Overview of the Defense Mechanisms of Plants

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All herbivores use plants as food and break pieces off to eat. Plants have various ways to prevent or kill herbivores that prey on them. The first line of the attack against the herbivores is the bark and the waxy cuticle. Other first line defenses are thorns, modified branches and shells. These defenses discourage the herbivores or give them allergic reactions to keep them from eating the plant. Some Acacia trees have a mutualistic relationship with certain ant species. The ants have shelter in the hollow thorns in exchange they protect the leaves of the tree.

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Chemical defenses: The first line of defense can be compromised and that could create an entry point for pathogens. The plant then needs to use different defenses like toxins and enzymes. Secondary metabolites are compounds not directly derived from photosynthesis and many metabolites are toxic and can kill animals if they ingest it. Some metabolites are alkaloids and this makes the herbivore lose interest by giving off foul odors or tastes. Other alkaloids affect herbivores by causing stimulation or lethargy associated with opioids. Some compounds become toxic after ingestion. Foxglove produces several toxic chemicals, and when ingested they can cause vomiting, hallucinations and death.

Timing: When the first defense is broken the plant will send a signal to the places around the wound to start a defense mechanism. Some defenses start within minutes others start within several hours. As tissue is damage Jasmonates promote the release of toxins that discourage or kill predators. If the damaged part is beyond repair the plant will close off on that part so it dies.

Defense responses against pathogens: Pathogens are microorganisms that live off the plant and damage the tissue and include are fungi and bacteria. Plants have various ways to discourage or kill these pathogens. After all defenses are breached the plant can close up stomata to prevent the spread of pathogens. If an infection has occurred in the roots, the plant will release chemicals that attract bacteria that will fight the infection.

Plants defend themselves by using thorns, bark has lignin which is difficult to chew and a leave is covered in a waxy cuticle. Thichomes are hair like structures on plants that if they are touched can create inflammation. Raphides are microscopic needles found in pineapple and kiwi causing wounds in the mouth which can the lead to an infection. The mimosa plant has a complex structure to its defense. When touched it rolls up. When the plant is being eaten the immune system will kick in. Toxins can spread in one area to kill the attacker.

The Combretum apiculatum is a small tree with multiple stems that is around 3-10 meters high that often has a curved stem. The branches are tin and long and so they hang down making the tree look willow like. This tree only grows in high rainfall areas. The bark colors on the main stem are different shades of grey and they become scaly and rough as the tree gets older. The simple leaves are yellow-green in color and have sharp, twisted tips. Young leaves are sticky. Each new shoot has 3-4 flowers. The petals are scented and have a yellow- green color. The flowering season is from September to February. Fruits are yellow-green and are round with wings up to 7mm. The tree Fruits from January to May and the fruit will stay on the tree up until a new flower emerges in August. The Fruit of the tree is toxic but the brown headed parrot eats it. The Combretuim Apiculatum can also grow in savannah areas where the rainfall is medium and the altitude is low. This tree grows on rock or in basalt soil. This tree prefers sandy soil, clay soil or rocky areas to grow in. This tree provides food for herbivores like Kudu, Eland and Elephants. Cattle will greys on the fallen leaves. This tree grows into a tree which provides shade but it grows very slow. An established tree is frost and drought resistant. The Bark of the tree is very hard making it difficult to eat or for termites to live in. The wood makes good fire wood, fencing poles and furniture. The leaves are used for steam baths and also as a remedy for stomach disorders.

The African Elephants do not consume just anything that comes its way. They are very picky with it comes to fruit, grasses, roots and bark. These large creatures can consume up to 300 kg per day. Their bodies only digest 40% of the food they consume in the day and the other 60% is undigested. Research led by Norman Owen-Smith and Jonas Chafota in the national park of Botswana looked at 80 000 elephants and focused their dietary patterns and what they prefer to eat. Some research suggested that larger animals are less picky about their food and could digest high-fibre and potentially toxic plant material. The African elephant the largest herbivore should be eating a variety of plant species but has only a few choices in its area. The elephant is found to eat 30% of stem, bark and roots in the wet season. They were found to eat 50% of this in the cool dry season and 94% of this in the hot dry season. What separates elephants from other animals is the selection of plant parts. This reduces the overlap of niches. Elephants prefer bark, roots and stem rather than foliage or fruit of plants. Due to what they eat their digestive strategy resembles that of rodents and lagomorphs.

For years, there has been a decrease of vegetation and trees in Africa. This has been the result of the African Elephant, a big eater. These animals topple trees as it comes naturally to their survival as well as majority of the tree losses occur in lowland areas as they have more moisture and possess soils that are high in nutrients, which is perfect for the standard elephant. According to a survey done by Discovery News, it has been proven that most African elephants prefer to overbalance trees in the 16- to 30-foot range, causing a great annual loss of up to 20 percent. A stronger elephant can overturn a tree in just a few seconds. This large overthrow of trees has caused researchers to discover that nearly 9 percent of the tree species overlooked decreased in height in the two years of the experiment. Discovery News did further testing by setting “high-tech lasers” across the park, causing a 3-D image to appear. Thus, furthermore giving the scientists a better insight. However, these trees are the main source of food, shelter, shade and for masses of other species. Due to these circumstances, huge African National Parks have selected a few areas in these parks to be closed off to these great herbivores.

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