Plant Hormones

Plant hormones can be defined as a chemical messenger, which plays a vital role in controlling the entire life processes and other activities in the plant cell like, seed germination, ripening of the fruit, growth, flowering and lot more. A plant hormone is also referred as “phytohormones” or “plant growth regulators.” Drought, nutrients, temperature, light, and stress are the environmental factors that affect the production of plant growth hormone.

Different types of Plant Hormones.

There are five different types of plant growth regulators known to affect the life processes and functions of plants.

Auxin

Auxins

Auxin is also referred as IAA- indole-3-acetic acid. It was the first phytohormones to be recognized. This plant growth regulator plays an important role in the growth and development of a plant. They play a vital role in:

  • Auxin helps in cell enlargement by increasing the plasticity of the cell wall.
  • Auxin helps in promoting the growth of the plant’s tissue.
  • Auxin is responsible for phototropism and geotropism.
  • Auxin helps in promoting root initiation and stem cutting in plants.
  • Auxin inhibits the growth of lateral bud by stimulating the production of ethylene.
  • Auxin controls the aging and senescence of plants.
  • Auxin helps in promoting the growth of many fruits without fertilization.
  • Auxin helps in delaying the shedding of leaves.

Cytokinins

Cytokinins

Cytokinins are involved in promoting cell division (cytokinesis) hence it is referred to as   Cytokinins. Zeatin is a naturally occurring plant growth hormone, which is isolated from corn plant. Cytokinins are present in all cell division sites including root tips, leaves, fruits, and seeds.

  • Cytokinins help in stomatal opening.
  • Cytokinins help in delaying the senescence of leaves.
  • Cytokinins promote the expansion of cotyledons.
  • Cytokinins are involved in the enlargement of stem cells.
  • Cytokinins are also involved in promoting chloroplast maturation.

Gibberellins

GIBBERELLINS

The plant hormone Gibberellin is the most abundant hormone in plants. There are more than 70 gibberellins isolated. They are GA1, GA2, GA3.. and so on. The GA3 Gibberellic acid is the most widely studied plant growth regulators.

  • Gibberellins help in controlling the elongation of the stem in young shoots.
  • Gibberellins help in seeds germination, enzyme production and food production, which is required for the new cells to grow.
  • Gibberellins help in promoting flowering, cell division, and growth of seeds after germination.
  • Gibberellins help in increasing the number of fruits, the size of a fruit and also delay the ripening of fruits in citrus plants.
  • Gibberellins are also involved in seed germination.

Ethylene

Ethylene

Ethylene is an organic compound and gaseous substance, which is produced from an amino acid. It is present in the large amount and is mainly produced by plants that are under stress.

  • Ethylene helps in stimulating fruit ripening and determining the sex of a flower.
  • Ethylene is involved in the production of female flowers in a male plant.
  • Ethylene promotes Apo-geotropism in roots.
  • Ethylene helps in the root initiation of plants.
  • Ethylene increases the speed of senescence.     

Abscisic acid

ASBISCIC-ACID

It is a chemical messenger produced by other parts of a plant to indicate whether the plant is experiencing water stress, this signaling is mainly produced by drought roots and leaves.

  • Abscisic acid helps in promoting bud dormancy and senescence of leaves.
  • Abscisic acid helps in the closing of stomata during Transpiration.
  • Abscisic acid inhibits detachment of plant parts.
  • Abscisic acid plays an important role in seed development, maturation and dormancy.
  • Abscisic acid helps in slowing the plant growth and protects undeveloped bud during the cold season.

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Practise This Question

Three sets of oat saplings named A, B and C were grown in different conditions in a box with a light from one side.
A: Tips were removed.
B: Tips were covered.
C: The saplings were left as they were.

Following observations were noted:-

i. Shoot bent towards the source of the light.
ii. Shoot growth ceased.
iii. Shoots grow normally in the vertical direction.

Match the seedlings with the correct observations.