Vegetables are not immobile living beings, they also respond, through movement, to external stimuli. This response to an external stimulus is called tropism, which derives from the Greek word trope and means turn, turn. When the vegetable grows towards the source of stimulus, we call it positive tropism, but when the vegetable grows in the opposite direction to the source of stimulus, we call it negative tropism.
Phototropism, gravitropism (or geotropism) and tigmotropism are the main types of tropisms, and the hydrotropism it's the chemotropism.
Phototropism is the plant's response when the stimulus is light. The stems tend to grow towards the light, thus showing positive phototropism. This movement takes place through the action of the hormone auxin in cell stretching. When the plant is lit from one side only, the auxin goes to the less lit side. In this way, the cells on that side are more elongated than the cells on the side with the highest incidence of light. That's why the plant bends towards the light source.
In gravitropism, also called geotropism by many, the factor that stimulates plant growth is the Earth's gravity. Auxins are also involved in this growth. In the case of roots, they have positive gravity, or positive geotropism, as they grow towards the ground. The stems, on the other hand, present negative gravity or negative geotropism, because they grow in the opposite direction to the force of gravity.
If we place a plant horizontally, the auxins produced in the apical bud will move to the region facing the ground, causing the cells on that side to elongate more than the cells on the side contrary. This way, the stem will curve upwards. This is an example of negative geotropism, as the stem grows in the opposite direction to the stimulus.
In the roots, the increase in the amount of auxin, on the side facing down, inhibits elongation cell, with the cells on the opposite side elongating more, causing the root to bend to low. Roots are an example of positive geotropism, as they tend to grow always in the direction of the stimulus.
Figure illustrating the negative geotropism of the stem.
O tigmotropism occurs when a plant comes in contact with a solid object and starts to grow around it. We can observe tigmotropism in climbing plants and plants that have tendrils, which grow curled in a support.
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Climbing plants and passion fruit are examples of vegetables that perform tigmotropism.
hydrotropism is the water-oriented movement, while the chemotropism it is movement oriented towards certain substances.
O nastism is also a movement performed by plants in response to external stimuli., but it differs from tropism in that it is a movement in which the direction of the stimulus does not influence its movement. In nastism, the stimulus causes the opening or closing of an organ regardless of the direction of the stimulus, its intensity being more important. We can cite as examples the poppy plant, which closes its leaflets based on a mechanical stimulus; and the eleven-hour plant flower, which opens its flowers in the hottest part of the day, when the light is most intense.
In nastism, the direction of stimulus does not influence the movement of the plant.
The movement of the opium plant is linked to alterations in the turgency of the cells of a structure located at the base of the leaflets, called pulvinus. When these cells are stimulated, cells in the upper part of the pulvins release potassium ions, causing a decrease in osmotic pressure, with loss of water to other cells. The decrease in turgescence of these cells causes the closing of the leaflets.
There are also other plants that open their leaves during the day and close them at night. These movements occur due to changes in the turgor of cells that are located on one side of the base of the petioles. During the day, these cells remain turgid, and the cells that are located on the opposite side of the petiole are practically wilted – thus, the leaves remain open. During the night, cells that were withered become turgid, and those that were turgid become withered, causing the leaves to close.
By Paula Louredo
Graduated in Biology
