Physical barriers in plantsSpec 5.9B
Plants can be attacked by pests (such as insects) and by pathogens. Some plants defend themselves with physical barriers that stop these entering.
- The leaf cuticle is a waxy layer on the surface of leaves. It forms a barrier that pathogens and pests find hard to get through.
- The cell wall is made of cellulose around every plant cell. It is a strong barrier that pathogens must get past before they can enter the cell.
Chemical defences in plantsSpec 5.10B
Some plants produce chemicals that defend them against attack. Some of these chemicals kill or slow the growth of microorganisms. Others are poisons that make the plant harmful or unpleasant for pests to eat.
Some of these plant chemicals can be used by humans to treat disease or relieve symptoms. Common examples are:
- willow bark contains a chemical that the painkiller aspirin was developed from
- foxgloves contain digitalis, a chemical used to treat some heart conditions
- the bark of the cinchona tree contains quinine, which was used to treat malaria
This is also why plant extracts are tested for antimicrobial properties, as in the core practical later in this topic.
Detecting and identifying plant diseasesSpec 5.11B
Plant diseases can be detected and identified in the field and in the laboratory. There are four approaches you need to be able to describe.
| Method | What is done | What it tells you |
|---|---|---|
| Eliminating environmental causes | Check conditions such as soil nutrients, water supply, temperature and pollution | Whether the symptoms could be caused by something other than a pathogen, such as a mineral deficiency or drought |
| Distribution analysis | Record where affected plants are, for example on a map | Whether the pattern points to spread of a pathogen (clusters around a source) or to an environmental cause (an area with poor soil) |
| Observing visible symptoms | Look for spots, discolouration, lesions, wilting, growths or leaf loss | Which disease it is likely to be, by comparing with known examples |
| Diagnostic testing | Test samples in the laboratory, or use testing kits in the field, for example kits that use antibodies to detect a pathogen's antigens, or tests that identify its DNA | Which pathogen is present |
Visible symptoms alone may not be enough, because different problems can look similar. For example, yellow leaves can be caused by a mineral deficiency or by a pathogen. That is why environmental causes are eliminated first and laboratory tests are used to confirm the pathogen.
Quick check
Name two physical barriers that protect plants from pathogens.
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The leaf cuticle and the cell wall.
Give one way plant chemicals can be useful to humans.
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Some can be used as medicines to treat disease or relieve symptoms, for example aspirin from willow.
Why do scientists eliminate environmental causes when investigating a plant disease?
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To check that the symptoms are not caused by something such as a mineral deficiency, rather than a pathogen.
What is distribution analysis?
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Recording where the affected plants are to look for a pattern that suggests the cause.