16.3 Plant responses to herbivory
Herbivores are animals that eat plants.
Response to herbivory
Plants cannot escape animals which want to eat them, so they have evolved a wide range of defences to prevent attack or minimise the damage they do.
Physical defences
include thorns, barbs, spikes, spiny leaves, fibrous and inedible tissue, hairy leaves, and even stings to protect themselves.
Chemical defences
Stinging nettles manages to include both physical and chemical defences it is trichomes (stinging hairs) Other chemicals include
Tannins - part of a group of compound called phenols produced by many plants, they can make up to 50% of the dry weight of the leaves. They have a very bitter taste which puts animals off eating the leave, they are toxic to insects - they bind to digestive enzymes produced in the saliva and inactivate them. Tea and red wine are both rich in plant tannins.
Alkaloids - a large group of very bitter tasting, nitrogenous compounds found in many plants. Many of them act as drugs, affecting the metabolism of animals that take them in and sometimes poisoning them. Alkaloids include caffeine, nicotine, morphine and cocaine. Caffeine is toxic to fungi and insects and the caffeine produced by coffee bush seedlings spreads through the soil and prevents the germination of the seeds of other plants - so caffeine protects against herbivores and plant rivals. Nicotine is a toxin produced in the roots of tobacco plants, transported to the leaves and stored in vacuoles to be released when the leaf is eaten.
Terpenoids - large group of compounds produced by plants which often form essential oils but also often act as toxin to insects and fungi that might attack the plant. Pyrethrin, produced by chrysanthemums, acts an as insect neurotoxin, interfering with the nervous system. Some terpenoids acts as insect repellents for example citronella produced by lemon grass repels insects.
Pheromones
A pheromone is a chemical made by an organism which affects the social behaviour of other member of the same species. Because plants do not behave socially, they do not rely on a lot of pheromones. There are a few instances where they could be regarded as using pheromones to defend themselves.
If a maple tree is attacked by insects, it releases a pheromone which is absorbed by leaves on other branches. These leaves then make chemicals such as callose to help protect them if they are attacked. Scientists have observed that leaves on the branes of nearby trees also prepare for attack in response to these chemical signals.
There is some evidence that plants communicate by chemicals produced in the root systems and one plant can tell another if it is under water stress.
However, plants do produce chemicals called volatile organic compounds (VOC’s) which act rather like pheromones between themselves and other organisms, particularly insects. They diffuse through the air in and around the plant. Plants use these chemical signals to defend themselves. They are usually only made when the plant detects attack by an insect pest through chemicals in the saliva of the insect. This may elect gene switching e.g
When cabbages are attacked by the caterpillars of the cabbage white butterfly, they produce a chemical signal which attracts the parasitic wasp. This insect lays its eggs in the caterpillars which are then eaten alive, protecting the plant. The signal from the plant also deters any other female cabbage while butterflies from laying their eggs. Scientists estimate up to 90% of cabbage white caterpillars are affected by the parasite. If the cabbage is attacked by cabbage greenly, a different signal which attracts the wasp that attack greenfly.
When apple trees are attacked by spider mites, they produce VCOs which attract predatory mites and come and destroy the apple tree pests
Some types of wheat seedling produce VOCs when they have been attacked by aphids and these repel other aphids from the plant.
Sometimes a VOC produced by a plant that has been attacked will not only attract predators of the pest organism - it may also act as a pheromone so that neighbouring plants begin to produce the VOC before they are actually attacked.