Plant viruses are viruses that affect plants. Like all other viruses, plant viruses are obligate intracellular parasites that do not have the molecular machinery to replicate without a host. Plant viruses can be pathogenic to higher plants. Most plant viruses are rod-shaped, with protein discs forming a tube surrounding the viral genome, they have an envelope. The great majority have an RNA genome, small and single stranded, but some viruses have double-stranded RNA, ssDNA or dsDNA genomes. Although plant viruses are not as well understood as their animal counterparts, one plant virus has become iconic: tobacco mosaic virus, the first virus to be discovered; this and other viruses cause an estimated U. S$60 billion loss in crop yields worldwide each year. Plant viruses are grouped into 49 families. However, these figures relate only to cultivated plants, which represent only a tiny fraction of the total number of plant species. Viruses in wild plants have been little studied, but the interactions between wild plants and their viruses do not appear to cause disease in the host plants.
To transmit from one plant to another and from one plant cell to another, plant viruses must use strategies that are different from animal viruses. Plants do not move, so plant-to-plant transmission involves vectors. Plant cells are surrounded by solid cell walls, therefore transport through plasmodesmata is the preferred path for virions to move between plant cells. Plants have specialized mechanisms for transporting mRNAs through plasmodesmata, these mechanisms are thought to be used by RNA viruses to spread from one cell to another. Plant defenses against viral infection include, among other measures, the use of siRNA in response to dsRNA. Most plant viruses encode a protein to suppress this response. Plants reduce transport through plasmodesmata in response to injury; the discovery of plant viruses causing disease is accredited to A. Mayer working in the Netherlands demonstrated that the sap of mosaic obtained from tobacco leaves developed mosaic symptom when injected in healthy plants; however the infection of the sap was destroyed.
He thought. However, after larger inoculation with a large number of bacteria, he failed to develop a mosaic symptom. In 1898, Martinus Beijerinck, a Professor of Microbiology at the Technical University the Netherlands, put forth his concepts that viruses were small and determined that the "mosaic disease" remained infectious when passed through a Chamberland filter-candle; this was in contrast to bacteria microorganisms. Beijerinck referred to the infectious filtrate as a "contagium vivum fluidum", thus the coinage of the modern term "virus". After the initial discovery of the ‘viral concept’ there was need to classify any other known viral diseases based on the mode of transmission though microscopic observation proved fruitless. In 1939 Holmes published a classification list of 129 plant viruses; this was expanded and in 1999 there were 977 recognized, some provisional, plant virus species. The purification of TMV was first performed by Wendell Stanley, who published his findings in 1935, although he did not determine that the RNA was the infectious material.
However, he received the Nobel Prize in Chemistry in 1946. In the 1950s a discovery by two labs proved that the purified RNA of the TMV was infectious which reinforced the argument; the RNA carries genetic information to code for the production of new infectious particles. More virus research has been focused on understanding the genetics and molecular biology of plant virus genomes, with a particular interest in determining how the virus can replicate and infect plants. Understanding the virus genetics and protein functions has been used to explore the potential for commercial use by biotechnology companies. In particular, viral-derived sequences have been used to provide an understanding of novel forms of resistance; the recent boom in technology allowing humans to manipulate plant viruses may provide new strategies for production of value-added proteins in plants. Viruses are small and can only be observed under an electron microscope; the structure of a virus is given by its coat of proteins.
Assembly of viral particles takes place spontaneously. Over 50% of known plant viruses are rod-shaped; the length of the particle is dependent on the genome but it is between 300–500 nm with a diameter of 15–20 nm. Protein subunits can be placed around the circumference of a circle to form a disc. In the presence of the viral genome, the discs are stacked a tube is created with room for the nucleic acid genome in the middle; the second most common structure amongst plant viruses are isometric particles. They are 25–50 nm in diameter. In cases when there is only a single coat protein, the basic structure consists of 60 T subunits, where T is an integer; some viruses may have 2 coat proteins. There are three genera of Geminiviridae that consist of particles that are like two isometric particles stuck together. A small number of plant viruses have, in addition to their coat proteins, a lipid envelope; this is derived from the plant cell membrane as the virus particle buds off from the cell. Viruses can be spread by direct transfer of sap by contact of a wounded plant with a healthy one.
