Allelopathy

Allelopathy

Allelopathy refers to all the biochemical interactions, both beneficial and harmful, among all types of plants, including microorganisms.

For an allelopathic interaction to occur, chemicals must be released into the environment by one plant that will affect the growth of another. In this way allelopathy differs from competition, which involves removal of some factor from the environment that is shared with other plants. Allelopathy was recognized as early as Theophrastus (300 b.c.e.), who pointed out that chick pea plants destroy weeds growing around them.

Methods of Action

Avariety of different allelochemicals are produced by plants, usu- ally as secondary metabolites that do not have a specific function in the growth and development of the host plant but that do affect the growth of other plants. Originally plant physiologists thought these secondary products were simply metabolic wastes which plants had to store because they do not have an excretory system as animals do. Their various functions are now beginning to be understood.

Biochemical Coevolution in Angiosperms

Biochemical Coevolution in Angiosperms

Flowering plants, or angiosperms, produce many compounds that are not directly related to growth and development. These secondary metabolites arise from primary metabolic pathways and act as antiherbivory mechanisms, allelochemicals, or attractants.

Secondary metabolites are biochemicals produced by plants in response to selection pressures. These pressures may be from herbivory, competition, or the need for pollination.

As plants produce compounds to enhance their survival, predators, competitors, and pollinators react and evolve means of adjusting to the plant’s efforts. Chemically simple secondary metabolites may be widespread throughout angiosperm (flowering plant) families, whereas more complex chemicals are often restricted to a single species.

Biological Invasions

Biological Invasions

Biological invasions are the entry of a type of organism into an ecosystem outside its historic range. In a biological invasion, the "invading" organism may be an infectious virus, a bacterium, a plant, an insect, or an animal.

Species introduced to an area from somewhere else are referred to as alien or exotic species or as invaders. Because an exotic species is not native to the new area, it is often unsuccessful in establishing a viable population and disappears.

The fossil record, as well as historical documentation, indicates that this is the fate of many species in new environments as they move from their native habitats. Occasionally, however, an invading species finds the new environment to its liking.

Biological Weapons

Biological Weapons

Biological weapons are biological agents that can be used to destroy living organisms. This general definition includes the use of virtually any kind of microorganism (bacterium or fungus) or biological agent (mycoplasma like organism, virus, viroid, or prion) to destroy any biologically important plant or animal.

There are two basic ways of using biological weapons against humanity. The first is to attack the food or water supply. This would produce hardship and the possible death from starvation of many individuals. In developed countries such as the United States, total devastation due to such an attack would likely be avoided, as there are considerable stores of food, in dispersed locations, that would mitigate against crop failure of moderate proportions.

In addition, most experts agree that the amount of biological contaminant required to overcome the effects of dilution and time in most water reservoirs makes poisoning of water supplies impractical, although not impossible.

The other way of using biological weapons against humanity is to attack individuals directly, using pathogens. Numerous known pathogens could be used, but the most effective biological weapons would creep upon the population with stealth. Naturally occurring agents that have promise as biological weapons are pathogens, which can infect and colonize a host. A pathogen causes disease, an alteration in the metabolism of a host.

Metabolites: Primary vs Secondary

Metabolites are compounds synthesized by plants for both essential functions, such as growth and development (primary metabolites), and specific functions, such as pollinator attraction or defense against herbivory (secondary metabolites).

Metabolites are organic compounds synthesized by organisms using enzyme-mediated chemical reactions called metabolic pathways. Primary metabolites have functions that are essential to growth and development and are therefore present in all plants. In contrast, secondary metabolites are variously distributed in the plant kingdom, and their functions are specific to the plants in which they are found.

Secondary metabolites are often colored, fragrant, or flavorful compounds, and they typically mediate the interaction of plants with other organisms. Such interactions include those of plant-pollinator, plant-pathogen, and plant-herbivore.

Mushrooms

Yellow mushroom

Members of the kingdom Fungi and division Mycota, mushrooms are members of the class Basidiomycetes. This class includes both the hymenomycetes, which reproduce spores using a layer structure called the hymenium, and the gasteromycetes, which include the stinkhorns and puffballs.

Fossil evidence suggests that mushrooms existed ninety million years ago. The order Agaricales represents approximately four thousand species in sixteen families of commonly found mushrooms.

The order Polyporus consists of fungi that grow on tree limbs or stumps. Although they prefer moist environments, mushrooms are sometimes found in deserts, beaches, and occasionally snowdrifts.

Parasitic Plants

Parasitic plants, lovevine embrace (Cuscuta spp.)

Parasitic plants are those symbiotic life-forms that form mutualistic relationships with other plants at their hosts’ expense.

The more is learned about plants and other organisms, the more apparent it is that many, if not most, plants are involved in symbiotic associations in which one or more other organisms live in close interaction with the plant—almost as if they are one. Symbiosis is frequently thought of as being a mutualistic relationship in which both the plant and its symbiont benefit.

Typical examples include the alga and fungus that make up a lichen and the bacteria living in a legume root nodule. Commensalistic relationships, such as Spanish-moss hanging from a live oak, are less commonly thought of; in this relationship, one plant obviously benefits, while the other seems to be little affected.

Poisonous and Noxious Plants

Poisonous plants

Poisonous plants have evolved toxic substances that function to defend them against herbivores and thereby better adapt them for survival.

After evolving adaptations that facilitated colonization of terrestrial habitats, plants were confronted with a different type of problem.

This was the problem of herbivory, or the inclination of many different types of organisms, from bacteria to insects to four-legged herbivores, to eat plants. Pressures from herbivory drove many different types of plants, from many different families, to evolve defenses.

Rusts

Rust fungi

Rust fungi belong to the taxonomic class Teliomycetes within the basidiosporic phylum and include some seven thousand known species, all plant parasites, with many being extremely important plant pathogens.

Rust diseases have caused serious problems for centuries and continue to cost billions of dollars annually worldwide.

Important examples include: leaf, stem, and crown rusts of wheat, barley, and oats; the blister, fusiform, and gall rusts of pines; coffee rust; many field crop rusts, such as soybean, peanut, sunflower, and flax rusts; and rusts of many horticultural crops and ornamental plants, such as cedar-apple rust, bean rust, and rose rust.