Control and Coordination | Control and Coordination in Plants
- Phytohormones (Plant Hormones)
Control and Coordination in Plants
Plants lack nervous system and sense organs as animals have them e.g. eye, ear, etc. Plants react to the environmental changes by using hormones called Phytohormones.
According to K.V. Thimann, phytohormone can be defined as “an organic substance produced naturally in plants controlling growth and other functions at a site remote from its place of production and active in minute amounts”.
Growth regulator is a broader term applied to all naturally occurring hormones along with those which are synthetic substances. All growth regulators are not phytohormones but all phytohormones are growth regulators.
Phytohormones (Plant Hormones)
In plants, the growth and development is controlled by various plant hormones i.e. phytohormones. The growth and development is coordinated and managed by one hormone by affecting or controlling the one or the other aspect of plant growth.
The growth of a plant is divided into following three stages :
- Cell division
- Cell enlargement
- Cell differentiation
The various aspects of plants which are controlled by plant hormones are as follows :
- Promotion of dormancy
- Breaking of dormancy
- Stomatal control
- Falling of leaves
- Fruit growth
- Ripening of fruits
- Ageing in plants
|1.||Auxin||· Promotes cell enlargement and cell differentiation.
· Promotes fruit growth.
|2.||Gibberellins||· Promotes cell enlargement and cell differentiation in presence of auxin.|
|3.||Cytokinin||· Promotes cell division i.e. cytokinesis.
· Helps in breaking the dormancy of seeds and buds.
· Promotes opening of stomata.
(A growth inhibitor)
|· Promotes the dormancy in seeds and buds.
· Promotes the closing of stomata.
· Promotes the wilting and falling of leaves.
Plant shows movement in its different parts, when it is subjected to any external stimuli such as light, force of gravity, water, etc.
Have you ever thought why a plant bends towards light when you put a potted plant in a dark room and allow light to enter in the room via window. Let us explore.
Recall the knowledge of phytohormones. Auxin is the hormone which shows cell enlargement and cell division. When a plant is subjected to the above mentioned condition, due to the direct sunlight on one side the auxin synthesis increases which enhances the growth of stem on the other side thereby bending the plant towards light, which we call “plant movement”.
Plant movement is broadly studied under following two heads.
- Tropism or Tropic movement
- Nasties or Nastic movement
Tropism or Tropic movements
Tropic movement is the directional movement of the part of plant in response to external stimuli. The direction of response is determined by the direction of stimulus. The direction of movement of plant can be towards the stimulus i.e. positive tropism or can be away from the stimulus. i.e. negative tropism.
(a) Types of Tropism: Depending upon the type of stimuli, different types of tropism are classified. There are five types of well recognized stimuli studied and tropisms are also studied in response to them. They are
Similarly, the types of tropisms are
|Phototropism||In response to light|
|Chemotropism||In response to chemical|
|Hydrotropism||In response to water|
|Geotropism||In response to gravity|
|Thigmotropism||In response to touch|
can be defined as “the movement of plant parts in response to light”. When the plant parts move towards the light it is called positive phototropism and if it moves away from light, it is called negative phototropism.
For example, stem / shoot shows movement towards the light, which means shoot shows positive phototropism. Root shows movement away from the light i.e. negative phototropism.
Phototropism in Plants
Experiment and explanation:
Let us discuss one experiment before discussing the role of hormones.
When a plant is grown in a dark room, with its window open (the only source of light to the plant) tends to move towards light coming through window. This is because of the positive phototropism shown by plant.
The principle hormone taking part in phototropism is “Auxin”. The flowchart explaining the mechanism of auxin action is as follows:
Mechanism of Auxin action
Mechanism of Auxin Action (Normal growth)
Mechanism of Auxin Action (in experimental condition)
In case of root: The effect of auxin on root is just opposite to that of shoot.
It can be defined as “the movement of plant parts in response to chemicals”. When the plant parts move towards the chemical it is called “positive chemotropism” and if it moves away from the chemical then it is called “negative chemotropism”.
For example, during the process of fertilization, the pollen tube grows towards the ovule, because stigma produces a chemical in response to which pollen tube grows towards ovule.
Pollen grains moving towards ovule (Chemotropism)
The growth of pollen tube towards ovule is one of the finest examples of chemotropism in plant kingdom. The process is as follows:
In the above discussed flowchart, the sugary secretion by stigma acts as chemical to which plants show tropic movement.
It can be defined as “the movement of plant parts in response to water”. When the plant parts move towards water, it is called positive hydrotropism and when it moves away from water, it is called negative hydrotropism.
Have you ever pulled off a plant? You will notice that roots grow in all directions, this is because in response to water they grow towards it, i.e. “hydrotropism”.
Let us try to understand the process of hydrotropism by a simple experiment.
Experiment and explanation: Take two glass troughs and label them 1 and 2. Plant a tiny seedling in both of them. In trough 2 make a small “well” and fill it with water. Water trough ‘1’ uniformly but in case of trough ‘2’, put water in the well.
After few days dig the soil without disturbing the root. We will notice that in case of trough 2 the root bends towards water where as in trough 1 root grows straight uniformly.
It can be defined as “the movement of plant parts in response to gravity”. When the plant parts move towards the direction of gravity then it is termed as “positive geotropism” and when it moves away from the gravity, then it is termed as “negative geotropism“.
Shoot shows negative geotropism and root shows positive geotropism.
Have you ever thought of why stem grows upward against the gravity and root grows against the gravity?
Experiment and explanation: Let us understand the phenomenon with the help of an experiment. Take two potted plant namely 1 and 2. Keep one of them say 1 in normal condition and put 2 in horizontal condition.
You will notice that after some days the plant in pot ‘2’ bends i.e. stem away from earth and root towards the earth.
It can be defined as “the movement of plant parts in response to touch“.
Stem tendril showing Thigmotropism
Have you ever seen a vine yard? The grape vine climbs on the provided support. Do you know why?
Experiment and explanation: The grape vine has a weak stem, so it cannot stand erect. To stand erect it needs support to which tendril (an outgrowth on the stem) binds. The tendril grows as it touches the support. The side of the tendril which touches the support grows slowly as compared to the other side which is not in contact. This phenomenon in response to touch is called as “thigmotropism”.
Usefulness of Tropic Movement: Considering all the tropic phenomenon we can conclude that these phenomenon are necessary for the plant to survive. If tendril would not have responded to the support, vine yard would not have been possible. If roots would not have responded to water, plant would have died.
Nasties or Nastic Movement
Nastic movement can be defined as, “the movement of plant part in response to an external stimuli in which the direction of response is not determined by the direction of stimulus“. It is usually shown by flat structure of plant such as leaf.
One of the best studied example of nastic movement is “response of Mimosa pudica on touch“.
Nastic movement may or may not be growth movement. The opening of flower by the action of sun light is a growth phenomenon where as folding of leaf on touch as in case of Mimosa pudica is not a growth phenomenon.
(a) Types of nastic movement
Thigmonasty is the nastic movement of a plant part in response to touch. For example – Mimosa pudica responds to touch by folding its leaves. The stimulus here is touch.
Photonasty (in sunflower)
Photonasty is the movement of plant part in response to light. The stimulus here is light. In case of dandelion flower it opens in the morning with the rising sun and as the sun sets flower also closes. But in case of moon flower it opens with the setting of sun and closes with the rising of sun. You can also see movement in sunflower which responds to the movement of sun.
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