Question

List the three types of tropisms that plants undergo and give a brief description of each.

Answer 1

A tropism  is a biological phenomenon, in which turning of all or part of an organism in a particular direction, in response to an external stimulus. It is found in both plants and animals. Usually it is associated with plants.

In plants the tropism is the growth or turning movement, toward or away from an environmental stimulus. When the movement is towards the stimulus, it is called positive tropism and when the movement is away from the stimulus, it is called negative tropism.

Common stimuli that influence plant growth include light, gravity, water, touch etc..

Several forms of tropisms are present in plants. They are Phototropism, Geotropism, Thigmotropism, Aero tropism, Heliotropism, Hydrotropism, etc.

1. Phototropism

From the name it is clear that, it is the tropism due to light (Photo). It is the the orientation of a plant in response to light,  either towards the source of light ( positive phototropism ) or away from it ( negative phototropism ).

Sunflowers show positive phototropism, their stems curve toward the light. Some vine shoot tips exhibit negative phototropism, which allows them to grow towards dark, solid objects and climb them.

Different photoreceptors are present in plants.They are phytochrome, cryptochrome,Zeit lupe etc. These receptors will absorbs light with different wavelength. This will leads to the expression of photogenic responses and suppression of skotogenic responses (Responses takes place in the absence of light). Mainly the action of different hormones are under the control of the phototropism.

Auxin is a plant hormone produced in the tip of the plant, that promotes cell elongation in shoots. So if the auxin is present in high concentration, cell elongation will happens there.

The auxin hormone synthesized at the tip of the stem spreads uniformly down the stem, when sunlight falls straight on the plant, and due to equal concentration of auxin, stem grows straight.
Similarly the auxin diffuses towards the shady side of shoot, when sunlight falls on only one side of the plant. The concentration of auxin stimulates the cells to grow longer. Therefore, the stem appears to bend towards the source of light.

This movement is mainly for optimizing photosynthetic light capture in the aerial portion. Differential cell elongation (Action of auxin) is exhibited by a plant organ in response to directional blue light. Photosynthetic activity is maximum in blue light.

2. Geotropism (Gravitropism.)

The directional growth of a plant in response to gravity is geotropism. In this case, the stimulus is gravity. Upward growth of plant parts(Shoot development), against gravity, is called negative geotropism, and downward growth of roots is called positive geotropism.

Geotropism also aids plant shoots in upright orientation, as they grow in opposition to the downward pull of gravity.

Root geotropism provides multiple advantages to a plant's vitality and survival.

The presence of gravity ensures the first root that emerges from any plant seed, called the radicle, grows downward into soil. Also it provides structural anchorage and integrity of the plant, especially once upright stems and leaves occur. Anchorage is needed to orient photosynthesizing leaves to the sun, and not risk plant collapse from wind or rain.

Just like phototropism, geotropism is also due to the distribution of auxin. In roots Auxin action is different from shoots. If auxin concentration is high that part will grow less in roots.  

In a root placed horizontally, the bottom side contains more auxin and grows less - causing the root to grow in the direction of the force of gravity. The opposite happens in a stem.

The combination of phototropism and geotropism allow plants to grow in the correct direction.

3. Thigmotropism

Plant alters its normal pattern or direction of growth or movement as the result of an external touch stimulus, that is thigmotropism.Thigmotropism is mainly used by climbing plants. It helps them direct the pattern of growth around an object that is in contact with the plant.

Here also both positive and negative thigmotropism are there. A positive thigmotropism is a response towards the touch stimulus. Examples of positive thigmotropism is the growth of ivy on walls. A negative thigmotropism is a response away from the touch stimulus.  For example, in a vertical bean root, the stimulus of touch is enough to change the direction of growth of the vertical roots.

Plant hormones auxin and ethylene have a role in it. Touched cells produce auxin which then transfers auxin to non-touched cells. These untouched cells then grow faster causing them to bend around the stimulus. The hormone ethylene helps in changing the shape or turgidity of the cell.

. All of these tropisms are crucial to the plants survival. . Their activities are largely controlled by external stimuli, which makes them good at adapting to their environment.