What is Tropism?
Tropism is the natural ability of an organism to transform or change in response to a stimulus. Natural responses are genetically programmed rather than acquired abilities. Tropism causes an organism to spontaneously move towards a stimulus. Individual tropisms can be any signal from the setting, which are also called after the stimulus that triggers the movement. In an optimistic tropism, the animal would transform towards stimulation. In a negative tropism, the animal would move away from the tropism. Since certain stimuli are either beneficial or harmful to an organism, they are genetically ingrained. Tropism triggers the taxis which are said to be in movements.
Tropism in Viruses
Viruses and other pathogens may also cause what is known as "host tropism," "tissue tropism," or "cell tropism," which refers to how various viruses/pathogens have evolved to preferentially target particular host organisms, tissues, or cell types inside those species. Tropisms are named for the stimulus they are reacting to (for example, a phototropism is a reaction to sunlight) and may be positive (towards the stimulus) or negative (against the stimulus) (away from the stimulus).
Different Types of Tropism
Phototropism
Gravitropism
Chemotropism
Thigmotropism
Hydrotropism
Thermotropism
Magnetotropism
Phototropism
In response to light plants generally grow towards or away from the light, this type of tropism is called phototropism. In plants, the stems and leaves show positive phototropism, and roots show negative phototropism.
Gravitropism
In response to gravity, certain plants show some growth in response to gravity, this type of tropism is called Gravitropism. Stems respond negatively to gravitropism and roots respond positively to gravitropism. This is also called geotropism. Among different parts of plants, the roots show positive geotropism when directed towards the center of gravity. The stems show negative geotropic as they grow against the center of gravity.
Chemotropism
The chemical substances in a plant that are responsible to bring a curvature movement in plant organs. When plants start to grow in response to certain chemicals, then it is called chemotropism. A few instances of chemotropic movements are the transformation of the flower into fruit, the tentacles movement in Drosera, etc.
Thigmotropism
The growth or development of movements made through plants in response to a solid object contact is called thigmotropism. These types of movements are common in tendrils and twiners. This movement is known as Haptotropism.
Hydrotropism
In relation to the stimulus of water, the movement or the growth of a plant is called hydrotropic movement is called hydrotropism. In this type of movement, roots respond positively, as they move and grow towards the water.
Thermotropism
In response to the changing atmospheric temperature, tropic movement of plants or a part of the plant is called Thermotropism. For example, the Rhododendron plant.
Magnetotropism
Many animals may be attracted to certain poles by magnetic fields that serve as a source of direction.
Types of Tropism in Virus
Wide Host Range: amphotropic (e.g. infects many species or cell types)
Small Host Selection: ecotropism (e.g. infects only one species or cell type)
HIV tropism refers to how a particular strain of HIV enters cells.
A virus that preferentially infects the nervous system of the host is known as neurotropism.
FAQs on Tropism
1. What is tropism in the context of plant biology?
In plant biology, tropism is a directional growth movement that a plant, or a part of a plant, makes in response to an external stimulus. This response can be either towards the stimulus (positive tropism) or away from it (negative tropism). These movements are crucial for a plant's survival, helping it find light, water, and physical support.
2. What are the main types of tropism explained with examples?
The main types of tropism in plants are based on the stimulus they respond to. Key examples include:
Phototropism: Growth in response to a light source. For example, a plant's stem bending towards a window.
Gravitropism (or Geotropism): Growth in response to gravity. A plant's roots grow downwards (positive), while its shoot grows upwards (negative).
Hydrotropism: Growth in response to water. Roots grow towards moist areas in the soil.
Thigmotropism: Directional growth in response to touch or contact. For example, the tendrils of a pea plant coiling around a support stick.
Chemotropism: Growth in response to chemical stimuli. A prime example is the growth of the pollen tube towards the ovule during fertilization.
3. How does phototropism help a plant to survive?
Phototropism is a vital survival mechanism that helps a plant maximise its exposure to sunlight. By bending its stem and leaves towards a light source, the plant ensures its chlorophyll-containing cells receive adequate light for photosynthesis. This process is essential for producing the energy the plant needs to grow, develop, and reproduce.
4. What is the difference between positive and negative gravitropism?
The difference lies in the direction of growth relative to the force of gravity. Positive gravitropism is growth towards the pull of gravity, as seen in plant roots that grow deep into the soil to anchor the plant and absorb water and nutrients. Negative gravitropism is growth against the pull of gravity, as demonstrated by plant shoots and stems that grow upwards, positioning leaves for optimal sunlight exposure.
5. What is the role of the plant hormone auxin in phototropism?
The plant hormone auxin plays a central role in phototropism. When light shines on a plant shoot from one side, auxin accumulates on the shaded side. This higher concentration of auxin stimulates cell elongation on the darker side of the stem. As the cells on the shaded side grow longer than the cells on the illuminated side, the stem bends towards the light source.
6. How is tropism different from nastic movement in plants?
The key difference is that tropic movements are directional and depend on the direction of the stimulus (e.g., bending towards light). In contrast, nastic movements are non-directional responses to a stimulus, meaning the direction of the response is independent of the stimulus's direction. For instance, the leaves of a touch-me-not plant (Mimosa pudica) fold inwards when touched, regardless of the direction of the touch.
7. Can you provide an example of thigmotropism in action?
A classic example of thigmotropism is seen in climbing plants with tendrils, such as grapevines or cucumber plants. When a tendril comes into contact with a solid object like a fence or another plant, the side of the tendril touching the object grows slower, while the opposite side grows faster. This differential growth causes the tendril to coil tightly around the support, helping the plant climb upwards.
8. Where is chemotropism most important in the life cycle of a flowering plant?
Chemotropism is most important during plant reproduction, specifically during fertilisation. After a pollen grain lands on a stigma, it grows a pollen tube down through the style to reach the ovule. This growth is not random; it is guided by specific chemical signals (sugars and calcium) released by the ovule, ensuring the pollen tube reaches its target for successful fertilisation.
9. Is the term 'tropism' used outside of plant biology?
Yes, the term 'tropism' is also used in microbiology and virology, but it has a different meaning. In this context, viral or tissue tropism refers to the specificity of a pathogen, like a virus, to infect a particular type of host cell, tissue, or species. This is determined by the presence of specific receptor molecules on the host cell surface that the virus can bind to, and is unrelated to directional growth.