Such contact may occur during agricultural practices, as by damage caused by tools or hands, or as by an animal feeding on the plant. TM
A fungus is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separate from the other eukaryotic life kingdoms of plants and animals. A characteristic that places fungi in a different kingdom from plants and some protists is chitin in their cell walls. Similar to animals, fungi are heterotrophs. Fungi do not photosynthesize. Growth is their means of mobility, except for spores, which may travel through the water. Fungi are the principal decomposers in ecological systems; these and other differences place fungi in a single group of related organisms, named the Eumycota, which share a common ancestor, an interpretation, strongly supported by molecular phylogenetics. This fungal group oomycetes; the discipline of biology devoted to the study of fungi is known as mycology. In the past, mycology was regarded as a branch of botany, although it is now known fungi are genetically more related to animals than to plants.
Abundant worldwide, most fungi are inconspicuous because of the small size of their structures, their cryptic lifestyles in soil or on dead matter. Fungi include symbionts of plants, animals, or other fungi and parasites, they may become noticeable when fruiting, either as molds. Fungi perform an essential role in the decomposition of organic matter and have fundamental roles in nutrient cycling and exchange in the environment, they have long been used in the form of mushrooms and truffles. Since the 1940s, fungi have been used for the production of antibiotics, more various enzymes produced by fungi are used industrially and in detergents. Fungi are used as biological pesticides to control weeds, plant diseases and insect pests. Many species produce bioactive compounds called mycotoxins, such as alkaloids and polyketides, that are toxic to animals including humans; the fruiting structures of a few species contain psychotropic compounds and are consumed recreationally or in traditional spiritual ceremonies.
Fungi can break down manufactured materials and buildings, become significant pathogens of humans and other animals. Losses of crops due to fungal diseases or food spoilage can have a large impact on human food supplies and local economies; the fungus kingdom encompasses an enormous diversity of taxa with varied ecologies, life cycle strategies, morphologies ranging from unicellular aquatic chytrids to large mushrooms. However, little is known of the true biodiversity of Kingdom Fungi, estimated at 2.2 million to 3.8 million species. Of these, only about 120,000 have been described, with over 8,000 species known to be detrimental to plants and at least 300 that can be pathogenic to humans. Since the pioneering 18th and 19th century taxonomical works of Carl Linnaeus, Christian Hendrik Persoon, Elias Magnus Fries, fungi have been classified according to their morphology or physiology. Advances in molecular genetics have opened the way for DNA analysis to be incorporated into taxonomy, which has sometimes challenged the historical groupings based on morphology and other traits.
Phylogenetic studies published in the last decade have helped reshape the classification within Kingdom Fungi, divided into one subkingdom, seven phyla, ten subphyla. The English word fungus is directly adopted from the Latin fungus, used in the writings of Horace and Pliny; this in turn is derived from the Greek word sphongos, which refers to the macroscopic structures and morphology of mushrooms and molds. The word mycology is derived from the Greek logos, it denotes the scientific study of fungi. The Latin adjectival form of "mycology" appeared as early as 1796 in a book on the subject by Christiaan Hendrik Persoon; the word appeared in English as early as 1824 in a book by Robert Kaye Greville. In 1836 the English naturalist Miles Joseph Berkeley's publication The English Flora of Sir James Edward Smith, Vol. 5. Refers to mycology as the study of fungi. A group of all the fungi present in a particular area or geographic region is known as mycobiota, e.g. "the mycobiota of Ireland". Before the introduction of molecular methods for phylogenetic analysis, taxonomists considered fungi to be members of the plant kingdom because of similarities in lifestyle: both fungi and plants are immobile, have similarities in general morphology and growth habitat.
Like plants, fungi grow in soil and, in the case of mushrooms, form conspicuous fruit bodies, which sometimes resemble plants such as mosses. The fungi are now considered a separate kingdom, distinct from both plants and animals, from which they appear to have diverged around one billion years ago; some morphological and genetic features are shared with other organisms, while others are unique to the fungi separating them from the other kingdoms: Shared features: With other euka
Powdery scab is a disease of potato tubers. It is caused by the cercozoan Spongospora subterranea f. sp. subterranea and is widespread in potato growing countries. Symptoms of powdery scab include small lesions in the early stages of the disease, progressing to raised pustules containing a powdery mass; these can rupture within the tuber periderm. The powdery pustules contain resting spores that release anisokont zoospores to infect the root hairs of potatoes or tomatoes. Powdery scab is a cosmetic defect on tubers. Potatoes which have been infected can be peeled to remove the infected skin and the remaining inside of the potato can be cooked and eaten. In general, not a lot is known about the life cycle of Spongospora subterranea f.sp subterranea. Most of the currently-proposed life cycle is based on that of Plasmodiophora brassicae, a related and better-studied protozoan, it has been proposed, due to this similarity, that there are two distinct stages that Sss can exist as. Asexual Stage: A zoospore infects root tissue and becomes a uninucleate plasmodium.
