Plant defences are those mechanisms employed by plants in response to herbivory and parasitism. According to Hanley et al. (2007), “the tissues of virtually all terrestrial, freshwater, and marine plants have qualities that to some degree reduce herbivory, including low nitrogen concentration, low moisture content, toxins or digestibility-reducing compounds”. The type of chemical defence may be species specific (Scott 2008). The defences that plants possess may be in the form of chemical production or in the form of physical defences such as thorns or spikes and even through reinforced, rigid leaves. “The compounds that are produced in response to herbivory can either have a direct effect on the attacker itself (e.g. toxins or digestibility reducers), or serve as indirect defenses by attracting the natural enemies of the herbivores” (Bezemer & van Dam 2005). This essay will focus on chemical plant defences and in particular the effects of terpenes, phenolics, nitrogen-based defences as well as allelopathy in plants. The first group of chemical compounds to be discussed are terpenes. Terpenes are a very comprehensive group of metabolites which are known to contain essential oils that produces well-known scents in herbs belonging to the Lamiaceae plant family (Scott 2008). These scented compounds are usually found in the trichomes of basil, rosemary, lavender, etc. (Scott 2008). Electron micrograph images of these trichomes can be seen below. According to Keeling & Boltmann (2006), there are about 30 000 known types of terpenes which originated from structures created from terpene synthases. These terpene synthases can be broken up into three groups, monoterpene synthases, sesquiterpene synthases, and diterpene synthases (Keeling &... ... middle of paper ... ...n-toxic, but becomes toxic once it reacts with air or soil compounds to become the allelopathic hydrojuglone (Appleton et al. 2009). Once this compound reaches the soil, it can affect neighbouring plants by “root contact, leakage or decay in the soil, falling and decaying leaves, or when rain leaches and drips juglone from leaves and branches onto plants below” (Appleton et al. 2009). This process is illustrated below. This essay has therefore discussed the most popular classes of plant chemical defences, namely terpenes, phenolics, nitrogen-based defences and allelopathy. The effects of these defences on herbivores has also been touched on. Many of these compounds produced by plants have later been extracted by humans for use as insecticides. Compounds such as tannins are also used in the wine industry. These chemicals thus have a very strong economic influence.
Dr. May Berenbaum, an entomologist at University of Illinois, has conducted over 40 years of research on parsnips and the parsnip web worm. During her lecture on March 22nd, she explained how parsnips are invasive weeds that are full of toxic furanocoumarins. Interestingly, the parsnip webworm only eats the parsnips and is able to process the toxin very well. The interactions between the web worm and parsnips cause the parsnips to select for plants with a higher amount of furanocoumarins in order to defend themselves from the worms. When the worms are not there, i.e. in New Zealand, the parsnip plants grow taller and develop more ways to attract pollinators, and they do not put energy into having as many furanocoumarins
Conium maculatum is a tall, branched, biennial plant, usually 1–2.5m high, and thought to be one of the most toxic members of the family Apiaceae (formerly Umbelliferae) of plant kingdom. The stems are rigid, smooth, and hollow except at the nodes. The plant has a bitter taste and white flower. The leaves are large, triangular; fern like, and alternate on the erect stem. The plant usually grows in waste places where moisture may accumulate and protected from cultivation, damp ground, and banks of streams, rivers, road sides, woodland and pastures. Poison hemlock (Conium maculatum) has a worldwide distribution and reported as a very common weed in Europe, North and South America, North Africa, Australia and New Zealand, and there are data about its occurrence in Ethiopia(Mekonnen, 1994), and in Pakistan (Ahmed et al.,1989).
Strychnine is a poisonous alkaloid, C21H22N2O2, obtained in colorless or white rhombic crystals. These have a bitter taste and melt at around 290( C (4 p.1). Alkaloids are any class of naturally occurring organic nitrogen containing bases, usually containing one or more of these nitrogen atoms in a ring of atoms called a cyclic system. Alkaloids are primarily found in plants and are predominant in flowering plant species. The function of alkaloids in plants is thought to be simply a waste product of the plants metabolic processes, but current research may suggest a specific biological function. This is evident in some plants as the levels of alkaloid increase just prior to seed formation and then drops off after the seed is ripe. This evidence suggests possible mechanisms of maturation and possible defense, against certain insect species (5 p.2). Strychnine, being an alkaloid, is slightly soluble in water, but is more soluble in alcohol and is released from its salts by alkalis. Many of the commercial alkaloids are found in the genus Strychnos.
