Despite not having sense organs plants can also sense things and respond to them. Plants respond to things like light, gravity, touch, water etc. These are environmental stimuli for a plant. Plants coordinate their behaviour by using their hormones and respond to the environmental changes. This they do by affecting the growth of a plant.
Therefore, pants use only hormones to coordinate their behaviour. They respond to the stimuli very slowly because they have no nervous system. Plant hormones are called phytohormones.
Dormancy: The inactive condition of a part of a plant in which metabolism almost stops is called dormancy. For example, seed, bud etc.
The control and coordination system in plants is done by plant hormones. They affect the growth of a plant in one or the other aspect. The growth of a plant is divided in three stages:
i) Cell division
ii) Cell enlargement
iii) Cell differentiation
So, a plant hormone not only controls these stages of plant growth but also promotion of breaking of dormancy, falling of leaves, fruit growth, ripening of fruits, ageing of plants etc.
The four types of plant hormones responsible for control and coordination in plants are:
4) Abscisic acid (ABA)
While auxins, gibberellins and cytokinins promote the growth of a plant, abscisic acid prevents or hampers the growth of a plant.
Auxins hormone controls a plant response to light and gravity. It is made by the cells present at the tip of a stem and roots. This hormone moves the plant away from light and towards gravity. It speeds up the growth of stem and slows down the growth of roots.
This diagram explains the bending of a plant stem or shoot towards light by the action of ‘auxin hormone’.
Auxins promote cell enlargement, cell differentiation and fruit growth.
Gibberellins hormone works in the presence of auxin hormone and promotes cell enlargement and cell differentiation. It also promotes fruit growth, elongation of shoots and in breaking the dormancy in seeds and buds.
This hormone promotes cells division in plants and breaks dormancy in seeds and buds. They also delay ageing in leaves and promotes the opening of stomata.
This hormone inhibits the growth of a plant. Therefore abscisic acid promotes dormancy in seeds and buds. It promotes closing of stomata, wilting and falling of leave and detachment of fruit and flower from the plant.
Plants are fixed at one place and cannot physically move from one place to another. But plants still show movement by using hormones under the influence of stimuli. This movement is noticed in the unequal growth of the two regions of a plant.
For example, auxin hormone speeds up the growth of a stem. So if one side of a stem has more auxin hormone than the other, then the side of stem which has more hormones will bend. This shows movement of a plant.
Plant movement due to external stimuli are of two types: tropism and nasties.
When the direction of external stimulus decides the direction of response in the form of growth it is called tropism. Thus, tropism is the directional growth movement of a plant part. It can be in the direction of the stimulus or away from the stimulus. Therefore,
Types of tropism
There are five stimuli in the environment. They are light, gravity, chemical, water and touch. These stimuli give five types of tropism. They are phototropism, geotropism, chemotropism, hydrotropism and thigmotropism.
When a plant part moves in response to light, it is called phototropism. If the stem of a growing plant moves or bends towards light, it is positive phototropism and if the root of a plant moves away from light, this means the roots of a plant shows negative phototropism.
When a plant part moves in response to gravity, it is called geotropism. For example, roots of a plant moves in a downward direction, so they show positive geotropism and stem moves in the upward direction, so it shows negative geotropism.
When a plant part moves in response to chemical stimulus, it is called chemotropism. Here the stimulus is chemical. For example, the growth of pollen tube towards the ovule during fertilisation in a flower is called positive chemotropism.
When a plant part moves in response to water, it is called hydrotropism. Here the stimulus is water. Roots of a plant moves towards water, so they show positive hydrotropism.
When a plant part shows directional movement in response to the touch of an object, it is called thigmotropism. For example, tendrils of a plant climb towards any support which they touch.
Response of plants to light: Phototropism
The stem and leaves of a plant moves in the direction of sunlight. So, when a plant is grown in an open ground where sunlight is coming from above then the stem of a plant grows straight in an upward direction. If, however, a plant is kept at a place where sunlight is coming from only one direction then the stem of a plant bends in that direction only. On the other hand roots of a plant bend away from the direction of sunlight.
We will perform an experiment to show this. A potted plant is kept in the open ground where sunlight is coming from above. So the stem of a plant grows straight up and roots also grow straight but in downward direction. Now, we keep this pot having straight stem and roots is kept in a dark room near a window with sunlight coming from only one side (right). After a few days we observe that stem of a plant responds to light and move towards right side and roots of the plant move towards left side that is away from sunlight.
