “turning”. This is a mechanism expressed by organisms as an expression of growth in a particular direction. Sometimes it is used to refer to the movement or turning of the organism. This biological course of action is predominantly observed in plants. But this process can also be observed in microorganisms like bacteria and viruses. The tropism of these microorganisms is studied to find their movement towards the host. And if they are inside the host their tropism is referred to as their movement towards the predilection site or the site of infection inside the host. This helps in countering the life cycles and modes of infections of pathogens. Similarly, in the case of plants, the process of tropism is beneficial as it is a mechanism of movement towards the sources of nutrients and certain factors which are essential for growth and survival.

Role of Stimulus in Tropism

Tropism can never occur without stimulus. Any factor which leads to action is called stimulus. This is a sort provocation for any organism to carry out a process. In the case of tropism, the plant or the organism either move towards the stimulus or away from the stimulus. However, there are situations in which the stimulus affects the movement in a repulsive way. So, to understand the proper effect of a stimulus on the movement of a plant, tropism is broadly classified into two types:

Positive Tropism

In this type of tropism, the organism (plant) follows the stimulus. It is observable that the plant will move towards the stimulus. For example, the roots of when grow to follow the direction of gravity. Similarly, the stem of the plants tends to grow towards the source of light. These examples are of positive tropism.

Negative Tropism

This tropism is believed to be the movement of the plant in the opposite direction of the stimulus. The movement is believed to occur against the inherent direction of the stimulus. For example, roots show repulsive behavior when exposed to light. As the plant starts to grow, the roots follow the inherent ability to oppose light by growing in a different direction, unlike stem. Similarly, the stem of the plant grows in the opposite direction of the gravitational pull of the earth. Unlike roots, the stem grows away from earth.

Now we have come to know the importance of stimulus and how it affects the growing movement of the plant. The study of tropism makes us believe that all the movement in plants is due to action caused by the signals (stimuli). These stimuli not only arise from outside, but there are several signals which arise from the inside of the plant and invoke growth movement in it. The stimulus which is caused due to external factors is known as exogenous stimuli. Whereas, the stimuli caused due to internal factors are called endogenous stimuli. Tropism, however, is specifically related to external (exogenous) stimuli. Stimuli vary, they are of very diverse kinds. Following are some examples of stimuli with a brief explanation:

Light

One of the important factors driving the tropic movements in plants is light. Plants tend to grow towards the light. This a basic mechanism of survival. It is well known that the rate of photosynthesis is directly proportional to the amount of light being absorbed by the plant. The fact that energy is required for survival makes it clear why plants tend to grow towards the light sources. The more directional a plant is towards the light source, the more likely it can absorb more light to obtain energy to carry out processes essential for survival.

Chemicals and Nutrients

Plants extract their food from the soil too. The chemical contents in the soil, for example, nutrients, provide the roots an incentive to grow towards them. This stimulus is governed by survival instincts too. The deeper the root grows the better exposure will be ensured to the nutrients, so that they may get easily absorbed by the plant. Which in turn makes plants better in growth and survival.

Gravity 

Gravity is also a stimulus for the organs of the plants to grow in certain directions. These directions provide them advantages. For example, the stem exhibits growth against the gravitational pull. This mechanism helps the plant to achieve height, so it can absorb light better. Similarly, roots grow towards the gravitational pull making them grow deeper, so that they may extract the nutrients from the soil more efficiently. This provides the plants a tactical advantage.

Water

Water is another important stimulus. Moisture makes the roots grow towards it. It provides the roots with a positive incentive. So, the roots may grow deeper. This helps in better absorption of water. This in turn promotes photosynthesis, as water is required in that process.

Physical Support

Many plants tend to grow on the surface or along surfaces. These surfaces provide the plants with physical support along which they can grow. It is observed that plants following this stimulus grow along with the support. For example, a plant is growing along an upright pole, it is observed the plant grows upright along the pole.

Types of Tropism 

The above types of stimuli provide us the knowledge of different types of tropism. These types are based on the inherent abilities of each stimulus. It also helps us learn the effect of a stimulus on the plants. Such types are given below:

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

Phototropism

Phototropism is a biological phenomenon that deals with the movement of the organism towards the source of light. This biological phenomenon is also observed in fungi and algae. But it is predominantly studied in plants. Plants, being the producers in an ecosystem, tend to show growth and movement patterns towards the light to maximize the amount of light that can fall on their leaves, and is absorbed. This makes the plants to obtain more energy to produce carbohydrates. Phototropism looks like a simple process. But there is a complex biological mechanism behind the simple idea that plants grow toward the light. Plants have unique organelles that work as light receptors. These receptors when detecting light, form specific channels and pathways to promote gene expression to synthesize proteins. The proteins which are produced, actually plant hormone to cause growth. This particular growth hormone is Auxin. The biological receptors which detect light are large molecules composed of protein and a light-absorbing pigment called a chromophore. 

