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Maize
Cultivars of maize
Cultivars of maize
Scientific classification
Kingdom: Plantae
Division: Magnoliophyta
Class: Liliopsida
Order: Poales
Family: Poaceae
Genus: Zea
Species: Z. mays
Zea mays
L.
Maize (Zea mays ssp. mays) is a cereal grain that was domesticated in Mesoamerica. It is called corn in the United States, Canada, and Australia, but in other countries that term may refer to other cereal grains. Hybrid maize is favored by farmers over conventional varieties for its high grain yield, due to heterosis ("hybrid vigor"). Maize is one of the first crops for which genetically modified varieties make up a significant proportion of the total harvest.While some maize varieties grow 7 m (23 ft) tall at certain locations, commercial maize has been bred for a high-end height of 2.5 m (9 ft). Sweetcorn is usually shorter than field corn varieties.

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Contents

Maize physiology
Genetics
Origin of maize
Cultivation
Pests of maize
Uses for maize



Maize physiology - Contents

The stems look like bamboo cane and the joints ( nodes) are about 40–50 cm (16–20 in) apart. Maize has a very distinct growth form, the lower leaves being like broad flags, 50–100 cm long and 5–10 cm wide (2–4 ft by 2–4 in); the stems are erect, from 2–3 m (7–10 ft) in height, with many nodes, casting off flag-leaves at every node. Under these leaves and close to the stem grow the ears.The ears are female inflorescences, tightly covered over by several layers of leaves, and so closed in by them to the stem, that they do not show themselves easily until the emergence of the pale yellow silks from the leaf whorl at the end of the ear. The silks are elongated stigmas that look like tufts of hair, at first green, and later red or yellow. Maize planted individually will develop 2–4 ears. Modern farming techniques in developed countries usually rely on dense planting, which reduce the number of ears per stalk to about 0.9 (that is, an average of one per plant with some so stressed they produce zero mature ears.) [1] Plantings for silage will be even denser to achieve an even lower percentage of ears and more plant matter. Certain varieties of maize have been bred to produce additional developed ears, and these are the source of the "baby corn" that is used as a vegetable in Oriental cooking.The apex of the stem ends in a male flower, the tassle. For each silk on which pollen from the tassle lands, one kernel of corn is produced. Young ears can be consumed raw, cob, silk, and all; as the plant matures (usually during the summer months) the cob becomes tougher and the silk dries to inedibility. By late August the kernels have dried out and become difficult to chew without cooking them tender first in boiling water.The kernel of corn has a pericarp of the fruit fused with the seed coat, typical of the grasses. It is close to a multiple fruit in structure, except that the individual fruits (the kernels) never fuse into a single mass. The grains are about the size of peas, and adhere in regular rows round a white pithy substance, which forms the ear. An ear contains from two to four hundred grains, and is from 10–25 cm (4–10 inch]]es) in length. They are of various colors, blackish, red, white and yellow. When ground into flour, it yields more flour, with much less bran, than wheat does. However, it lacks the protein gluten, and therefore makes baked goods with poor raising capability.A genetic variation that accumulates more sugar and less starch in the ear is consumed as a vegetable and is called sweetcorn.Immature maize shoots accumulate a powerful antibiotic substance, DIMBOA (2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one). DIMBOA is a member of a group of hydroxamic acids (also known as benzoxazinoids) that serve as a natural defense against a wide range of pests including insects, pathogenic fungi and bacteria. DIMBOA is also found in related grasses, particularly wheat. A maize mutant (bx) lacking DIMBOA is highly susceptible to attack by aphids and fungi. DIMBOA is also responsible for the relative resistance of immature maize to the European corn borer. As maize matures, DIMBOA levels and resistance to the corn borer both decline.


