Coordination in Plants: Phytohormones, Tropic & Nastic Movements

All living beings respond and react to the changes that occur in the environment. These responses are mainly reflected in the movements of their body parts. We have often seen humans react against various stimuli such as hot, cold, pain, foul & pleasant smells, different tastes, etc. Like humans, have you ever observed any reaction or response in plants against any action or stimulus? Yes, plants do respond to environmental changes.

However, coordination in plants is not as elaborated as in humans. They do not have developed nervous systems or thinking power to respond to the stimulus in the same manner as animals and humans. Sunflower is named so because it bends towards the sunlight that we have often seen. It is a type of coordination in plants where the sunlight is a stimulus. Likewise, there are many more examples that you might have not been being familiar with. Let’s read the article to know the secret behind the coordination in plants that reflect in their movements.

What is Coordination in Plants?

The responses to external stimuli or environmental changes that are produced by the working of various organs in a systematic and controlled manner are known as coordination. Plants lack a nervous system. They use a chemical system for coordination that comprises different types of hormones. The chemical coordination within the plant body brings about growth and movements in plants and plant parts.

Phytohormones

Plant hormones are called phytohormones. These are the chemical substances controlling and coordinating plant responses to several stimuli such as water, sunlight, gravity, etc. Plants have different types of hormones. Phytohormones are active in small concentrations. They are capable of influencing physiological activities leading to promotion, inhibition, and modification of growth. These growth-regulatory substances are generally grouped under five major classes, namely auxin, gibberellins, cytokinins, ethylene, and abscisic acid.

Fig: Classification of Plant Hormones

1. Auxin

The term auxin is applied to indole-3-acetic acid IAA. Auxins are generally produced by the growing apices of the stem and roots, from where they migrate to the region of their action. Auxins like IAA (Indole Acetic Acid) and IBB (Indole Butyric Acid) have been isolated from plants. Auxin plays the following role in plant coordination:

a. Auxin helps to induce meristematic activity and the formation of new cells. This leads to the formation of adventitious roots at the base of the cuttings. This fact is of great practical importance and has been widely utilised to promote root development in economically useful plants, which are propagated by cuttings.
b. Auxins control preharvest fruit drop.
c. The synthetic auxins can be used to kill broadleaf weeds.
d. Initiation and promotion of cell division by auxin are very useful in tissue culture and callus formation.

2. Gibberellins

A Japanese biologist studied a disease of rice in which the young seedlings grew extremely tall, fell over, and died. The principle responsible for the disease’s symptoms in rice was later identified from extracts of a fungus Gibberella fujkuroi, hence named gibberellins. Subsequently, gibberellins were identified from other plants as well. There are more than 100 gibberellins reported from both fungi and higher plants. They are denoted as GA1, GA2, GA3, and so on. The major sites of gibberellin production in plants are embryos, roots, and young leaves. Gibberellins contribute to plant growth and coordination in the following ways:

a. Gibberellins stimulate stem elongation and leaf expansion but have no effect on roots. Cabbage is a good example of this in which leaf development is profuse.
b. The most striking effect of gibberellins is the elongation of genetically dwarf mutants (varieties) of plants such as corn and pea.
c. The natural dormancy of buds, tubers, rhizomes, and some seeds is overcome by gibberellins.
d. Gibberellins induce the production of enzymes, like amylases, proteases, lipases, and ribonucleases for mobilising storage reserves during seed germination and early seedling growth.
e. Gibberellins cause parthenocarpy and control flowering in long-day plants.
f. Gibberellins promote the production of male flowers.

3. Cytokinins

Cytokinins are a group of plant hormones that are synthesised in areas where cell division is occurring, for example, root apices, developing shoot buds, young fruits, etc. Cytokinins never act alone. There are the following functions of cytokinins:
a. Cytokinins in conjugation with auxins stimulate cell division even in non-meristematic tissues.
b. The ratio of cytokinins to auxins controls cell differentiation. When both are present in equal amounts, cells divide but do not differentiate. If there is more cytokinin than auxin, shoot buds develop from the callus. If there is more auxin than cytokinins, the root develops.
c. Cytokinins can retard the ageing of plant organs by controlling protein synthesis.
d. Cytokinins induce flowering and break the dormancy of some seeds.

4. Ethylene

Ethylene is a gaseous hormone that causes the following effects on plant growth and functioning:
a. Ethylene is highly effective in inducing fruit ripening.
b. Exposure of plants to ethylene causes drooping of leaves and flowers. This phenomenon is known as epinasty.
c. It modifies growth by inhibiting stem elongation and stimulating the transverse expansion of the stem.
d. Ethylene causes loss of pigmentation in leaves.

5. Abscisic Acid (ABA)

It is the growth inhibitor hormone, also called the stress hormone. There are the following functions of abscisic acid:
a. It enables the seeds to withstand desiccation and to become dormant.
b. It helps the plant to cope with adverse environmental conditions. For example, under severe drought, abscisic acid prevents water loss by promoting stomatal closure.
c. Abscisic acid application to the leaf causes the yellow colouration of leaves.

Fig: Effect of Different Types of Plant Hormones on Growth of Plant Parts.

Plant Movements

Plants show movements. Although, unlike animals, plants are incapable of locomotion. Plant organs move when they are subjected to some external stimuli like water, sunlight, gravity, chemical substances, touch, etc. In normal circumstances, the plant movements are in the direction best suited for their development. These movements are caused by the action of plant hormones under the influence of the stimulus.

