Plant Hormones or Growth Regulators or Phytohormones

Q.N. 1. What are phytohormones?  Mention the different categories of plant hormones.

Phytohormones are the chemicals that are produced by the plants to regulate their own growth and the development. They are produced in very low concentration and are transported to the various parts of the plant. They control the activities like cell division, enlargement, flowering, seed formation, dormancy and abscission.

Based on their action, phytohormones are grouped as plant growth promoters and plant growth inhibitors. Plant growth inhibitors are abscisic acid and ethylene.

Plant growth promoters

Plant growth promoters are auxins, cytokinins, and gibberellins.

Auxins

Q.N. 2. What are auxins? List the physiological effects of auxins.  

Auxins are plant growth regulators that are responsible for the elongation of the cells in the shoots. Auxin was isolated from the coleoptiles of oat seedlings by F. W. Went for the first time.

Auxins are synthesized in the meristems and transported to the growing regions of the plant by diffusion. IAA (Indole-3-Acetic Acid) is the principal natural hormone that is found in the highest concentration at the tips of the stem and the roots, in the young growing leaves, and in the flowers and the fruits.

Auxins are natural auxins viz., Indole-3-acetic acid (IAA) and Indole butyric acid (IBA) and synthetic auxins viz., 2,4-D (2,4-Dichlorophenoxyacetic acid), and NAA (Naphthalene acetic acid).

Figure 1 Structure of Indole-3-Acetic Acid

Physiological effects or functions of auxins (IAA)

Some of the many functions of auxins are as follows:

1. Cell division or callus formation: Auxins form the undifferentiated mass of the cells, called the callus, from tissues by initiating and promoting the cell division.

2. Cell elongation: Auxins make cell wall elastic and cause the elongation of cell wall during endosmosis. They also bring about the enlargement of the shoot and the root tips especially, behind the apical meristems.

3. Apical dominance: Auxins in the apical bud inhibit the development of the lateral buds; the phenomenon is called apical dominance. When an apical bud is removed, one or more lateral buds grow from the lower part of the removed apical bud.

4. Suppression of abscission layer: Auxins inhibit the abscission of the leaves and the fruits. During abscission, a zone of abscission is present at the base of the leaves or fruits, cutting off the supply of nutrients and water; the leaf and the fruit start falling. Auxins in the cells near or within the abscission zone prevent the formation of the abscission zone.

5. Eradication of weeds: As roots are extremely sensitive to auxins, the application of high concentration of 2,4-D stimulates the growth promoting activities of the cells of the roots; roots get distorted and the sieve tubes get blocked. Then, roots decay and plants die.

6. Root initiation: Auxins activate root initials when they are applied. To develop cuttings into new plants during vegetative propagation, the rapid formation of adventitious roots can be achieved by applying auxins.

7. Flower initiation: In general, auxins inhibit the flowering. However, auxins like 2,4-D and NAA promote uniform flowering in plants like litchi and pineapple.

8. Production of parthenocarpic fruits: When the plants are treated with low concentration of auxins, they develop seedless fruits even in the absence of pollination. Such seedless fruits are called parthenocarpic fruits. The process of developing such fruits is called parthenocarpy.

Cytokinins

Q.N. 3. What are cytokinins? List the physiological effects of cytokinins.

Cytokinins are the phytohormones that stimulate cell division and influence the growth of plants. They are produced in the dividing cells throughout the plant. In mature plants, they are produced in the root tips and travel to the shoots in the transpiration stream. They move upward in the xylem and pass into the leaves and fruits, where they are essential for a normal growth and cell differentiation.

Cytokinins are normally derived from adenine, the nitrogen-containing compound. Naturally occurring cytokinin was identified from young maize (Zea mays) for the first time in 1963. That cytokinin is called zeatin. The common cytokinin are kinetin, zeatin, and 6-benzylaminopurine.

Auxins and cytokinins act antagonistically i.e. auxins stimulate the growth of apical bud whereas cytokinins promote the growth of lateral buds.

Physiological effects or functions of cytokinins

Some of the many physiological effects of cytokinin are as follows:

1. Cell division: Cytokinins promote cell division in both the apical meristems and non-meristematic tissues.

2. Cell enlargement and differentiation: Cytokinins have a vital role in morphogenesis and differentiation of the shoot and the root meristems.

3. Initiation of interfascicular cambium: Cytokinins induce  the formation of interfascicular cambium in the plants.

4. Counteraction of apical dominance: Auxins and cytokinins act antagonistically in the control of apical dominance. Auxins stimulate the growth of the apical bud whereas the cytokinins promote the growth of the lateral buds.

  

5. Breaking of dormancy: Cytokinins break the dormancy of seeds to promote their germination.

6. Flowering: Cytokinins induce flowering in certain species of the plants.

7. Delay senescence: Cytokinins slow down the process of senescence. Cut leaves when dipped into cytokinins solution, they stay green longer. The retarding effect on the aging of plants by cytokinins is called the Richmond-Lang Effect.

Gibberellins

Q.N. 4. What are gibberellins? Lists the functions of gibberellins.

Gibberellins are the phytohormones that regulate the various plant developmental processes like stem elongation, germination, dormancy, flowering, flower development, leaf and fruit senescence. Gibberellic acid is the commercially available gibberellin.

Gibberellins were first discovered by Japanese scientist Kuroswa in 1926 while studying Bakanae disease, caused by Giberella fujikuroi, in rice. Later on, the gibberellic acid was isolated by Yabuta and Sumiki in 1935 for the first time.  

Now, more than 100 different types of gibberellins have been isolated. From Gibberella fujikkuroi only, more than 15 types of gibberellins have been isolated. Some of the well known gibberellins are GA1, GA2, GA3, GA4, GA5, GA6, GA7, GA8, and GA9.

GA3, the most thoroughly studied gibberellin, is present in the immature seeds, the root and the shoot apex, the young leaves, the buds, and the embryos in high concentration. From those parts, they are transported to the different parts of the plants through the xylem and phloem.

Physiological effects or functions of gibberellins

Some of the many functions of gibberellins are listed as follows:

1. Stem elongation: Genetic dwarf varieties of pea and corn plants attain normal height when they are treated with gibberellins. The normal height is due to the stem elongation by enlargement of the cells. Gibberellins induce rapid cell division and cell elongation.

2. Bolting of rosette plants: Plants such as cabbage have rosette appearance due to reduced internodal length and a number of leaves around the shoot apex. When those plants with rosette appearance are treated gibberellins, their dwarf stem is converted into the tall by the enormous elongation of internodes. This rapid and enormous elongation of the stem of rosette plants is known as bolting.

3. Leaf expansion: In many plants, the leaves become broad and elongated, when treated with gibberellic acid.

4. Breaking of dormancy: During winter, many plants have buds with reduced metabolic activity and growth. Such buds are called dormant buds and the condition is called dormancy of buds. Seeds of many plants do not germinate immediately after they mature; they germinate after a certain duration when they receive a suitable environment. Such a condition is known as dormancy of seeds. Gibberellins break the dormancy of buds and seeds.

5. Parthenocarpy: Gibberellins are more effective than auxins in inducing parthenocarpy in fruits like apple, tomato, and pear.

6. Sex expression: The treatment of gibberellins induces the formation of male flowers in place of female flowers in plants like cucurbits and hemps.

7. Reversal of dwarfism: Gibberellins reverse the genetic dwarfness of plants like corn and pea.

8. Flowering: Application of gibberellins promotes the flowering in the long day plants under the unfavourable short day conditions.