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Auxin

A plant hormone that regulates growth

Plant Hormones

A plant stem bends as it grows toward the light
Plant growing
towards the light

There are several different plant hormones. Those controlling the growth of roots and shoots are called auxins which stimulate cell growth.

Shoots grow upwards and roots downwards
Shoots grow upwards and roots downwards

Auxins can diffuse from cell to cell in solution or travel greater distances in the plant in the vascular bundles.

If a plant has light falling on it from one side only, it will grow towards the source of the light. This is as a result of auxin accumulating on the side of the plant away from the light, causing extra growth in this region and so causing the plant to bend towards the light.

Roots are not sensitive to light but grow downwards in response to the stimulus of gravity. Auxins are again involved, although scientists think another plant hormone might actually cause slowing of the growth of cells on one side of the root causing it to bend downwards.

Commercial Use of Plant Hormones

Plants hormones are used widely in agriculture in the following ways:

A plant cutting with initial root growth
Some time later new
roots are grown

A plant cutting with the base of its stem being dipped in rooting hormone powder
Cutting dipped in
rooting hormone powder
  • As selective weedkillers which are not harmful to the surrounding plants or to animals.
  • To produce new plants from cuttings. A cutting is part of a plant which, when removed and given suitable conditions, develops shoots and roots and becomes an independent plant. With the use of hormone and rooting powder, cut stems can develop roots and grow into clones of the parent plant.
  • To promote the ripening of fruit or induce flowering when required.
  • To speed up or slow down the growth of plants.
  • To produce fruit without seeds. Unpollinated flowers are treated so that they grow fruit without seeds.

Question 8