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Responses to environmental changes

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What are plant hormones?

Plants need a method of responding to their environment to cope with predation and abiotic stresses, such as drought.

Plants do not have a nervous system, so a major way in which they respond to the environment is by using plant hormones, often called plant growth factors.

Unlike animal hormones, plant growth factors are not made in specialized organs but in many tissues all over the plant. They may be used where they are made, or transported within the plant. They usually exert their influence by affecting growth.

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Effects of plant growth factors

Plant growth can be divided into three main types:

These types of growth can all be controlled by plant growth factors.

cell differentiation (specialization).

cell elongation / enlargement

cell division

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Tropisms

A tropism is the movement of part of a plant in response to, and directed by, an external stimulus. The movement is by growth and is therefore slow. It is described as positive or negative depending on whether the growth is towards or away from the stimulus respectively.

light

gravity

chemical

water

air (oxygen)

phototropism

geotropism

chemotropism

hydrotropism

aerotropism

shoots positive, roots negative

roots positive, shoots negative

pollen tubes positive

roots positive

pollen tubes negative

Stimulus Tropism Examples

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Difficulties in studying growth factors

The effects of a plant growth factor can depend on:

The fact that growth factors are only present in minute amounts adds to the difficulties in studying plant responses.

what other growth factors are present. Another growth factor can increase the effect of the growth factor being studied (synergism) or reduce it (antagonism).

the species of plant

the developmental stage of the plant

the tissue being acted on, e.g. stems or roots

the concentration of the growth factor

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Types of plant growth factor

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Which growth factor?

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