PLANT HORMONES
BOTANY

PLANT HORMONES | FOUR MAJOR TYPES OF PLANT HORMONES


What is Plant Hormones? The special substances produced by the plants which influence the growth and plant responses to various stimuli are called plant hormones.Let’s discuss in detail different types of plant hormones are as under;

TYPES OF PLANT HORMONES

There are four types of plant hormones;

  • Auxins
  • Gibberellins
  • Cytokinins
  • Abscisic acid

AUXINS

These are Indole Acetic Acid (IAA) or their variants.

CHARACTERISTICS

1)         Cell enlargement: In the stem, promote cell enlargement in the region behind apex.

2)         Cell division: Promote cell division in cambium.

3)    Growth of roots: In root, promote growth at very. low concentrations. Inhibits growth at higher concentrations e.g., geotropism. Promote growth of roots from cuttings and calluses.

4)         Bud initiation: Promote bud initiation. in shoots but sometimes  function opposite to cytokinins and inhibitory.

5)         Apical dominance: Promote apical dominance and fruit growth.

6)         Parthenocarpy: Can sometimes induce parthenocarpy (fruit (seedless) formation without pollination and fertilization).

7)         Leaf senescence: Cause delay in leaf senescence (senesce    grow old, aging) in a few species.

8)         Abscission: Inhibit abscission (leaf fall).

Commercial applications

Discovery of IAA (Indole acetic acid) led to the synthesis of wide range of compounds by chemists. The synthetic. auxins are economical  than IAA to produce and often more active because plants generally 40 not have necessary enzymes to break them down.

GIBBERELLINS

Gibberellins are produced commercially from fungal cultures:

1)         Cell enlargement: Promote, ceil enlargement in presence Of auxins.

2)         Cell division: Promote cell division in apical meristem and cambium.

3)         Bolting: Promote ‘bolting’ of some rosette plants.

4)         Bud initiation: Promote bud initiation in shoots of chrysanthemum callus.

5)         Leaf and fruit growth: Promote leaf growth and fruit growth.

6)         Parthenocarpy: Sometimes may induce parthenocarpy.

7)         Apical dominance: apical dominance, enhance the action of  8) Break dormancy: Break bud and seed dormancy.

9)         Flowering in long-day plants: Sometimes substitute for red light.  Therefore promote flowering in long-day plants, while inhibit in   short-day plants.

10)       Leaf senescence: Causes delay in leaf senescence (senesce    grow old) in a few species.

Commercial applications

Some of their commercial applications are as under;

1. Fruit setting: GA promotes fruit setting e.g., in .tangerines (a small  sweet orange-coloured citrus fruit with a thin skin) and pears and are used for growing seedless grapes (parthenocarpy) and also increasing the berry size.

2. Used in brewing industry: GA3 is used in the brewing industry to stimulate a-amylase production in barley and this promotes malting.

3.Delay fruit ripening: It is used to delay ripening of fruits.

4. Improve storage life of fruits: It is used to improve storage life of bananas and grape fruits.

CYTOKININS

1)         Stem growth: Promote stem growth by cell division in apical meristem and cambium.

2)         Root growth: Inhibit primary root growth and promote lateral root growth.

 3)        Bud initiation: Promote bud initiation

4)         Leaf growth: Promote leaf growth.

 5)        Fruit growth: Promote fruit growth but can rarely induce

parthenocarpy.

 6)        Bud growth: Promote lateral bud growth.

7)         Bud dormancy: Break bud dormancy.

8)         Leaf senescence: Cause delay in leaf senescence.

9)         Stomatal opening: Promote stomatal opening.

 Commercial application: Cytokinins delay the aging of fresh leaf crops such as cabbage and lettuce (delay of senescence) as well as keeping flowers fresh. They can also be used to break dormancy of some seeds.

ABSCISIC ACID

1)         Stem growth: Inhibits stem growth notably during physiological stress, e.g., drought, water-logging.

2)         Bud dormancy: Promotes bud dormancy.

3)         Seed dormancy: Promotes seed dormancy.

4)         Flowering: Promotes flowering in short day plants, and inhibits in long day plants (antagonistic to gibberellins).

5)         Leaf senescence: Sometimes promotes leaf senescence.

 6)        Abscission: Promotes abscission.

7)         Closing of stomata: Promotes closing of stomata under conditions of water stress (wilting).

Commercial application: Abscisic acid can be sprayed on tree crops to regulate fruit drop at the end of the season. This removes. the need for picking over a large time-span.

Ethene

 1)        Stem growth: Inhibits. stem growth, notably during physiological stress.

2)         Root growth: Inhibits root growth.

3)         Bud dormancy: Breaks dormancy of bud.

4)         Flowering: Promotes flowering in pineapple.

5)         Fruit ripening: Promotes fruit ripening.

Commercial application: Ethene induces flowering in pineapple. Stimulates ripening of tomatoes and citrus fruit. The commercial compound ethephon breaks down to release ethane in plants and is applied to rubber plant to stimulate the flow of latex (a milky white liquid produced by the rubber tree). It is brought about in higher animals by nervous and chemical coordination.