Plant chemical defences: antibacterial chemicals and poisons

Infection and response • Plant disease (biology only)

Flashcards

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How do deterrent chemicals influence herbivore feeding behaviour?

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Deterrent chemicals create unpleasant taste or adverse effects after ingestion, which causes herbivores to avoid those plants in future.

Key concepts

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Definition of chemical defences

Chemical defences are secondary metabolites and other compounds produced by plants that reduce infection or herbivory. These compounds either act directly on pathogens (for example by killing or inhibiting microbial growth) or act on herbivores (for example by causing sickness, bitter taste or reduced digestion). Limiting factors include the amount produced, the plant's energy budget, and whether the compound is available at the site of attack. Effectiveness depends on concentration, location and the sensitivity of the attacker.

Antibacterial chemicals in plants

Some plant species produce naturally occurring antibacterial compounds in leaves, bark or sap that limit bacterial infection. Plants such as mint and witch hazel possess antibacterial compounds that prevent bacterial growth on plant tissues and can be isolated for antiseptic use. Cause → effect: antibacterial compound synthesis → disruption of bacterial cell processes or walls → reduced bacterial colonisation and lower infection spread.

Poisons and deterrent chemicals

Many plants synthesise toxic secondary metabolites that deter herbivores by causing illness, reduced feeding or death. Examples include deadly nightshade and poison ivy, and the foxglove produces digitalis, a compound toxic to many animals and used medically in controlled doses. Cause → effect: ingestion of toxin by herbivore → physiological damage, bad taste or sickness → lower herbivory and improved plant survival.

Forms and location of chemical defences

Chemical defences occur as surface compounds (waxy layers, oils), internal metabolites (alkaloids, glycosides, tannins) or compounds released after damage. Surface compounds reduce microbial attachment; internal metabolites reduce palatability or act systemically. Limiting factors include transport to the damaged tissue and dilution after rainfall or sap flow.

Human relevance and experimental evidence

Some plant antibacterial chemicals and essential oils show measurable inhibition of bacteria when tested in laboratory assays. Experimental data demonstrate varying effectiveness between different plant extracts, which supports the identification of useful antiseptic compounds from plants. Cause → effect: extract application to bacterial culture → decrease in bacterial growth/turbidity → identification of antimicrobial activity.

Key notes

Important points to keep in mind

Chemical defences are molecules that reduce infection or deter herbivores.

Some plants (mint, witch hazel) produce antibacterial compounds that limit bacterial growth.

Poisons such as those from deadly nightshade and foxglove reduce herbivore feeding and can be toxic.

Effectiveness depends on concentration, location and the attacker’s sensitivity.

Production of chemical defences requires energy and can trade off with growth.

Surface compounds (oils, waxes) can be washed away by rain, reducing protection.

Laboratory tests with plant extracts and essential oils show variable antibacterial activity.

Human uses of plant chemicals include antiseptics and medicines when used at safe, controlled doses.