This plasmodium turns into a multinucleate plasmodium. The multinucleate plasmodium forms zoosporangium, which release more zoospores; this process can happen quickly and can act as an important source of secondary inoculum within a field. Sexual Stage: This stage follows a similar pattern to the asexual stage, but with a few exceptions, it is hypothesized that two zoospores fuse together to form a dikaryotic zoospore and infect the roots. Once the infection occurs, the dikaryotic zoospore develops into a binucleate plasmodium. Similar to the asexual stage, this plasmodium will replicate its nucleus to create a multinucleate plasmodium; the second main different between stages occurs here. The pairs of nuclei will fuse by karyogamy, the plasmodium will divide into numerous resting spores within a sporosori; these resting spores have three-layered walls and are resistant to the environment, allowing them to persist in the soil for longer than 10 years. As a reminder, most of the life cycle is still unclear.
However, the presence of zoospores, plasmodia and resting spores have been observed in the field and lab. The ploidy levels and karyogamy events have yet to be proven. Spongospora subterranea pathogenesis is most effective in cool, damp environments, such as northern Britain, the Columbia Basin of south-central Washington, north-central Oregon; the environmental condition is critical during the release of infective agents into the soil-environment. Upon release from resting spores, zoospores require moisture to swim towards the host tuber or roots. One study, found powdery scab was more common on plants grown in constant dampness compared to plants grown with varying moisture levels. In this same study she concluded disease risk was related more to the environment, or moisture level, than the level of inoculum present. Inoculum may be present but not able to disperse due to environmental conditions, therefore does not reach host tissue to infect. Other environmental factors that affect Spongospora subterranea infection are directly related to agronomic practices.
Increased use of fertilizers containing nitrate or ammonium nitrogen increase the incidence and severity of powdery scab. It is thought that the fertilization increases root growth, thus provides more tissue for infection and disease cycling to occur. Reduced cellulose within the cell walls caused by excess nitrogen may increase susceptibility of host to infection, it is apparent that the environment can directly affect both the host susceptibility and the dispersal of the pathogen setting the pace for the disease cycle. S. subterranea is an obligate parasite slime mold that infects the below ground structures of the host. Infection leads to hyperplasia of the host cells and eventual bursting. However, the mechanism behind this is still unknown. Zoospores infect the root hairs by attaching to the outer surface and penetrating the epidermis through lenticels and stomata. Once inside, the multinucleate plasmodium divides to produce more zoospores; the plasmodium causes the infected host cells to multiply and enlarge into a gall.
This rapid multiplication produces uninucleate cells that aggregate together as sporosori. The sporosori look like a powdery mass within the gall; the gall swells and bursts out the epidermis of the tuber, releasing the spores back into the soil. Gall severity depends on inoculum level and potato skin type. Infection is most prevalent in the early stages of tuber formation while the potato tissue is unsuberized. But, infection can occur at all stages on development. White and red skinned potatoes and susceptible while russet skinned are somewhat resistant. Russet skin is thicker and has higher levels of the LOX protein, used as a marker for resistance. There is little known about variation and sexual recombination within S. subterranea, therefore high priority is given to researching the variations within potato cultivars for researching host/pathogen relationships and management. Powdery Scab has important implications for commercial farming. Not only does the pathogen itself cause harm, but the pathogen is a vector for potato mop-top virus, another plant pathogen.