To show the response of a plant to light (Phototropism)
This happens because of the action of auxin hormone.
Response of plants to gravity: Geotropism
Roots of the plant grow towards the earth which shows positive geotropism and stem grows upward away from the pull of gravity which shows negative geotropism. This can be shown through an experiment.
This diagram shows the response of a plant to gravity or geotropism.
Response of plants to chemicals: Chemotropism
Sugary substance acts as a stimulus for the growth of pollen tube towards the ovule. This happens because ripe stigma in the carpel of a flower secrets a chemical substance into the style towards the ovary. This sugary substance acts as a stimulus for the pollen grain which responds by growing a pollen tube in the downward direction into the style of a carpel and reaches the ovule of the flower for fertilisation.
This diagram shows the response of a plant part ‘pollen’to chemical secreted by stigma or chemotropism.
Response of plants to water: Hydrotropism
Roots always grow in the direction of water, even if they have to grow against the pull of gravity or sideways. This can be shown through an experiment.
Let’s take two glass troughs and fill two-third of them with soil. In both the troughs we plant a tiny seedling. In the second trough we place a small clay pot inside the soil. Let’s water the soil in the first trough daily and put water in the clay pot placed inside the soil in second trough.
An experiment to show the response of a plant to water or hydrotropism.
After a few days, dig up the seedling from both troughs. We will notice that roots of the seedling of first trough are straight and that of the second trough in which clay pot filled with water was placed have bent towards the side at which clay pot was placed.
This shows that roots of a plant grow towards water.
Directional response of a plant to the touch of an object: Thigmotropism
There are some plants which cannot stand upright on their own and need support. These plants are called climbing plants and have organs called tendrils. Tendrils are the thin thread-like growth on the stems and leaves of the climbing plants. Tendrils are sensitive to the touch of other object. When tendrils come in contact with other object, it bends towards the object by growing towards it, wind around the object and cling to it. This winding movement of a climbing plant is called thigmotropism.
This diagram shows the response of a plant part ‘tendril’to the touch of an object here a bamboo stick.
For example, bitter gourd, bottle gourd, grape vine etc., have stem tendrils and peas, glory lily etc., have leaf tendrils.
The Usefulness of Tropic Movements
Tropic movements help the plant to survive. For example, roots of the plant are positively geotropic and that is the reason that they will always downward into the earth even if they are planted upside down. Also the root will grow towards water as it is positively hydrotropic. Also the shoot will grow upward as it is negatively geotropic.
Nasties or Nastic Movements
When the direction of response in a plant is not determined by the direction of stimulus, it is called nastic movement. Nastic movement is not the directional movement of the plant part.
In nastic movement, whatever be the direction of stimulus, all the parts of the plant equally move in the same direction. This type of movement is generally seen in leaves, flower petals etc.
This diagram shows the nastic movements in the leaves of sensitive plant (Mimosa pudica) caused by ‘touch’.
Following are the examples of nastic movement:
Nastic movement may or may not be a growth movement.
The movement of a plant part in response to the touch of an object is called thigmonasty. In this case the movement of a plant part is non-directional. The stimulus for thigmonasty is touch.
The sensitive plant mimosa pudica which is also known as touch-me-not is an example of thigmonasty. If we touch the leaves of this plant they fold up immediately and droop.
The sensitive plant has pulvini at the base of each leaf. This pulvini contains a lot of water in their cells and due to internal pressure of water they are firm and keep the leaf upright. So, as soon as we touch the leaves there is sudden loss of water from pad-like swellings (pulvini). This is why pulvini lose its firmness and leaves droop.
When we touch the sensitive place electric impulses travel through the cells of the plant and act on plant hormone. The plant hormone migrate water from the cells of the pulvinus to the intercellular spaces of the other half of pulvinus. This loss of water makes pulvini lose firmness and thus its leaves droop.
After 15 to 30 minutes water comes back to the cells and leaf gets back to its original shape.
The leaves of sensitive plant fold due to the loss of water from pulvinus at their base.
The movement of a plant part, usually flower petals, in response to light is called photonasty. The movement is non-directional. The stimulus in photonasty is light. Dandelion and moonflower are examples of photonasty.
The opening and closing of flower petals in response to light are growth movements. This is because petals open when their outer surface grows and petals close when their outer surface grows.
Function of Plant Hormones
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