Genetics - Contents

Maize has 10 chromosomes (n=10). The combined length of the chromosomes is 1500 cM. Some of the maize chromosomes have what are known as "chromosomal knobs". They are highly repetitive heterochromatic domains that stain darkly. Individual knobs are polymorphic among strains of both maize and teosinte. Barbara McClintock used these knob markers to prove her transposon theory of "jumping genes".There is a stock center of maize mutants, The Maize Genetics Cooperation - Stock Center, located in the Department of Crop Sciences at the University of Illinois, Urbana/Champaign. The total collection has nearly 80,000 samples. The bulk of the collection consists of several hundred named genes, plus additional gene combinations and other heritable variants. There are about 1000 chromosomal aberrations (e.g., translocations and inversions) and stocks with abnormal chromosome numbers (e.g., tetraploids).In 2005 the US National Science Foundation (NSF), Department of Agriculture (USDA) and the Department of Energy (DOE) formed a consortium to sequence the maize genome. The resulting DNA sequence data will be deposited immediately into GenBank, a public repository for genome-sequence data. Sequencing the corn genome has been considered difficult because of its large size and complex genetic arrangements. The genome has 50,000–60,000 genes scattered among the 2.5 billion bases—molecules that form DNA—that make up its 10 chromosomes. (By comparison, the human genome contains about 2.9 billion bases and 26,000 genes.)


Origin of maize - Contents

Corn plants showing ears
Corn plants showing ears
There are several theories about the origin of maize:
  1. It is a direct domestication of a Mexican annual teosinte, Zea mays ssp. parviglumis, native to the Balsas River valley of southern Mexico, with up to 12% of its genetic material obtained from Zea mays ssp. mexicana through introgression;
  2. It derives from hybridization between a small domesticated maize—a slightly changed form of a wild maize—and a teosinte of section Luxuriantes, either Z. luxurians or Z. diploperennis;
  3. It underwent two or more domestications either of a wild maize or of a teosinte;
  4. It evolved from a hybridization of Z. diploperennis by Tripsacum dactyloides. (The term "teosinte" describes all species and subspecies in the genus Zea, excluding Zea mays ssp. mays.) In the late 1930s, Paul Mangelsdorf suggested that domesticated maize was the result of a hybridization event between an unknown wild maize and a species of Tripsacum, a related genus. However, the proposed role of tripsacum (gama grass) in the origins of maize has been refuted by modern genetic analysis, negating Mangelsdorf’s model and the fourth listed above.
The third model (actually a group of hypotheses) is unsupported. The second parsimoniously explains many conundrums but is dauntingly complex. The first model was proposed by Nobel Prize winner George Beadle in 1939, and it has experimental support, but it has not explained:
  1. how the immense diversity of the species of sect. Zea originated,
  2. how the tiny archaeological specimens of 3500–2700 BC (uncorrected) could have been selected from a teosinte,
  3. how domestication could have proceeded without leaving remains of teosinte or maize with teosintoid traits until ca. 1100 BC,
  4. and other problems.
The domestication of maize is of particular interest to researchers— archaeologists, geneticists, ethnobotanists, geographers, etc. The process is thought by some to have started from 7,500 to 12,000 years ago (corrected for solar variations). It is unknown what precipitated its domestication, because the edible portion of the wild variety is too small and hard to obtain to be eaten directly, as each kernel is enclosed in a very hard bi-valve shell. However, George Beadle demonstrated that the kernels of teosinte are readily "popped" for human consumption, like modern popcorn. Some have argued that it would have taken too many generations of selective breeding in order to produce large compressed ears for efficient cultivation. However, studies of the hybrids readily made by intercrossing teosinte and modern maize suggest that this objection is not well-founded. Archaeological remains of early maize cobs, found at Guila Naquitz Cave in the Oaxaca Valley of Mexico, date back roughly 6,250 years (corrected; 3450 BC, uncorrected); the oldest cobs from caves near Tehuacan, Puebla, date ca. 2750 BC Little change occurred in cob form until ca. 1100 BC when great changes appeared in cobs from Mexican caves: maize diversity rapidly increased and archaeological teosinte was first deposited.Perhaps as early as 1500 BC, maize began spread widely and rapidly. As it was introduced to new cultures, new uses were developed and new varieties selected to better serve in those preparations. Maize was the staple food, or a major staple, of most the pre-Columbian North American, Mesoamerican, South American, and Caribbean cultures. During the 1st millennium CE (AD), maize cultivation spread from Mexico into the US Southwest and a millennium later into northeastern US and southeast Canada, transforming the landscape as Native Americans cleared large forest and grassland areas for the new crop.


