This response lowers the surface area available to herbivores, which are presented with the underside of each leaflet, and results in a wilted appearance. It may also physically dislodge small herbivores, such as insects. Some plants mimic the presence of insect eggs on their leaves, dissuading insect species from laying their eggs there. Because female butterflies are less likely to lay their eggs on plants that already have butterfly eggs, some species of neotropical vines of the genus Passiflora Passion flowers contain physical structures resembling the yellow eggs of Heliconius butterflies on their leaves, which discourage oviposition by butterflies.
Another category of plant defenses are those features that indirectly protect the plant by enhancing the probability of attracting the natural enemies of herbivores. Such an arrangement is known as mutualism , in this case of the " enemy of my enemy " variety. One such feature are semiochemicals , given off by plants. Semiochemicals are a group of volatile organic compounds involved in interactions between organisms.
One group of semiochemicals are allelochemicals ; consisting of allomones , which play a defensive role in interspecies communication , and kairomones , which are used by members of higher trophic levels to locate food sources. When a plant is attacked it releases allelochemics containing an abnormal ratio of these herbivore-induced plant volatiles HIPVs.
The subsequent reduction in the number of herbivores confers a fitness benefit to the plant and demonstrates the indirect defensive capabilities of semiochemicals. Plants sometimes provide housing and food items for natural enemies of herbivores, known as "biotic" defense mechanisms, as a means to maintain their presence.
For example, trees from the genus Macaranga have adapted their thin stem walls to create ideal housing for an ant species genus Crematogaster , which, in turn, protects the plant from herbivores. Similarly, several Acacia tree species have developed stipular spines direct defenses that are swollen at the base, forming a hollow structure that provides housing for protective ants. These Acacia trees also produce nectar in extrafloral nectaries on their leaves as food for the ants.
Plant use of endophytic fungi in defense is common. Most plants have endophytes , microbial organisms that live within them. While some cause disease, others protect plants from herbivores and pathogenic microbes. Endophytes can help the plant by producing toxins harmful to other organisms that would attack the plant, such as alkaloid producing fungi which are common in grasses such as tall fescue Festuca arundinacea. There have been suggestions that leaf shedding may be a response that provides protection against diseases and certain kinds of pests such as leaf miners and gall forming insects.
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Defensive structures and chemicals are costly as they require resources that could otherwise be used by plants to maximize growth and reproduction. Many models have been proposed to explore how and why some plants make this investment in defenses against herbivores. The optimal defense hypothesis attempts to explain how the kinds of defenses a particular plant might use reflect the threats each individual plant faces.
The first factor determining optimal defense is risk: how likely is it that a plant or certain plant parts will be attacked?
This is also related to the plant apparency hypothesis , which states that a plant will invest heavily in broadly effective defenses when the plant is easily found by herbivores. The second factor is the value of protection: would the plant be less able to survive and reproduce after removal of part of its structure by a herbivore? Not all plant parts are of equal evolutionary value, thus valuable parts contain more defenses. Experimentally, the fitness value of a plant structure is determined by removing that part of the plant and observing the effect.
For example, the seeds of many edible fruits and nuts contain cyanogenic glycosides such as amygdalin. This results from the need to balance the effort needed to make the fruit attractive to animal dispersers while ensuring that the seeds are not destroyed by the animal. The final consideration is cost: how much will a particular defensive strategy cost a plant in energy and materials? This is particularly important, as energy spent on defense cannot be used for other functions, such as reproduction and growth.
Plant defense against herbivory - Wikipedia
The optimal defense hypothesis predicts that plants will allocate more energy towards defense when the benefits of protection outweigh the costs, specifically in situations where there is high herbivore pressure. The carbon:nutrient balance hypothesis, also known as the environmental constraint hypothesis or Carbon Nutrient Balance Model CNBM , states that the various types of plant defenses are responses to variations in the levels of nutrients in the environment.
For example, plants growing in nitrogen -poor soils will use carbon -based defenses mostly digestibility reducers , while those growing in low-carbon environments such as shady conditions are more likely to produce nitrogen-based toxins. The hypothesis further predicts that plants can change their defenses in response to changes in nutrients. For example, if plants are grown in low-nitrogen conditions, then these plants will implement a defensive strategy composed of constitutive carbon-based defenses.
If nutrient levels subsequently increase, by for example the addition of fertilizers , these carbon-based defenses will decrease. The growth rate hypothesis, also known as the resource availability hypothesis , states that defense strategies are determined by the inherent growth rate of the plant, which is in turn determined by the resources available to the plant.
A major assumption is that available resources are the limiting factor in determining the maximum growth rate of a plant species. This model predicts that the level of defense investment will increase as the potential of growth decreases. A recent test of this model involved a reciprocal transplants of seedlings of 20 species of trees between clay soils nutrient rich and white sand nutrient poor to determine whether trade-offs between growth rate and defenses restrict species to one habitat.
When planted in white sand and protected from herbivores, seedlings originating from clay outgrew those originating from the nutrient-poor sand, but in the presence of herbivores the seedlings originating from white sand performed better, likely due to their higher levels of constitutive carbon-based defenses.
