Plant Responses to Herbivory
This lesson covers:
- Physical defences plants use against herbivores
- Chemical defences plants use against herbivores
- How plants use volatile organic compounds for pest control
- Plant movements in response to touch
Physical defences against herbivores
Herbivory is the process by which herbivores eat plants.
Plants have developed a variety of physical structures to protect themselves from herbivores. These adaptations make it more challenging and potentially harmful for animals to consume them.
Examples of physical defences include:
- Thorns, barbs, or spikes - These sharp structures can cause physical harm to herbivores, deterring them from eating the plant.
- Tough, fibrous tissue - This makes the plant hard to chew and digest.
- Stinging hairs - These inject irritating substances into herbivores.
- Densely hairy leaves - These create a physical barrier that is difficult for many herbivores to overcome.
The effectiveness of these defences varies depending on the herbivore's size and feeding habits.
Chemical defences against herbivores
Many plants produce chemicals that are unpalatable or toxic to herbivores. These substances can deter herbivores or interfere with their digestion or other biological processes.
Key types of chemical defences include:
- Tannins
- Alkaloids
- Terpenoids
These chemicals are commonly found in parts of the plant that are most vulnerable to herbivory, like leaves and seeds.
Alkaloids
The main chemical defence you need to know are alkaloids, which are bitter nitrogen-containing compounds.
Alkaloids can have a variety of effects:
- Alkaloids like morphine and cocaine act as drugs and affect the metabolism of animals, discouraging some herbivores from consuming the plant that produces them.
- Alkaloids like caffeine and nicotine are toxic or poisonous to animals, discouraging some herbivores from consuming the plant or even killing them.
Volatile organic compounds for pest control
Plants can release volatile organic compounds (VOCs) into the air when they detect herbivore attacks.
VOCs are similar to pheromones, which are chemicals made by an organism that affect the social behaviour of other members of the same species, but VOCs can affect members of different species too.
These VOCs serve several purposes:
- Attracting predators - For example, VOCs from pest-damaged cabbages can attract wasps that prey on the pests.
- Repelling herbivores - VOCs can discourage other herbivores from feeding on the plant.
- Signalling neighbouring plants - VOCs can alert nearby plants to prepare their defences, even before they are attacked themselves.
Plant movements in response to touch
Some plants can move rapidly in response to touch.
This movement serves as a defence mechanism against small insect herbivores.
An example in Mimosa pudica
When touched, the leaves of Mimosa pudica quickly fold up and the plant droops. This can dislodge or deter small insects, and make the plant appear smaller and less appetising.
This movement is caused by rapid changes in cell water pressure and ion distribution.
The leaves will eventually reopen as the cells undergo osmotic changes.