Plant Growth Regulators, Types of Plant Growth Regulators, Abscisic Acid: Discovery and Physiological Effects, Practice Problems and FAQs

You have seen germinating seeds. In lower classes we have done seed germination as an activity in Biology. That time after germination we used to measure the length of the plant for one week at least and used to make data of that. But do you know how seeds germinate? Who will help them in this process? Yes, plant growth regulators or plant hormones will help them in germination.

                                  Fig: Seed germination

If the conditions are not favourable, what will happen to the seed? Will it germinate in the same way? No. Then how is the growth suppressed in this context?

In plants, there are some chemicals called plant growth hormones present that either stimulate or inhibit the various developmental processes. They are of different types that normally help in different metabolic processes. During unfavourable conditions, abscissic acid allows accumulation of reserve food that protect and provide nourishment to the developing embryos. It inhibits seed germination. So, let’s take a deep dive into the details of plant growth regulators and understand more about abscisic acid in this article.

Table of contents

• Plant growth regulators
• Types of Plant growth regulators on the basis of chemical composition
• Types of plant growth regulators on the basis of function
• Characteristics of plant growth regulators
• Abscisic acid
• Discovery of abscisic acid
• Physiological effects of abscisic acid
• Practice Problems
• FAQs

Plant growth regulators

Plant growth regulators are also known as phytohormones. They are organic molecules that are different in chemical compositions. They are produced in one part and translocated to their area of action. They are naturally produced by plants under appropriate conditions. They can accelerate or retard the growth of plants.

Types of Plant growth regulators on the basis of chemical composition

On the basis of chemical composition, plant growth regulators are of five types. These are as follows:

• Indole compounds
• Adenine derivatives
• Carotenoid derivatives
• Terpenes
• Gases

Indole compounds

Auxin is an indole derivative. Indole has a bicyclic structure, consisting of a benzene ring fused to a five-membered ring.

                Fig: Structure of auxin hormone

Adenine derivatives

It includes the plant growth regulators like cytokinin and adenine.

Cytokinin

It is an adenine derivative. Distinct substitutions are attached to the N₆ position of the adenine ring here. The common class of cytokinins have isoprenoid (derived from 5-carbon compound isoprene) side chains. They are chemically amino purines or phenyl urea derivatives.

                          Fig: Structure of cytokinin

Auxin

It is an indole derived phytohormone which is weakly acidic. It has an unsaturated ring structure. Indole has a bicyclic structure, consisting of a six-membered benzene ring fused to a five-membered ring.

                               Fig: Structure of auxin

Carotenoid derivatives

Abscisic acid is a carotenoid derivative. It is commonly considered as the plant stress hormone. Therefore, plants are able to survive in dry conditions. Carotenoids are plant pigments having yellow to orange colour.

                                 Fig: Abscisic acid

Terpenes

Gibberellic acid is a terpene derived phytohormone. Gibberellins possess a gibberellane ring structure. They are tetracyclic diterpenes. They possess 19 - 20 carbon atoms in their structure. It promotes seed germination and stimulates stem growth.

                               Fig: Gibberellic acid

Gases

Ethylene is a gaseous phytohormone. The chemical formula of ethylene is C₂H₄. It breaks seed dormancy and initiates germination.

                               Fig: Structure of ethylene

Types of plant growth regulators on the basis of function

On the basis of function in the plant body, plant growth regulators are of two types as follows:

• Growth promoting phytohormones
• Growth inhibiting phytohormones

Growth promoting phytohormones

These hormones promote growth in the plant body and are of the following types:

• Auxins
• Gibberellins
• Cytokinins
• Ethylene

                                    Fig: Growth promoting hormones

Common growth promoting functions of plant growth regulators

The growth promoting functions of the plant growth hormones are as follows:

• Cell division - It is required for growth. Mitotic divisions result in an increase in the number of cells. Examples of hormones include cytokinins.
• Cell elongation - It is required for growth and development. Examples include auxins.
• Tropic growth - Hormones help in tropic movements like phototropism (light), chemotropism (chemicals), thigmotropism (touch), gravitropism (gravity), thermotropism (temperature) and hydrotropism (water). These are types of responses of the plants towards the different environmental factors. Examples include auxins.
• Organ development - It is required for growth and development. It includes mainly the production of fruits, leaves, flowers, seeds etc. Examples include cytokinins.
• Formation of reproductive structures - It is required for proper yield and propagation. Examples include auxins.
• Flowering - It is required for propagation of generations and yield. Examples include gibberellins.
• Seed formation - It is required for propagation of generations and yield. Examples include gibberellins.
• Fruiting - It is required for propagation of generations and yield. Examples include gibberellins.

Growth inhibiting hormones

These hormones inhibit growth in the plant body and are as follows:

• Ethylene
• Abscisic acid

                                 Fig: Growth inhibiting hormones

Common growth inhibiting functions of the plant growth regulators

The common growth inhibiting functions of the hormones are as follows:

• Dormancy - It is considered as the temporary inability of a viable seed to germinate under unfavourable conditions. This is required to overcome adverse conditions. Examples include abscisic acid.
• Stress tolerance - It is required to overcome the unfavourable conditions. Examples include abscisic acid.
• Abscission - It is considered as the process by which plants shed organs such as leaves, fruits, or flowers. Ethylene accelerates abscission.

