Plant Defences

Plants face constant threats from herbivores (animals that eat them), pathogens (disease-causing microorganisms) and environmental damage. Unlike animals, plants cannot run away, so they have evolved a range of defence mechanisms. This lesson covers the physical, chemical and mechanical defences of plants, as well as their wound responses, as required by Edexcel GCSE Biology (1BI0) Topic 6.

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Why Do Plants Need Defences?

Plants are under constant attack from:

• Herbivores — animals (including insects) that feed on plant tissue
• Pathogens — bacteria, viruses and fungi that cause disease
• Parasites — organisms that live on or in the plant and take nutrients from it

Without defences, plants would be consumed, infected and destroyed. Natural selection has led to the evolution of many defence mechanisms that help plants survive and reproduce.

Exam Tip: When discussing plant defences, always classify them as physical, chemical or mechanical. This shows the examiner you understand the different categories and can organise your knowledge clearly.

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Physical Defences

Physical defences are structural barriers that prevent pathogens from entering the plant and deter herbivores from feeding.

1. Cellulose Cell Walls

• Every plant cell has a cell wall made of cellulose
• Cellulose is a strong, rigid material that acts as a physical barrier
• Pathogens must penetrate the cell wall to infect the cell
• The cell wall is the first line of defence at the cellular level

2. Bark

• Trees and woody plants produce bark — a tough, outer layer of dead cells
• Bark protects the living tissues beneath (including the phloem) from:
  • Physical damage (abrasion, impacts)
  • Pathogen entry — bark is difficult for microorganisms to penetrate
  • Water loss — bark is relatively waterproof
• If bark is damaged, the tree becomes vulnerable to infection

3. Waxy Cuticle

• The leaves and stems of most plants are covered by a waxy cuticle
• This serves as a waterproof barrier that:
  • Prevents excessive water loss through evaporation
  • Creates a physical barrier that pathogens find difficult to penetrate
  • Repels water from the leaf surface, reducing the conditions that favour fungal growth (fungi thrive in moist conditions)

4. Thorns, Spines and Prickles

• Many plants produce thorns (modified stems), spines (modified leaves) or prickles (outgrowths of the epidermis)
• Examples: roses (prickles), cacti (spines), hawthorn (thorns)
• These structures deter herbivores — animals are less likely to eat a plant that will cause them pain or injury
• Some thorns are large enough to cause serious injury to large herbivores

5. Hair-like Structures (Trichomes)

• Some plants have trichomes — tiny hair-like structures on the surface of leaves and stems
• Trichomes can deter small herbivores (such as insects) by making it physically difficult for them to reach the leaf surface
• They can also trap insects (as in some carnivorous plants)

Physical defence	Location	How it protects
Cellulose cell wall	All plant cells	Barrier to pathogens at cellular level
Bark	Trunk and branches of trees	Tough outer layer protects against damage and pathogens
Waxy cuticle	Leaf and stem surfaces	Waterproof barrier; blocks pathogen entry; reduces water loss
Thorns/spines/prickles	Stems, leaves, branches	Deter herbivores by causing pain
Trichomes (hairs)	Leaf and stem surfaces	Deter small herbivores; may trap insects

Exam Tip: If asked for physical defences, list at least three different types. Remember that the waxy cuticle has a dual role: it reduces water loss AND acts as a barrier to pathogens.

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Chemical Defences

Plants produce a wide range of chemicals that deter herbivores, kill pathogens or make the plant unpalatable.

1. Antimicrobial Substances

• Many plants produce chemicals that can kill or inhibit the growth of microorganisms
• These substances are often found in the plant's tissues and are released when the plant is damaged
• Examples:
  • Mint produces menthol — has antibacterial and antifungal properties
  • Witch hazel produces tannins — have antimicrobial effects
  • Tea tree produces tea tree oil — well known for its antiseptic properties
  • Garlic produces allicin — has antibacterial properties

2. Poisons (Toxins)

• Some plants produce toxic chemicals that are harmful or fatal if eaten
• This deters herbivores from consuming the plant
• Examples:
  • Deadly nightshade (Atropa belladonna) — contains atropine, which is highly toxic to humans and many animals
  • Foxglove (Digitalis purpurea) — contains digitalis (cardiac glycosides), which affects the heart and can be fatal
  • Ricin — a highly toxic substance found in castor bean seeds
  • Nicotine — produced by tobacco plants as an insecticide (it is toxic to insects)

3. Insect Repellents

• Some plants produce chemicals that repel insects rather than killing them
• Examples:
  • Citronella — produced by lemongrass; well-known insect repellent
  • Pyrethrin — produced by chrysanthemum flowers; used as a natural insecticide
  • Neem — produces azadirachtin, which repels and disrupts insect feeding

4. Bitter or Unpleasant Taste

• Many plants produce tannins and other compounds that give their leaves and bark a bitter taste
• Herbivores learn to avoid plants that taste unpleasant
• This is a form of learned avoidance — animals that eat the plant once and experience the bitter taste are less likely to eat it again

Chemical defence	Examples	How it protects
Antimicrobial substances	Mint (menthol), witch hazel (tannins)	Kill or inhibit pathogens
Poisons/toxins	Deadly nightshade (atropine), foxglove (digitalis)	Deter/kill herbivores
Insect repellents	Citronella, pyrethrin	Drive away insect herbivores
Bitter taste (tannins)	Oak bark, tea	Herbivores avoid eating the plant

Exam Tip: You do not need to memorise every example, but you should be able to give at least two examples of chemical defences with specific plant names. The examiners reward specific knowledge.

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Mechanical Adaptations

Mechanical defences involve movement or structural changes in response to threats. They go beyond simple physical barriers by involving active responses.

1. Leaves That Curl When Touched (Mimosa)

• The sensitive plant (Mimosa pudica) has leaves that fold inward and droop rapidly when touched
• This sudden movement may startle herbivores and cause them to move on
• The collapsed leaves also appear less appetising and expose thorns on the stem
• The mechanism involves rapid changes in turgor pressure in specialised cells at the base of each leaflet

2. Thorns (Modified Leaves or Stems)

• While thorns were listed under physical defences, they can also be considered mechanical because some plants produce more thorns in response to herbivory
• When a plant is repeatedly browsed by animals, it may produce denser, sharper thorns on new growth
• This is a mechanical response to damage

3. Stinging Hairs (Nettles)

• Stinging nettles (Urtica dioica) have hollow, needle-like hairs (trichomes) on their leaves and stems
• When an animal or person brushes against the hairs, the fragile tips break off
• The broken hair acts like a hypodermic needle, injecting a mixture of chemicals including histamine, acetylcholine and formic acid into the skin
• This causes a painful, stinging sensation and a rash
• The pain teaches herbivores to avoid nettles in the future

4. Leaf Folding and Rolling

• Some plants roll or fold their leaves in response to touch or damage
• This can hide the leaf surface from herbivores and make the leaf less accessible