Conium maculatum is a tall, branched, biennial plant, usually 1–2.5m high, and thought to be one of the most toxic members of the family Apiaceae (formerly Umbelliferae) of plant kingdom. The stems are rigid, smooth, and hollow except at the nodes. The plant has a bitter taste and white flower. The leaves are large, triangular; fern like, and alternate on the erect stem. The plant usually grows in waste places where moisture may accumulate and protected from cultivation, damp ground, and banks of streams, rivers, road sides, woodland and pastures. Poison hemlock (Conium maculatum) has a worldwide distribution and reported as a very common weed in Europe, North and South America, North Africa, Australia
Theophrastus Bomabastus Von Hoheneheim (Paracelsus) said “sola dosis facet venenum,” translated as, “all things are poisonous and nothing is without poison.” (Wink & Wyk, 2008) For people living in the Great Lakes regions of Northern Michigan, it often feels the opposite, “in everything there is medicine and nothing is without medicine.” Undoubtedly, some plants can externally or internally irritate the body affecting homeostasis, including instances of photo-sensitization and allergies; but generally these symptoms are not life threatening. According to herbalists and toxicologists, it is important to “distinguish between poisons that can kill in minute amounts, toxins that are less toxic than poisons, and toxicants that are toxic in high concentrations only.” (Wink & Wyk, 2008).
In a study conducted by Appel and Cocroft at the University of Missouri, it was found that some small plants (Arabidopsis thaliana) have chemical defenses against predators, specifically caterpillars (Pieris rapae). These plants can sense when a predator is near and can communicate with others about the impending danger. When a caterpillar is near the plant can “hear” the vibrations and change it’s chemical makeup. By changing the chemicals in the leaves the plant creates a bitter taste in the caterpillar's mouth to stop the caterpillar from feasting on any more leaves. The experiment consisted of a large sample of Arabidopsis thaliana as well as Pieris rapae (caterpillars).
“(Silent Spring) spells out in memorable detail through out the book the effects of synthetic insecticides and herbicides on water, soil, plants, wildlife, fish and human beings. But in the book’s final chapter she suggests alternative courses of action for mankind —- a way out of this march toward death.” (Holmes, Pg. 123)
Invasive plant species such as Ailanthus altissima (Tree of heaven), Allaria petiolata (garlic mustard), and once again Celastrus orbiculatus, use allelopathy to deter away competitors surrounding a plant species to make room for it to grow and expand. What allelopathy does is produce chemical compounds that change the soil and not allow other species of plants to grow, intake nutrients or germinate (Pisula & Meiners 2010). A net benefit is created when chemical compounds are released in the soil because it leaves more nutrients or resources available to the invasive plant species when native plants aren’t
Design: Botanical chemical constituents were identified from the Natural Products Alert Database for the periods coverings at least 1975 through 2003. This database includes
Interactions in between plant species are of great importance and often result in fierce competition within the community. Over the past decades, increasing in human activity and movement allowed certain species to migrate beyond their origins and vastly expand in population, plants being one of the most complicated organisms on earth were highly involved in such activity. This enabled invasions by alien plants which poses serious threats and may inflict negative impacts to native species of plants in an certain ecosystem. Due to this nature, it may be concluded that there are certain traits and characteristics in these plants that allowed for the dominance to occur. It is crucial to be able to identify these traits to prevent
Plants like many eukaryotic composed organisms have the ability to detect and protect themselves against microorganisms known as pathogens. Plant fossils have recorded that land plant’s existence was established 480 million years ago, but molecularly, plant evolution began 700 million years ago. Molecular interaction with microbes and other organisms gave the shape and structure of plants, giving us an idea that microbes also evolve according to its host. Plants lack mobility depriving themselves from a somatic secondary immune response like many mammals giving pathogens the ability to easily attack. Pathogenic microbes can access plants by penetrating through the leaves, entering through plant wounds, or by using the stomata a natural pore on plants that opens and closes for gas exchange. To detect and stop from extensive damage from microbes, plants developed an immune system through its structure, chemicals, and defense proteins.
Qualitative and quantitative difference exists among plant allelochemicals in plant species. Moldenke et al. (1983) focused on the induction effect of three monoterpenes found in peppermint on the sixth instar of Peridoia saucia; pulegone, menthone and menthol. These phytochemicals were fed to the Peridoia saucia at 0.01% - 0.1% of diet. There was no significant difference in the level of induction caused by the three allelochemics; probably due to their closeness in chemical nature. Furthermore, the mixture of the three compounds did not yield greater P450 induction. This is also suggestive that qualitative difference among plant allelochemics might also have effect on induction of P450 and other detoxification enzymes.
The condensed tannins are even further categorized into many different sub-types pertaining to the compounds structure and linkages of their flavonoid groups (Barbehenn and Constabel 2011). Plant species with secondary or woody growth have been found to contain generous amounts of condensed tannins (Hattenschwiler and Vitousek 2000). Conversely, these condensed tannins are generally absent within the herbaceous plants (Hattenschwiler and Vitousek 2000).
Zimmerman, H. G. and Neser, S. 1999. Trends and prospects for biological control of weeds
Botanicals are not very selective because they target a broad range of insect pests. Plant insecticides act in several ways: as repellents by driving the insects away due to smell or taste, as anti feedants which cause insects on the plants to reduce their food intake and hence starve them to death; as oviposition deterrents, by preventing insects from laying eggs; or as inhibitors by interfering with the life cycle of the insects. Plant insecticides have several advantages. However, the regulatory environment and public health needs should create opportunities for the use of safer botanicals since human and animal health is paramount.