Responses to environmental changes

2. Responses to environmental changes

3. 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.

4. Effects of plant growth factors Plant growth can be divided into three main types: • cell division • cell elongation / enlargement • cell differentiation (specialization). These types of growth can all be controlled by plant growth factors.

5. Tropisms Stimulus Tropism Examples 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 phototropism shoots positive, roots negative gravity geotropism roots positive, shoots negative pollen tubes positive chemical chemotropism water hydrotropism roots positive air (oxygen) aerotropism pollen tubes negative

6. Difficulties in studying growth factors The effects of a plant growth factor can depend on: • the concentration of the growth factor • the tissue being acted on, e.g. stems or roots • the developmental stage of the plant • the species of plant • 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 fact that growth factors are only present in minute amounts adds to the difficulties in studying plant responses.

7. Types of plant growth factor

8. Which growth factor?

9. Fill in the missing words