Role of Auxin in Phototropism

As described above, auxin is a growth increasing hormone that is produced by plants when light is detected by the light receptors. Auxin is a unique chemical compound due to its properties. Firstly, auxin initiates the mechanism of proton pump in the cells of the plants which are being exposed to light. This process causes a decrease in the pH in the internal environment of plant cells. The lower pH level provides the enzymes called expansions. These enzymes are specialized for breaking down the Hydrogen bonding which is present in the cell wall. When the bonds are broken the overall structure of the cell wall becomes quite flexible. Due to this flexibility in the cell walls, each cell elongates, and it becomes easier for the plant cells to grow. This phenomenon remarkably increases the growth of the plant when exposed to light.

Heliotropism

The nature of this phenomenon is akin to phototropism. This is the behavioral movement of flowers of certain plant species while facing the sun. For example, sunflowers and daisies. The flowers of these plants show a unique process of following the sun all day. When the night falls these flowers randomly arrange their faces, and when the sun rises again, they follow the sunlight by facing the center of the flowers anti-parallel to the rays coming from the sun. There are also certain species of plants, which exhibit leaf heliotropism ( leaves tracing sunlight).

Chemotropism

Chemotropism is a common process that can be observed in plants, fungi, as well as animals. This is the movement of an organism due to incentives produced by chemicals. The movement can be encouraged by the chemicals or it can be discouraged. Or more simply, a positive chemotropism can be possible as well as negative chemotropism. Bacteria, viruses, and fungi are found to move in response to many chemicals. This process helps them reach the host or their mating partner. This phenomenon also helps them evade certain chemicals that can be harmful to them. Similarly, the roots of the plant tend to flow the concentration of nutrients that can be present deep beneath the soil. This negative chemotropism finds its application in weed killers. This chemical makes the roots of unwanted plants weak. This makes it easier to uproot them.

Gravitropism

This process is the movement of the plant in response to the gravitational pull of the earth. Gravitropism is also known as geotropism. This phenomenon can also be achieved by artificial gravity. That is why the study of gravitropism also finds its application in growing plants in space.

Role of Auxin in Gravitropism

Auxin promotes growth and elongation where it is released in the plant. So, due to the pull of gravity, it is observed that auxin accumulates in the tips of the roots of the plants. This causes the roots to grow and elongate downwards. 

Hydrotropism

This is the movement of plants in response to water. Roots of the plants show hydrotropism. This movement is mostly positive. The tips of the roots are embedded with certain receptors. These receptors are designed for detecting the water gradient. Water gradient detection leads to a series of biological events that cause growth in the roots towards the water sources. This helps plants achieve the maximum amount of water as it is necessary for carrying out photosynthesis.

Thigmotropism

This type of movement is widely observed in vine plants. This is a type of growth movement in which the plants grow along with support. During the growth process, plants follow the surface of the support. For example, if a vine plant is sown and tied to a pole it will start growing along the pole. Plants expressing thigmotropic growth patterns also exhibit curls. This happens due to partial growth. As one side of the stem grows faster as compared to the other side. However, these curls also provide enough friction to the stem to keep growing along the supporting surface. Roots show negative thigmotropism. They show a negative response to the stimulus of touch. This makes them penetrate the soil. 

Summary 

Tropism is a biological mechanism that is observed in all living organisms. The observation is obvious as locomotion and movements are hallmarks of life.

In plants, the phenomenon of tropism is of great interest. Plants show a unique pattern of growth movements when they receive stimuli in different forms like light, gravity, water, chemicals, nutrients, and contact.

The growth patterns show that either a stimulus positively impacts the growth movement of the plant or its effects negatively.

Tropism helps plants in obtaining maximum sunlight, absorbing water, extraction of minerals, and reproduction.

Sperm reaches the pollen tube through chemotropism. We cannot deny the fact that tropism is a hallmark of life as it helps in the reproduction and survival of plants.

References

  1. Haga, Ken; Takano, Makoto; Neumann, Ralf; Iino, Moritoshi (January 1, 2005). “The Rice COLEOPTILE PHOTOTROPISM1 Gene Encoding an Ortholog of Arabidopsis NPH3 Is Required for Phototropism of Coleoptiles and Lateral Translocation of Auxin(W)”. Plant Cell. 17 (1): 103–15. doi:10.1105/tpc.104.028357PMC 544493PMID 15598797.
  2. Cassab, Gladys I.; Eapen, Delfeena; Campos, María Eugenia (2013-01-01). “Root hydrotropism: An update”. American Journal of Botany. 100 (1): 14–24. doi:10.3732/ajb.1200306ISSN 0002-9122PMID 23258371