3.6 PLANT HORMONES

LEARNING OUTCOMES:1. State what plant hormones are A plant hormone is a chemical substance which is produced by the plant and influences the growth and development of the plant. It brings effect near the site of its formation or it may be transported to other parts of the plant where it acts.

2. Give examples of plant hormonesAuxins and ethylene (ethene)

LEARNING OUTCOMES:3. Infer the effects of auxins on the growth responses

Plant respond to stimuli by growing to a certain direction. This growth response is called tropism. These responses are controlled by hormones called auxins.

4. Explain the role or auxins in tropismAuxins control the growth movements of shoots and roots in plants. There are 2 types of tropism :

Tropism Stimulus Response

Phototropism Direction of light from one side

Shoots bend towards light (Positive phototropism)Roots grow away from light (Negative phototropism)

Geotropism Force of gravity

Shoots grow upwards against the force of gravity (Negative geotropism)Roots grow downwards in the direction of gravity (Positive geotropism)

PhototropismThe growth movement of a plant in response to light is called phototropism.

GeotropismThe growth movement of a plant in response to gravity is called geotropism. Role of auxins in geotropism:

1. Gravity causes the tip of growing shoots and roots to produce auxins.

2. Auxins diffuse into zone of elongation.3. Owing to gravity, auxins move to lower side of

shoot and root.4. The lower side of shoot and root has a higher

concentration of auxins than the upper side.

5. A high concentration of auxins in the shoots promotes elongation of cells. Hence the lower side of shoot with higher concentration of auxins will grow faster than the upper side. As a results, the shoot curves and grows upwards, showing negative geotropism.

6. A high concentration of auxins inhibits the elongation of cells in root. The upper side of the root grows faster than the lower side. The young root curves and grows downwards, showing positive geotropism

USE OF HORMONES IN AGRICULTUREHORMONE

USE OF HORMONE

EXPLANATION

auxins In hormones rooting powders

Promote the growth of adventitious roots from the cut ends of stems. It helps cutting to produce roots more quickly.

In selective weed killers

Kill the dicot weeds while leaving cereal crops or grasses (monocot) of a lawn largely unharmed.

In fruit setting Prevent young developing fruits from falling off prematurely.Certain flowers that are sprayed with auxins can form fruits without being fertilised. Such fruits are seedless, for example, seedless grapes. The process is called parthenocarpy

In producing bushy plants

Gardeners cut off the apical bud to reduce level of auxins so that the lateral buds can grow into side shoots and produce bushier plants

In promoting growth of plants and fruits

Promote growth of plant cells. Gardeners use auxins to stimulate growth of fruits, to delay fruit ripening and prevent fruit from falling off

Ethylene In promoting the ripening of fruits

Fruits are usually imported in an unripe state to prevent damage. However the fruits can then be ripened quickly for sale by exposing them to ethylene gas.