These finding suggest that defensive strategies limit the habitats of some plants. The growth-differentiation balance hypothesis states that plant defenses are a result of a tradeoff between "growth-related processes" and "differentiation-related processes" in different environments. The GDBH also accounts for tradeoffs between growth and defense over a resource availability gradient.
In situations where resources e. As resource availability increases, the requirements needed to support photosynthesis are met, allowing for accumulation of carbohydrate in tissues. As resources are not sufficient to meet the large demands of growth, these carbon compounds can instead be partitioned into the synthesis of carbon based secondary metabolites phenolics, tannins, etc. In environments where the resource demands for growth are met, carbon is allocated to rapidly dividing meristems high sink strength at the expense of secondary metabolism.
Thus rapidly growing plants are predicted to contain lower levels of secondary metabolites and vice versa. In addition, the tradeoff predicted by the GDBH may change over time, as evidenced by a recent study on Salix spp. Much support for this hypothesis is present in the literature, and some scientists consider the GDBH the most mature of the plant defense hypotheses. The variation of plant susceptibility to pests was probably known even in the early stages of agriculture in humans. In historic times, the observation of such variations in susceptibility have provided solutions for major socio-economic problems.
Riley, with J. Planchon, helped save the French wine industry by suggesting the grafting of the susceptible but high quality grapes onto Vitis labrusca root stocks. Fresh growth of grass is sometimes high in prussic acid content and can cause poisoning of grazing livestock. The production of cyanogenic chemicals in grasses is primarily a defense against herbivores. The human innovation of cooking may have been particularly helpful in overcoming many of the defensive chemicals of plants.
Many enzyme inhibitors in cereal grains and pulses , such as trypsin inhibitors prevalent in pulse crops, are denatured by cooking, making them digestible. It has been known since the late 17th century that plants contain noxious chemicals which are avoided by insects. These chemicals have been used by man as early insecticides; in nicotine was extracted from tobacco and used as a contact insecticide.
In , insect infested plants were treated with nicotine fumigation by heating tobacco and blowing the smoke over the plants. In later years, the applications of plant resistance became an important area of research in agriculture and plant breeding , particularly because they can serve as a safe and low-cost alternative to the use of pesticides.
Natural materials found in the environment also induce plant resistance as well. The selective breeding of crop plants often involves selection against the plant's intrinsic resistance strategies. This makes crop plant varieties particularly susceptible to pests unlike their wild relatives.
In breeding for host-plant resistance, it is often the wild relatives that provide the source of resistance genes. These genes are incorporated using conventional approaches to plant breeding, but have also been augmented by recombinant techniques, which allow introduction of genes from completely unrelated organisms.
The most famous transgenic approach is the introduction of genes from the bacterial species, Bacillus thuringiensis , into plants. The bacterium produces proteins that, when ingested, kill lepidopteran caterpillars. The gene encoding for these highly toxic proteins, when introduced into the host plant genome, confers resistance against caterpillars, when the same toxic proteins are produced within the plant.
This approach is controversial, however, due to the possibility of ecological and toxicological side effects. Many currently available pharmaceuticals are derived from the secondary metabolites plants use to protect themselves from herbivores, including opium , aspirin , cocaine , and atropine. However, many of these biochemical pathways are conserved in vertebrates, including humans, and the chemicals act on human biochemistry in ways similar to that of insects.
It has therefore been suggested that the study of plant-insect interactions may help in bioprospecting. There is evidence that humans began using plant alkaloids in medical preparations as early as B. For example, to combat herbivory by the larvae of some Lepidoptera species, Cinchona trees produce a variety of alkaloids, the most familiar of which is quinine.
Quinine is extremely bitter, making the bark of the tree quite unpalatable, it is also an anti- fever agent, known as Jesuit's bark , and is especially useful in treating malaria. Throughout history mandrakes Mandragora officinarum have been highly sought after for their reputed aphrodisiac properties. However, the roots of the mandrake plant also contain large quantities of the alkaloid scopolamine , which, at high doses, acts as a central nervous system depressant , and makes the plant highly toxic to herbivores.
Scopolamine was later found to be medicinally used for pain management prior to and during labor ; in smaller doses it is used to prevent motion sickness.
Plant defense against herbivory
Repellent companion planting , defensive live fencing hedges , and "obstructive-repellent" interplanting, with host-plant resistance species as beneficial 'biological control agents' is a technique in biological pest control programs for: organic gardening , wildlife gardening , sustainable gardening , and sustainable landscaping ; in organic farming and sustainable agriculture ; and in restoration ecology methods for habitat restoration projects. From Wikipedia, the free encyclopedia. Main article: Evolutionary history of plants. Further information: Toxalbumin.
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Further information: Medicinal plants. Further information: Mimicry in plants. Anti-predator adaptation Aposematism Biopesticide Chemical ecology Canavanine Druse botany Laticifer Lectin List of beneficial weeds List of companion plants List of pest-repelling plants Plant disease resistance Plant tolerance to herbivory Pollination Phytoalexin Raphide Rapid plant movement Seed predation Tritrophic interactions in plant defense. Rice University.
Current Chemical Biology-. Raven December Dilcher, D. Davis, D.