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The nematodes or roundworms constitute the phylum Nematoda. They are a diverse animal phylum inhabiting a broad range of environments. Taxonomically, they are classified along with insects and other moulting animals in the clade Ecdysozoa, unlike flatworms, have tubular digestive systems with openings at both ends. Nematode species can be difficult to distinguish from one another. Estimates of the number of nematode species described to date vary by author and may change over time. A 2013 survey of animal biodiversity published in the mega journal Zootaxa puts this figure at over 25,000. Estimates of the total number of extant species are subject to greater variation. A referenced article published in 1993 estimated there may be over 1 million species of nematode, a claim which has since been repeated in numerous publications, without additional investigation, in an attempt to accentuate the importance and ubiquity of nematodes in the global ecosystem. Many other publications have since vigorously refuted this claim on the grounds that it is unsupported by fact, is the result of speculation and sensationalism.
More recent, fact-based estimates have placed the true figure closer to 40,000 species worldwide. Nematodes have adapted to nearly every ecosystem: from marine to fresh water, from the polar regions to the tropics, as well as the highest to the lowest of elevations, they are ubiquitous in freshwater and terrestrial environments, where they outnumber other animals in both individual and species counts, are found in locations as diverse as mountains and oceanic trenches. They are found in every part of the earth's lithosphere at great depths, 0.9–3.6 km below the surface of the Earth in gold mines in South Africa. They represent 90% of all animals on the ocean floor, their numerical dominance exceeding a million individuals per square meter and accounting for about 80% of all individual animals on earth, their diversity of lifecycles, their presence at various trophic levels point to an important role in many ecosystems. They have been shown to play crucial roles in polar ecosystem; the 2,271 genera are placed in 256 families.
The many parasitic forms include pathogens in animals. A third of the genera occur as parasites of vertebrates. Nathan Cobb, a nematologist, described the ubiquity of nematodes on Earth as thus:In short, if all the matter in the universe except the nematodes were swept away, our world would still be dimly recognizable, if, as disembodied spirits, we could investigate it, we should find its mountains, vales, rivers and oceans represented by a film of nematodes; the location of towns would be decipherable, since for every massing of human beings, there would be a corresponding massing of certain nematodes. Trees would still stand in ghostly rows representing our highways; the location of the various plants and animals would still be decipherable, had we sufficient knowledge, in many cases their species could be determined by an examination of their erstwhile nematode parasites. Modern Latin compound of nemat- "thread" + -odes "like, of the nature of". In 1758, Linnaeus described some nematode genera included in the Vermes.
The name of the group Nematoda, informally called "nematodes", came from Nematoidea defined by Karl Rudolphi, from Ancient Greek νῆμα and -eiδἠς. It was treated as family Nematodes by Burmeister. At its origin, the "Nematoidea" erroneously included Nematodes and Nematomorpha, attributed by von Siebold. Along with Acanthocephala and Cestoidea, it formed the obsolete group Entozoa, created by Rudolphi, they were classed along with Acanthocephala in the obsolete phylum Nemathelminthes by Gegenbaur. In 1861, K. M. Diesing treated the group as order Nematoda. In 1877, the taxon Nematoidea, including the family Gordiidae, was promoted to the rank of phylum by Ray Lankester; the first clear distinction between the nemas and gordiids was realized by Vejdovsky when he named a group to contain the horsehair worms the order Nematomorpha. In 1919, Nathan Cobb proposed, he argued they should be called "nema" in English rather than "nematodes" and defined the taxon Nemates, listing Nematoidea sensu restricto as a synonym.
However, in 1910, Grobben proposed the phylum Aschelminthes and the nematodes were included in as class Nematoda along with class Rotifera, class Gastrotricha, class Kinorhyncha, class Priapulida, class Nematomorpha. In 1932, Potts elevated the class Nematoda to the level of phylum. Despite Potts' classification being equivalent to Cobbs', both names have been used and Nematode became a popular term in zoological science. Since Cobb was the first to include nematodes in a particular phylum separated from Nematomorpha, some researchers consider the valid taxon name to be Nemates or Nemata, rather than Nematoda, because of the zoological rule that gives priority to the first used term in case of synonyms; the phylogenetic relationships of the nematodes and their close relatives among the protostomian Metazoa are unresolved. Traditionally, they were held to b
Agriculture is the science and art of cultivating plants and livestock. Agriculture was the key development in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that enabled people to live in cities; the history of agriculture began thousands of years ago. After gathering wild grains beginning at least 105,000 years ago, nascent farmers began to plant them around 11,500 years ago. Pigs and cattle were domesticated over 10,000 years ago. Plants were independently cultivated in at least 11 regions of the world. Industrial agriculture based on large-scale monoculture in the twentieth century came to dominate agricultural output, though about 2 billion people still depended on subsistence agriculture into the twenty-first. Modern agronomy, plant breeding, agrochemicals such as pesticides and fertilizers, technological developments have increased yields, while causing widespread ecological and environmental damage. Selective breeding and modern practices in animal husbandry have increased the output of meat, but have raised concerns about animal welfare and environmental damage.
