5.9B–5.10BPlant defence responses
Biology only — not on the Combined Science specification
Plants cannot run away or make antibodies, so their defences are built in. The specification splits them three ways, and a question asking for "physical and chemical defence responses" wants them kept apart.
Mimicry is the odd one out and worth a sentence of its own: some plants have markings that look like insects already feeding, or like eggs, so a passing insect goes elsewhere; others resemble a species that is poisonous. Nothing is physically stopping the animal — it is deterred by what it thinks it sees.
Common mistake
Bark is a physical defence, not a chemical one, and the point is that the layers fall off — taking any pathogens growing on them with it. Thorns are mechanical rather than physical in this classification, so use the specification's own three groups rather than inventing your own.
- Mimicry
- a plant defence in which markings resemble insects already feeding, eggs, or a poisonous species, so that an animal is deterred by what it appears to see
5.11BDetection and identification of plant diseases
Biology only — not on the Combined Science specification
Plants can be infected by a range of viral, bacterial and fungal pathogens, as well as by insects. Knowledge of plant diseases is restricted to three: tobacco mosaic virus as a viral disease, black spot as a fungal disease, and aphids as insects.
Aphids are worth a second look, because they are the one that is not a pathogen at all. They are insects that pierce the plant and feed on the sugars in the phloem, weakening it and stunting growth — and they can carry viruses from plant to plant while they do it.
Higher tier only: plant diseases can be detected by stunted growth, spots on leaves, areas of decay (rot), growths, malformed stems or leaves, discolouration, and the presence of pests.
Identification can then be made by reference to a gardening manual or website, by taking infected plants to a laboratory to identify the pathogen, or by using testing kits that contain monoclonal antibodies — which is where monoclonal antibodies turn up again in a greenhouse.
Not every sick plant has an infection. Plants can also be damaged by ion deficiency conditions, and knowledge of ions is limited to two:
• Stunted growth caused by nitrate deficiency. Nitrate ions are needed for protein synthesis, and therefore for growth. • Chlorosis caused by magnesium deficiency. Magnesium ions are needed to make chlorophyll, so without them the leaves turn yellow.
Both are worth knowing as a pair with their reason attached, because the reason is the mark.
This is knowledge with a job. Understanding ion deficiencies allows horticulturists to provide optimum conditions for their plants: reading the symptoms tells a grower which mineral the soil is short of, so the right fertiliser is added rather than a general one — cheaper, and better for the surrounding water.
Common mistake
Stunted growth and yellow leaves are not automatically a disease. If there are no spots, no pests and no rot, a mineral ion deficiency is the more likely answer — and the exam sets this trap deliberately.
- Chlorosis
- yellowing of the leaves caused by magnesium deficiency, because magnesium ions are needed to make chlorophyll
- Aphid
- an insect that pierces a plant and feeds on the sugars in the phloem, weakening it and stunting growth, and which can carry viruses from plant to plant
- Ion deficiency condition
- damage to a plant caused by a shortage of a mineral ion rather than by infection, such as stunted growth from nitrate deficiency or chlorosis from magnesium deficiency
Before you practise
- Physical defences: the waxy cuticle, the cellulose cell wall, and layers of dead bark that fall away.
- Chemical defences are poisons and antibacterial compounds — several are the origin of human drugs.
- Mechanical adaptations — thorns, hairs, drooping or curling leaves, mimicry — deter animals rather than pathogens.
- Identify a disease in the field from the symptoms and by eliminating environmental causes such as drought or a mineral deficiency.
- Identify it in the laboratory by testing with monoclonal antibodies or by analysing the DNA of the pathogen.