Plant Responses to Herbivory
Physical Defenses
- Adaptations in the anatomy of the plant which prevent them being eaten.
- Thorns, spiny leaves, inedible tissue, hairy leaves, stingers.
- Thorns, spiny leaves, inedible tissue, hairy leaves, stingers.
- Substances inside/secreted by the plant which deter or kill herbivores.
- Tannins
- Part of a group of compounds called Phenols.
- Very bitter tasting to deter large animals.
- Toxic to insects - bind to digestive enzymes and inhibit them.
- Alkaloids
- A group of bitter tasting nitrogenous compounds.
- Poison and affect the metabolism of insects which ingest them.
- Examples are caffeine, nicotine, morphine and cocaine.
- Caffeine is toxic to insects and fungi, and prevents the germination of other plants. This means it is protected from herbivores and any plant competition.
- Nicotine is a toxin produced in the roots of tobacco plants, transported to the leaves and stored in vacuoles.
- Terpenoids
- A group of essential oils produced by plants.
- Toxic to insects and fungi.
- Pyrethrin is a terpenoid produced by chrysanthemums, which acts as an insect neurotoxin.
- Citronella is an insect repellent found in lemongrass.
- A pheromone is a chemical which affect the social behaviour of the same species.
- As plants do not behave socially, they use pheromones to protect themselves.
- When attacked by insects, Maple trees can release pheromones to be absorbed into the leaves on other tree branches, which then stimulates the production of callose to block off plasmodesmata between cells, pre-emptively killing them.
- Pheromones can be secreted by a plant's root in order to tell a neighbouring plants about a lack of water.
- Volatile Organic Compounds (VOCs) are similar to pheromones, but act between themselves an other organisms.
- They are usually only made when the plant detects attack by an insect pest through chemicals in saliva.
- Cabbages produce a VOC when attacked by caterpillars which attracts the parasitic wasp, Cotesia glomerata. This wasp lays its eggs inside the caterpillar who is then eaten alive by the larvae, protecting the plant. The VOC also repels female cabbage white butterflies from laying their eggs on the cabbage. Cabbages can send out different pheromones in response to different insects, with the same result.
- Apple trees produce VOCs which attract predatory mites when attacked by spider mites.
- Some wheat seedlings produce VOCs to repel aphids once they have been attacked.
- VOCs often not only attract the predators to their pests, but can act as pheromones to stimulate VOC production in neighbouring plants.
- A minority of plants can move at a speed noticeable by the naked eye.
- Plants such as the Mimosa pudica have leaves which are able to fold and collapse to frighten larger herbivores and dislodge smaller pests.
- The leaf folds within seconds, and recovers over 10-12 minutes as potassium ions move back into cells, followed by the osmosis of water.
- The leaf folding appears to be a result of electrochemical change in the cells similar to that of an action potential, causing potassium ions to move into the upper, flexible side of the pulvinus (thickened region around the vascular bundle) and potassium ions move out of the lower, extensor side.
- Water osmoses as a result of this increasing turgor in the top and reducing it in the bottom, with elastic tissue allowing it to bend.