Environmental issues include contributions to global warming, depletion of aquifers, antibiotic resistance, growth hormones in industrial meat production. Genetically modified organisms are used, although some are banned in certain countries; the major agricultural products can be broadly grouped into foods, fibers and raw materials. Food classes include cereals, fruits, meat, milk and eggs. Over one-third of the world's workers are employed in agriculture, second only to the service sector, although the number of agricultural workers in developed countries has decreased over the centuries; the word agriculture is a late Middle English adaptation of Latin agricultūra, from ager, "field", which in its turn came from Greek αγρός, cultūra, "cultivation" or "growing". While agriculture refers to human activities, certain species of ant and ambrosia beetle cultivate crops. Agriculture is defined with varying scopes, in its broadest sense using natural resources to "produce commodities which maintain life, including food, forest products, horticultural crops, their related services".
Thus defined, it includes arable farming, animal husbandry and forestry, but horticulture and forestry are in practice excluded. The development of agriculture enabled the human population to grow many times larger than could be sustained by hunting and gathering. Agriculture began independently in different parts of the globe, included a diverse range of taxa, in at least 11 separate centres of origin. Wild grains were eaten from at least 105,000 years ago. From around 11,500 years ago, the eight Neolithic founder crops and einkorn wheat, hulled barley, lentils, bitter vetch, chick peas and flax were cultivated in the Levant. Rice was domesticated in China between 11,500 and 6,200 BC with the earliest known cultivation from 5,700 BC, followed by mung and azuki beans. Sheep were domesticated in Mesopotamia between 11,000 years ago. Cattle were domesticated from the wild aurochs in the areas of modern Turkey and Pakistan some 10,500 years ago. Pig production emerged in Eurasia, including Europe, East Asia and Southwest Asia, where wild boar were first domesticated about 10,500 years ago.
In the Andes of South America, the potato was domesticated between 10,000 and 7,000 years ago, along with beans, llamas and guinea pigs. Sugarcane and some root vegetables were domesticated in New Guinea around 9,000 years ago. Sorghum was domesticated in the Sahel region of Africa by 7,000 years ago. Cotton was domesticated in Peru by 5,600 years ago, was independently domesticated in Eurasia. In Mesoamerica, wild teosinte was bred into maize by 6,000 years ago. Scholars have offered multiple hypotheses to explain the historical origins of agriculture. Studies of the transition from hunter-gatherer to agricultural societies indicate an initial period of intensification and increasing sedentism. Wild stands, harvested started to be planted, came to be domesticated. In Eurasia, the Sumerians started to live in villages from about 8,000 BC, relying on the Tigris and Euphrates rivers and a canal system for irrigation. Ploughs appear in pictographs around 3,000 BC. Farmers grew wheat, vegetables such as lentils and onions, fruits including dates and figs.
Ancient Egyptian agriculture relied on its seasonal flooding. Farming started in the predynastic period at the end of the Paleolithic, after 10,000 BC. Staple food crops were grains such as wheat and barley, alongside industrial crops such as flax and papyrus. In India, wheat and jujube were domesticated by 9,000 BC, soon followed by sheep and goats. Cattle and goats were domesticated in Mehrgarh culture by 8,000–6,000 BC. Cotton was cultivated by the 5th-4th millennium BC. Archeological evidence indicates an animal-drawn plough from 2,500 BC in the Indus Valley Civilisation. In China, from the 5th century BC there was a nationwide granary system and widespread silk farming. Water-powered grain mills were in use followed by irrigation. By the late 2nd century, heavy ploughs had been developed with iron mouldboards; these spread westwards across Eurasia. Asian rice was domesticated 8,200–13,500 years ago – depending on the molecular clock estimate, used – on the Pearl River in southern China with a single genetic origin from the wild rice Oryza rufipogon
The potato is a starchy, tuberous crop from the perennial nightshade Solanum tuberosum. In many contexts, potato refers to the edible tuber, but it can refer to the plant itself. Common or slang terms include tater and spud. Potatoes were introduced to Europe in the second half of the 16th century by the Spanish. Today they are a staple food in many parts of the world and an integral part of much of the world's food supply; as of 2014, potatoes were the world's fourth-largest food crop after maize and rice. Wild potato species can be found from the United States to southern Chile; the potato was believed to have been domesticated independently in multiple locations, but genetic testing of the wide variety of cultivars and wild species traced a single origin for potatoes. In the area of present-day southern Peru and extreme northwestern Bolivia, from a species in the Solanum brevicaule complex, potatoes were domesticated 7,000–10,000 years ago. In the Andes region of South America, where the species is indigenous, some close relatives of the potato are cultivated.