Roots move downward to obtain water and mineral nutrients. Shoot moves upward to be exposed to sunlight. Carnivorous plants exhibit movement to trap insects to fulfil their nutritional needs. Plant movements are generally growth movements that are divided into two main categories:
1. Tropic movement (tropism)
2. Nastic movement (nasties)

Tropic Movements

Tropic movements are growth-dependent movements. Growth brings about a permanent change in any plant or its part with respect to size, form, weight, and volume. Tropism shows a definite direction in relation to stimuli (either towards or away from the stimulus), therefore called directional movements.

Types of Tropic Movements

Based on different stimuli, tropic movements can be categorised into the following categories:

Phototropism

The movement of plant parts (roots, shoots) in response to sunlight or artificial light is called phototropism. Roots are negatively phototropic, and the stem (shoot) is positively phototropic. Auxin hormone is primarily responsible for the growth of roots and shoots. In a condition when the light is provided from the right side of the plant, the roots will bend and grow towards the left, while the shoot bends towards the right (in the direction of light). It happens because the auxin in the shoot accumulates on the left side, and the left side grows faster due to auxin compared to the right side, and the shoot bends towards the light.

Fig: Phototropism in the Plant

Geotropism

The response of a plant part to the force of gravity is called geotropism or gravitropism. Based on the direction of growth, it could either be positive or negative. The roots grow downward. Therefore, roots are called positively geotropic. The stem or shoot grows against the pull of gravity. Therefore, the stem is negatively geotropic. The diagram represents the response of plant parts to gravity when kept straight and horizontally.

Fig: Geotropism in the Plant

Hydrotropism

The response of plant parts to the water is called hydrotropism. Roots are positively hydrotropic moving towards the water source and shoot is negatively hydrotropic. The figure represents the attraction of roots towards the water and thereby showing the positive hydrotropism in roots.

Fig: Hydrotropism in the Plant.

Chemotropism

The growth of the plant part in response to a chemical stimulus is called chemotropism. The growth of pollen tubes towards the ovule in flowers for fertilisation is the best-known example of chemotropism.

Fig: Pollen Grain Shows Chemotropism to the Chemical Secreted by the Stigma

Thigmotropism

Thigmotropism is a directional movement in plants in response to touch. It is also called haptotropism. Climbers have a weak stem that needs support to grow upward. The stem leaves or petiole of climbers modify into a thread-like green structure called tendrils. The tendrils of plants like sweet peas, grapes, and Cuscuta are sensitive to touch. When these tendrils come in contact with any support, the tendrils then start to circle around the object and cling to the available support.

Fig: Thigmotropism in the Climber (plant).

Nastic Movements

Nastic movements are non-directional movements of plant parts in response to external stimuli. Nastic movements can be categorised as follows:

Thigmonasty

It is the non-directional movement of plant parts in response to touch, physical, and mechanical injuries. It is also called seismonasty. The leaves of Mimosa pudica (touch-me-not plant) fold up and droop when touched. The bending of tendrils of insectivorous plants in contact with insects is also an example of thigmonasty.

Fig: Thigmonasty in the leaves of Mimosa pudica.

Photonasty

It is the non-directional movement of plant parts in response to light. The examples of photo nasty are as follows:
a. The petals of the dandelion flower open up in the morning in bright light but close in the evening.
b. The petals of the moonflower close during the day in the bright light but open up at night.

Nictinasty

It is the bending of leaves due to nightfall. It is also called sleep movement—for example Amaranthus, Balsam, etc.

Fig: Photonasty in Dandelion Flower During Day and Night

Difference between Coordination in Plants and Animals

Summary

Coordination refers to the ability of different parts of the organisms to work together smoothly and efficiently. Like humans, plants also respond to stimuli. However, the responses in plants are slow. The plants use hormones to coordinate their movements in response to several stimuli. There are five hormones reported in plants, namely auxin, gibberellins, cytokinins, ethylene, and abscisic acid.

All the physiological activities leading to promotion, inhibition, and modification are regulated by these phytohormones, which in turn brought about the growth in plant parts. The growth movements can be divided into two categories; tropisms and nasties. Tropic movements can be observed in the roots and shoots of the plant. The leaflets of touch-me-not and petals of some flowers exhibit nastic movement. This article covers the different types of phytohormones, their functions, and different types of plant movements.

FAQs

Q.1. How does coordination take place in plants?
Ans: Coordination in plants occurs with the help of chemical substances called plant hormones or phytohormones such as auxin, cytokinins, etc.

Q.2. What are the three directional movements in plants?
Ans: The directional movements are called tropic movements. Phototropism, hydrotropism, and geotropism are the three directional movements.

Q.3. How do plants show their movements?
Ans: Plants cannot move from one place to another. Instead, they can move their parts. Plants show the movements in response to environmental factors such as sunlight, water, gravity, touch, etc. These movements can be noticed due to the unequal growth of plant parts.

Q.4. Which type of coordination is present in plants?
Ans: The plants do not have a nervous system and exhibit chemical coordination.

Q.5. Why are tropic movements in plants called directional movements?
Ans: The tropic movements in plant parts can be seen either towards the stimulus or away from the stimulus. Therefore tropic movements are called directional movements.