Cultivation - Contents

Corn production in Colorado
Corn production in Colorado
Maize is widely cultivated throughout the world, and a greater weight of maize is produced each year than any other grain. While the United States produces almost half of the world's harvest, other top producing countries are as widespread as China, India, Brazil, France, Indonesia, and South Africa. Worldwide production was over 600 million metric tons in 2003—just slightly more than rice or wheat. In 2004, close to 33 million hectares of maize were planted worldwide, with a production value of more than billion.Maize is planted in the spring to take advantage of spring rains. Its root system is generally shallow, so the plant is dependent on soil moisture. As a C4 plant (a plant that uses C4 photosynthesis), maize is a considerably more water-efficient crop than a C3 plant, like the small grains, alfalfa and soybeans. Maize is most sensitive to drought at the time of silk emergence, when the flowers are ready for pollination. In the United States, a good harvest was predicted traditionally if the corn was "knee-high by the Fourth of July", although modern hybrids often exceed this growth rate. Maize used as silage is harvested while the plant is green and the fruit unmatured. Otherwise, maize is left in the field very late in the autumn in order to dry thoroughly. In fact, it is sometimes not harvested until winter or even early spring. The importance of regular rain is shown in many parts of Africa, where periodic drought regularly causes famine by causing maize crop failure.
Field of maize in Liechtenstein
Field of maize in Liechtenstein
Maize was planted by the Native Americans in hills, in a complex system known to some as the Three Sisters: beans used the corn plant for support, and squashes provided ground cover to stop weeds. This method was replaced by single species hill planting where each hill 60–120 cm (2–4 ft) apart was planted with 3 or 4 seeds, a method still used by the home gardener. A later technique was checked corn where hills were placed 40 inches apart in each direction, allowing cultivators to run through the field in two directions. In more arid lands this was altered and seed were planted in the bottom of 10–12 cm (4–5 in) deep furrows to collect water. Modern technique plants maize in rows which allows for cultivation while the plant is young.In North America, fields are often planted in a two-crop rotation with a nitrogen-fixing crop, often soybeans. Sometimes a third crop, winter wheat, is added to the rotation. Fields are usually plowed each year, although no-till farming is increasing in use. Nearly all maize cultivars grown in the United States and Canada are hybrids. Over half of the corn acreage planted in the United States has been genetically modified using biotechnology to express agronomic traits desired by farmers.Before about World War II, most maize was harvested by hand. This often involved large numbers of workers and associated social events. Some one- and two-row mechanical pickers were in use but the corn combine did not get adopted until after the War. By hand or mechanical picker, the entire ear is harvested which then requires a separate operation of a corn sheller to remove the kernels from the ear. Whole ears of corn were often stored in corn cribs and these whole ears are a sufficient form for some livestock feeding use. Some modern farms store maize in this manner and later shell it for sale in the off-season to capture better prices. The combine with a corn head (with points and snap rolls instead of a reel) cuts the stalk near the base and then separates the ear of corn from the stalk so that only the ear and husk enter the machinery. The combine separates the husk and the cob, keeping only the kernels.


Pests of maize - Contents



Insect pests
Exotic varieties of maize are collected to add genetic diversity when selectively breeding new domestic strains.
Exotic varieties of maize are collected to add genetic diversity when selectively breeding new domestic strains.
  • Corn earworm (Heliothis zea)
  • Fall armyworm (Spodoptera frugiperda)
  • Common armyworm (Pseudaletia unipuncta)
  • Stalk borer (Papaipema nebris)
  • Corn leaf aphid (Rhopalosiphum maidis)
  • European corn borer (Ostrinia nubilalis) (ECB)
  • Corn silkfly (Euxesta stigmatis)
  • Lesser cornstalk borer (Elasmopalpus lignosellus)
  • Corn delphacid (Peregrinus maidis)
The susceptibility of maize to the European corn borer, and the resulting large crop losses, led to the development of transgenic Bt corn expressing the Bacillus thuringiensis (Bt) toxin. Bt corn is widely grown in the United States and has been approved for release in Europe.

