Plant diseases also cause mineral deficiencies: nitrate deficiency → stunted growth (no amino acid synthesis). Magnesium deficiency → yellow leaves (no chlorophyll).
Detection: look for symptoms, test kits with monoclonal antibodies, send to laboratory.
Plant defence mechanisms
Physical: cellulose cell walls, bark, waxy cuticle (prevents entry of pathogens), thorns and spines (prevent grazing).
Chemical: antibacterial chemicals (e.g. mint produces menthol), poisons to deter predators (nicotine, poison ivy), attract insects that prey on pests.
Mechanical: leaves that droop or curl when touched to deter insects, mimicry (some plants look like other less palatable plants).
Exam questions — 3 questions · 11 marks
⚡ Extended questions included. Read the hint. Write in full sentences. AI marks against the AQA mark scheme.
1 markMineral deficiencyAQA 8461 style
A plant has yellow leaves and poor growth. Which two mineral deficiencies could explain these symptoms?
A Calcium and potassium deficiency
B Nitrate deficiency (stunted growth — needed for amino acids) and magnesium deficiency (yellow leaves — needed for chlorophyll)
C Phosphate deficiency only
D Potassium and calcium deficiency
4 marksPlant defence mechanismsAQA 8461 P1 style
Describe three ways in which plants defend themselves against pathogens and pests. Include physical, chemical and mechanical examples. (3 marks)
Hint: One example from each category. For each: name the defence AND explain how it protects the plant.