Responding to the environment - PLANTS

Definitions, explanations and terms.

Hormone

A chemical messenger produced in one part of the plant and has an effect on another part of the stem

Tropism

Growth or turning movement of a plant or part of a plant in response to external stimulus

Phototropism

Growth of part of a plant in response to the stimulus of light

Geotropism

Growth of a plant in response to gravity

Unilateral light

Light coming from one side or direction

Herbicide

A substance that is toxic to plants and destroy unwanted vegetation (weeds)

Apical bud

The growth point of the stem located at the tip of the stem

Apical dominance

When auxins produced at the tip of the stem inhibit the growth of the branches closer to the tip of the stem

Auxins

The main hormones that cause plants to grow. They stimulate or can prevent (inhibit) growth.

Main functions of Auxins

Apical dominance

Growth regulation (responsible for phototropism and geotropism)

Other functions of Auxin

Cell division (mitosis)

Formation of adventitious roots in cuttings

Development of flowers and fruits

Abscission of leaves and ripe fruits (causes leaves and fruit to fall off)

Negative tropism

When the movement of a plant is away from the stimulus

Positive tropism

When the movement of a plant is toward the stimulus

Role of auxins in phototropism (sunlight directly overhead)

Auxins produced at the tip of the stem/shoot

Auxins move downwards evenly

This even distribution brings about equal growth on all sides of the stem/shoot

As a result the stem grows upward

Role of auxins in phototropism (unilateral light)

The auxin concentration will be high on the dark side - light destroys auxins

More growth occurs on the dark side because auxins stimulate growth on the dark side

As a result the stem/shoot bends towards the light

Positively phototropic

Are Stems for they bend toward the light

Negatively phototropic

Are Roots for they grow away from the light

Advantages of stems being positively phototropic

Favourable position for leaves to receive sunlight needed for photosynthesis

Allows easy pollination of flowers

Allows easier seed dispersal

Role of auxins in geotropism (Upright)

Auxin is produced at the tip of the roots

Auxin move upwards evenly

The even distribution brings about growth on all sides of the root

As a result the root grows downward

Role of auxins in geotropism (Horizontally)

horizontally (only one side exposed to gravity)

The auxin concentration will be high on the lower side of the root - gravity attracts auxins

More growth occurs on the upper side of the root because auxins on the lower side inhibit growth

As a result the root bends downwards