Hormones in Plants (Phytohormones)

Hormones in Plants

Plant hormones are signal particles, produced within plants, that take place in very low concentrations. Plant hormones manage all aspects of plant development and growth, from embryogenesis, the regulation of organ size, pathogen defense, tension tolerance, and through to reproductive development. Some of the unique substances produced by the plants which influence the development and plant response to various stimuli are given below.

(a) Auxins:

These are indole acetic acid (IAA) or its variants.

  • In stem, promote cell augmentation in the region behind apex. Promote cellular division in the cambium.
  • In the root, promote growth at very low concentrations. Prevent growth at higher concentrations, e.g., geotropism. Promote the growth of roots from cuttings and calluses.
  • Promote bud initiation in shoots however often antagonistic to cytokinin’s and is repressive.
  • Promote apical dominance and fruit growth. They can sometimes induce parthenocarpy.
  • Cause delay in leaf senescence (aging) in a couple of species.
  • Inhibit abscission.
Further Reading:  PAGE - Poly Acrylamide Gel Electrophoresis
Synthetic auxins

NAA (Naphthalene acetic acid) Indole propionic acid

  • Stimulates fruiting – help natural fruit set. In some cases, causes fruit setting in lack of pollination (parthenocarpy).

2,4 D (2,4 Dichloro phenoxy acetic acid)

  • Selective weed killer. Eliminates broad-leaved species (dicots). Utilized in cereal crops and grass to eliminate weeds. Prevents the growing of potatoes. Prevents early fruit drop (slows down abscission).
Commercial Applications

The Discovery of IAA resulted in the synthesis of the vast array of substances by chemists. The artificial auxins are economical than IAA to produce and frequently more active because plants normally do not have the needed enzymes to break them down.

(b) Gibberellin:

These are produced commercially from fungal cultures.

  • Promote cell enhancement in the existence of auxins. Also, promote cell division in apical meristem and cambium.
  • Promote ‘bolting’ of some rosette plants.
  • Promote bud initiation in shoots of chrysanthemum callus.
  • Promote leaf growth and fruit growth. May induce parthenocarpy.
  • In apical supremacy, improve the action of auxins.
  • Break bud and seed dormancy.
  • Often might alternative to red light. For that reason, promote lowering in long-day plants, while preventing in short-day plants.
  • Cause delay in leaf senescence in a few types.
Further Reading:  Disorders of Human Circulatory System
Commercial Applications

A few of their industrial applications are as under.

  1. GA promotes fruit setting e.g., in tangerines and pears, and is used for growing seedless grapes (parthenocarpy) and likewise increases the berry size.
  2. GA3 is used in the brewing industry to promote a-amylase production in barley and this promotes malting.
  3. To delay ripening and enhance the storage life of bananas and grapefruits.
(c) Cytokinins
  • Promote stem growth by cell division in apical meristem and cambium.
  • Hinder primary root development.
  • Promote lateral root growth.
  • Promote bud initiation and leaf development.
  • Promote fruit development but can seldom cause parthenocarpy.
  • Promote lateral bud growth, also break bud dormancy.
  • Cause delay in leaf senescence.
  • Promote the stomatal opening.
Commercial Application

Cytokinins postpone the aging of fresh leaf crops, such as cabbage and lettuce (delay of senescence) along with keeping flowers fresh. They can also be used to break the dormancy of some seeds.

(d) Abscisic acid:
  • Inhibits stem and root growth notably during physiological stress, e.g., dry spell, and waterlogging.
  • Promotes bud and seed dormancy.
  • Promotes flowering in short-day plants, and inhibits in long-day plants (antagonistic to gibberellins).
  • Often promotes leaf senescence.
  • Promotes abscission.
  • Promotes the closing of stomata under conditions of water stress (wilting).
Further Reading:  Glycolysis, Pyruvic Oxidation, Krebs cycle, and Respiratory Chain
Commercial Application:

Abscisic acid can be sprayed on tree crops to regulate fruit drop at the end of the season. This removes the requirement for choosing over a large time-span.

(e) Ethene:
  • Inhibits stem growth, significantly during physiological stress.
  • Inhibits root growth.
  • Breaks dormancy of bud.
  • Promotes flowering in pineapple.
  • Promotes fruit ripening.
Commercial Application

Ethene induces flowering in pineapple. Promotes ripening of tomatoes and citrus fruit. The industrial substance ethephon breaks down to release ethene in plants and is applied to the rubber plant to promote the flow of latex.