Characteristics of plant growth regulators

Different plant growth regulators show different properties. Plant growth regulators exhibit the following common characteristics:

• Differentiation and elongation of cells - Examples include auxins.
• Formation of leaves, flowers and stems - Examples include cytokinins.
• Wilting of leaves - Examples include abscisic acid.
• Ripening of fruits - Examples include plant growth regulators like ethylene.
• Seed dormancy - Examples include abscisic acid.
• Cell division and cell enlargement - Examples include cytokinins.
• Delayed senescence - Examples include gibberellins.

Abscissic acid

Abscisic acid is a carotenoid derivative. It is commonly considered as the plant stress hormone.

Discovery of Abscissic acid

In the 1960’s, different groups of scientists discovered various growth inhibitors and named them as inhibitor β, abscisin II, and dormin. Later, it was proved that all the three growth inhibitors were chemically identical and were named abscissic acid.

Ohkum in 1965 isolated an inhibitor substance from cotton fruits and named it as 'abscisin II'. Cornforth and his associates in 1965 demonstrated that Dormin and Abscisin II are chemically identical. Addicot discovered this hormone from cotton balls. Hence they gave the common name abscisic acid or ABA due to its known effects on stimulating the process of abscission.

                  Fig: Discovery of abscisic acid

Functions of abscisic acid

The following are the major functions of abscisic acid.

Resistance

It increases tolerance to stress and helps the plant to cope up with the stressful condition. It acts as a growth inhibitor under unfavourable conditions and it slows down metabolic activities. During unfavourable conditions like extreme cold, the rate of photosynthesis decreases. In such conditions, ABA decreases the metabolic activity to conserve energy. This inhibits plant growth. It inhibits cambium activity also.

                                Fig: Inhibition of plant growth

Rooting

It is required for promoting root initiation in stem cuttings in some plants. Examples include Ivy (Hedera).

Development of seeds

It helps in seed development and maturation. This occurs by the accumulation of reserve food.

                                             Fig: Seed germination

Seed dormancy

It induces seed dormancy and inhibits germination. For example, it inhibits gibberellin mediated amylase formation during germination in cereal grains.

                                     Fig: Dormant date palm seed

Closure of stomata

During desiccation it stimulates the closure of stomata and helps to conserve water by minimising transpiration. This is done by inhibiting the uptake of potassium and promoting the leakage of malic acid. In this way it allows the plant to overcome the stress conditions. Hence this hormone is called abscisic acid.

                GIF: Opening and closing of stomata

Controlling growth

It is an antagonist to growth promoting hormones like gibberellins, auxins and cytokinins. It counteracts the effects of growth promoting hormones and keeps their activities under check. This is required for balance.

Senescence

Abscisic acid stimulates senescence of leaves by causing destruction of chlorophylls. It also inhibits protein and RNA synthesis.

                                                       Fig: Senescence

Promote abscission

Abscisic acid promotes abscission of fruits and flowers.

                                       GIF: Abscission

Practice Problems

1. Determine the incorrect statement regarding functions of abscissic acid.

1. Tolerant to stress
2. Seed development and maturation
3. Stimulates the opening and closing of stomata
4. Delays senescence

Solution: Abscissic acid increases tolerance to stress and helps the plant to cope up with the stressful condition. It helps in seed development and maturation. This occurs by the accumulation of reserve food. It induces seed dormancy and inhibits germination. It stimulates the closure of stomata and helps to conserve water by minimising transpiration. Abscisic acid stimulates senescence of leaves by causing destruction of chlorophylls. It also inhibits protein and RNA synthesis. Hence, the correct option is d.

2. Which is a stress hormone in plants?

1. Cytokinin
2. Auxin
3. Abscissic acid
4. Ethylene
    

Solution: Abscissic acid is also known as stress hormone. This is because it increases tolerance to stress and helps the plant to cope up with the stressful condition. It acts as a growth inhibitor under unfavourable conditions, it slows down metabolic activity. During unfavourable conditions like extreme cold, the rate of photosynthesis decreases. In such conditions, ABA decreases the metabolic activity to conserve energy. Hence, the correct option is c.

3. What is the site of producing abscissic acid in plants?

1. Roots of a plant
2. Terminal buds of a plant
3. Leaves of a plant
4. Both a and b
    

Solution: Abscissic acid is produced in the roots and terminal buds of a plant. Abscisic acid is a carotenoid derivative. It is also considered as the stress hormone. Hence, the correct option is d.

FAQs

1. What is the difference between growth promoting and growth inhibiting hormones?
Answer: The growth promoting hormones promote growth in the plant body. This includes auxins and gibberellins. On the contrary, the growth inhibiting factors inhibit growth of a plant body. This includes ethylene and abscissic acid.

2. Is abcissic acid considered as a growth promoter or growth inhibitor?
Answer: Abscissic acid is considered as a growth inhibitor. It acts as a growth inhibitor under unfavourable conditions, it slows down metabolic activity. During unfavourable conditions like extreme cold, the rate of photosynthesis decreases. In such conditions, ABA decreases the metabolic activity to conserve energy. This inhibits plant growth.

3. In which category does abscissic acid belong to?
Answer: The abscissic acid belongs to the carotenoid derivatives. It is considered a plant stress hormone. Therefore, plants are able to survive in dry conditions. Carotenoids are plant pigments having yellow to orange colour.

4. How is abscissic acid antagonistic to gibberellins?
Answer: Abscissic acid is an antagonist to gibberellins. Gibberellin is a growth promoting hormone, but abscisic acid is a growth inhibitory hormone.

5. What is meant by dormin?
Answer: Dormin is the old name of the growth inhibiting hormone found in buds of plants which is undergoing dormancy. This hormone is called abscisic acid or ABA now.