Following millennia of selective breeding, there are now over 1,000 different types of potatoes. Over 99% of presently cultivated potatoes worldwide descended from varieties that originated in the lowlands of south-central Chile, which have displaced popular varieties from the Andes; the importance of the potato as a food source and culinary ingredient varies by region and is still changing. It remains an essential crop in Europe eastern and central Europe, where per capita production is still the highest in the world, while the most rapid expansion in production over the past few decades has occurred in southern and eastern Asia, with China and India leading the world in overall production as of 2014. Being a nightshade similar to tomatoes, the vegetative and fruiting parts of the potato contain the toxin solanine and are not fit for human consumption. Normal potato tubers that have been grown and stored properly produce glycoalkaloids in amounts small enough to be negligible to human health, but if green sections of the plant are exposed to light, the tuber can accumulate a high enough concentration of glycoalkaloids to affect human health.
The English word potato comes from Spanish patata. The Spanish Royal Academy says the Spanish word is a hybrid of the Taíno batata and the Quechua papa; the name referred to the sweet potato although the two plants are not related. The 16th-century English herbalist John Gerard referred to sweet potatoes as "common potatoes", used the terms "bastard potatoes" and "Virginia potatoes" for the species we now call "potato". In many of the chronicles detailing agriculture and plants, no distinction is made between the two. Potatoes are referred to as "Irish potatoes" or "white potatoes" in the United States, to distinguish them from sweet potatoes; the name spud for a small potato comes from the digging of soil prior to the planting of potatoes. The word has an unknown origin and was used as a term for a short knife or dagger related to the Latin "spad-" a word root meaning "sword", it subsequently transferred over to a variety of digging tools. Around 1845, the name transferred to the tuber itself, the first record of this usage being in New Zealand English.
The origin of the word "spud" has erroneously been attributed to an 18th-century activist group dedicated to keeping the potato out of Britain, calling itself The Society for the Prevention of Unwholesome Diet. It was Mario Pei's 1949 The Story of Language. Pei writes, "the potato, for its part, was in disrepute some centuries ago; some Englishmen who did not fancy potatoes formed a Society for the Prevention of Unwholesome Diet. The initials of the main words in this title gave rise to spud." Like most other pre-20th century acronymic origins, this is false, there is no evidence that a Society for the Prevention of Unwholesome Diet existed. Potato plants are herbaceous perennials that grow about 60 cm high, depending on variety, with the leaves dying back after flowering and tuber formation, they bear white, red, blue, or purple flowers with yellow stamens. In general, the tubers of varieties with white flowers have white skins, while those of varieties with colored flowers tend to have pinkish skins.
Potatoes are cross-pollinated by insects such as bumblebees, which carry pollen from other potato plants, though a substantial amount of self-fertilizing occurs as well. Tubers form in response to decreasing day length, although this tendency has been minimized in commercial varieties. After flowering, potato plants produce small green fruits that resemble green cherry tomatoes, each containing about 300 seeds. Like all parts of the plant except the tubers, the fruit contain the toxic alkaloid solanine and are therefore unsuitable for consumption. All new potato varieties are grown from seeds called "true potato seed", "TPS" or "botanical seed" to distinguish it from seed tubers. New varieties grown from seed can be propagated vegetatively by planting tubers, pieces of tubers cut to include at least one or two eyes, or cuttings, a practice used in greenhouses for the production of healthy seed tubers. Plants propagated from tubers are clones of the parent, whereas those propagated from seed produce a range of different varieties.