Diseases
  • Corn smut or common smut (Ustilago maydis): a fungal disease, known in Mexico as huitlacoche, which is prized by some as a gourmet delicacy in itself.
  • Maize Dwarf Mosaic Virus
  • Stewart's Wilt (Pantoea stewartii)
  • Common Rust (Puccinia sorghi)



Uses for maize - Contents

In the United States and Canada, the primary use for maize is as a feed for livestock, fed either as silage or as grain. Silage is made by fermentation of chopped green cornstalks. The grain also has many industrial uses. Some is hydrolyzed and enyzmatically treated to produce syrups, particularly high fructose corn syrup, a sweetener, and some is fermented and distilled to produce grain alcohol, or ethanol. Grain alcohol from maize is traditionally the source of bourbon whiskey. Increasingly ethanol is being used as an additive in gasoline ( gasohol) for motor fuels to increase the octane rating, lower pollutants, and reduce petroleum use.
Multicolored varieties of corn
Multicolored varieties of corn
Human consumption of corn and corn meal constitutes a staple food in many regions of the world. Corn meal is made into a thick porridge in many cultures: from the polenta of Italy and the mămăligă of Romania to mush in the US or the food called sadza, nshima, ugali and mealie pap in Africa. It is the main ingredient for tortilla and many other dishes of Mexican food, and for chicha, a fermented beverage of Central and South America.Sweetcorn is a genetic variation that is high in sugars and low in starch that is served like a vegetable.Maize can also be prepared as hominy, in which the kernels are bleached with lye; or grits, which are simply coarsely ground corn. These are commonly eaten in US Southern States, foods handed down from Native Americans. Another common food made from maize is corn flakes. The flour of maize ( cornflour or masa) is used to make cornbread and Mexican tortillas. Teosinte is used as fodder, and can also be popped as popcorn. As a food, maize (Zea mays ssp. mays) is used in various forms, with several major Cultivar Groups. The most important Cultivar Groups are:
  • Flour corn - Zea mays L. subsp. mays Amylacea Group
  • Popcorn - Zea mays L. subsp. mays Everta Group
  • Dent corn - Zea mays L. subsp. mays Indentata Group
  • Flint corn - Zea mays L. subsp. mays Indurata Group
  • Sweetcorn - Zea mays L. subsp. mays Saccharata Group
  • Waxy corn - Zea mays L. ceratina Kuleshov
  • Pod corn - Zea mays L. var. tunicata Larrañaga ex A. St. Hil
Some forms of the plant are occasionally grown for ornamental use in the garden. For this purpose, variegated and coloured leaf forms, as well as those with colourful cobs are used.
Corn male flower, a.k.a. corn tassel
Corn male flower, a.k.a. corn tassel
Corn female flower, a.k.a. corn silk
Corn female flower, a.k.a. corn silk
Corncobs can be hollowed out and treated to make inexpensive smoking pipes, first manufactured in the United States in 1869. Corn cobs are also used as a biomass fuel source. Maize is relatively cheap and home heating furnaces have been developed which uses maize kernels as a fuel. They feature a large hopper which feeds the uniformly sized corn kernels (or wood pellets or cherry pits) into the fire.An unusual use for maize prior to harvest is for a maze. In the US, these are called "corn mazes" and are popular in many farming communities. The first modern corn maze was designed by Adrian Fisher, who is in the Guinness Book of World Records for several of his maze designs.In 1983, Barbara McClintock received the Nobel Prize in Physiology or Medicine for discovery of transposons while studying maize. Maize is still an important model organism for genetics and developmental biology today.In 2005, research by the USDA Forest Service indicated that the rise in maize cultivation 500 to 1,000 years ago in the southeastern United States contributed to the decline of freshwater mussels, which are very sensitive to environmental changes. [2]
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