There are about 5,000 potato varieties worldwide. Three thousand of them are found in the Andes alone in Peru, Ecuador and Colombia, they belong to eight or nine species, dependin
Tubers are enlarged structures in some plant species used as storage organs for nutrients. They are used for the plant's perennation, to provide energy and nutrients for regrowth during the next growing season, as a means of asexual reproduction. Stem tubers form thickened stolons. Common plant species with stem tubers include yam; some sources treat modified lateral roots under the definition. The term originates from Latin tuber, meaning "lump, swelling"; some sources define the term "tuber" to mean only structures derived from stems. A stem tuber forms from thickened stolons; the top sides of the tuber produce shoots that grow into typical stems and leaves and the under sides produce roots. They tend to form at the sides of the parent plant and are most located near the soil surface; the underground stem tuber is a short-lived storage and regenerative organ developing from a shoot that branches off a mature plant. The offsprings or new tubers are attached to a parent tuber or form at the end of a hypogeogenous rhizome.
In the autumn the plant dies, except for the new offspring stem tubers which have one dominant bud, which in spring regrows a new shoot producing stems and leaves, in summer the tubers decay and new tubers begin to grow. Some plants form smaller tubers and/or tubercules which act like seeds, producing small plants that resemble seedlings; some stem tubers are long-lived, such as those of tuberous begonia, but many plants have tubers that survive only until the plants have leafed out, at which point the tuber is reduced to a shriveled-up husk. Stem tubers start off as enlargements of the hypocotyl section of a seedling but sometimes include the first node or two of the epicotyl and the upper section of the root; the stem tuber has a vertical orientation with one or a few vegetative buds on the top and fibrous roots produced on the bottom from a basal section the stem tuber has an oblong rounded shape. Tuberous begonia and Cyclamen are grown stem tubers. Mignonette vine produces aerial stem tubers on 12-to-25-foot-tall vines, the tubers fall to the ground and grow.
Plectranthus esculentus of the mint family Lamiaceae, produces tuberous under ground organs from the base of the stem, weighing up to 1.8 kg per tuber, forming from axillary buds producing short stolons that grow into tubers. Potatoes are stem tubers. Enlarged stolons thicken to develop into storage organs; the tuber has all the parts including nodes and internodes. The nodes are the eyes and each has a leaf scar; the nodes or eyes are arranged around the tuber in a spiral fashion beginning on the end opposite the attachment point to the stolon. The terminal bud is produced at the farthest point away from the stolon attachment and tubers thus show the same apical dominance as a normal stem. Internally, a tuber is filled with starch stored in enlarged parenchyma like cells; the inside of a tuber has the typical cell structures of any stem, including a pith, vascular zones, a cortex. The tuber is produced in one growing season and used to perennate the plant and as a means of propagation; when fall comes, the above-ground structure of the plant dies, but the tubers survive over winter underground until spring, when they regenerate new shoots that use the stored food in the tuber to grow.
As the main shoot develops from the tuber, the base of the shoot close to the tuber produces adventitious roots and lateral buds on the shoot. The shoot produces stolons that are long etiolated stems; the stolon elongates during long days with the presence of high auxins levels that prevent root growth off of the stolon. Before new tuber formation begins, the stolon must be a certain age; the enzyme lipoxygenase makes a hormone, jasmonic acid, involved in the control of potato tuber development. The stolons are recognized when potato plants are grown from seeds; as the plants grow, stolons are produced around the soil surface from the nodes. The tubers form close to the soil surface and sometimes on top of the ground; when potatoes are cultivated, the tubers are planted much deeper into the soil. Planting the pieces deeper creates more area for the plants to generate the tubers and their size increases; the pieces sprout shoots. These shoots generate short stolons from the nodes while in the ground.
When the shoots reach the soil surface, they produce roots and shoots that grow into the green plant. A tuberous root or storage root, is a modified lateral root, enlarged to function as a storage organ; the enlarged area of the root-tuber, or storage root, can be produced at the end or middle of a root or involve the entire root. It is thus similar in function and appearance to a stem tuber. Examples of plants with notable tuberous roots include the sweet potato and dahlia. Root tubers are perennating organs, thickened roots that store nutrients over periods when the plant cannot grow, thus permitting survival from one year to the next; the massive enlargement of secondary roots represented by sweet potato, have the internal and external cell and tissue structures of a normal root, they produce adventitious roots and stems which again produce adventitious roots. In root-tubers, there are reduced leaves. Root tubers have one end called the proximal end, the end