2007 Schools Wikipedia Selection. Related subjects: Plants
Cultivars of maize
Maize (Zea mays ssp. mays), also known as corn, is a cereal grain that was domesticated in Mesoamerica and then spread throughout the American continents. It spread to the rest of the world after European contact with the Americas in the late 15th century and early 16th century. The term maíze derives from the Spanish form of the Arawak Indian term for the plant. However, it is popularly called corn in the United States, English Canada (in French Canada it is called maïs), New Zealand, and Australia. Corn is a shortened form of "Indian corn", i.e. the Indian grain. The English word "corn" originally referred to a granular particle, most commonly cereal grains. It is called mealies in southern Africa. 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 [Kuleshov 1933], commercial maize has been bred for a height of 2.5 m (8 ft). Sweetcorn is usually shorter than field-corn varieties.
The stems superficially resemble bamboo canes and the joints ( nodes) are about 20–30 cm (8–12 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, conventionally 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. They grow about 3 cm a day.
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 develops 2 to 4 ears. Modern farming techniques in developed countries usually rely on dense planting, which produces on average only about 0.9 ears per stalk because it stresses the plants. Plantings for silage are even denser, and achieve an even lower percentage of ears and more plant matter. Certain varieties of maize have been bred to produce many additional developed ears, and these are the source of the "baby corn" that is used as a vegetable in Asian cuisine.
Maize cultivars grown in the temperate zone are considered day-neutral and flower after a certain number of days at > 50° F (10°C). However, maize cultivars from tropical locations typically have a short-day requirement for flowering and generally do not produce seed in the long summer days at higher latitudes. The day-length requirement for flowering is controlled genetically and regulated by the phytochrome system.
The apex of the stem ends in the tassel, an inflorescence of male flowers. Each silk may become pollinated to produce one kernel of corn. Young ears can be consumed raw, with the cob and silk, but 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 in) in length. They are of various colors: blackish, bluish-gray, red, white and yellow. When ground into flour, maize yields more flour, with much less bran, than wheat does. However, it lacks the protein gluten of wheat and therefore makes baked goods with poor rising 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 (family Crambidae). As maize matures, DIMBOA levels and resistance to the corn borer decline.
Many forms of maize are used for food, once classified as various subspecies:
- 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
- Amylomaize - Zea mays
- Pod corn - Zea mays L. var. tunicata Larrañaga ex A. St. Hil.
This system has been replaced over the last 60 years by multi-variable classifications based on ever more data. Agronomic data was supplemented by botanical traits for a robust initial classification, then genetic, cytological, protein and DNA evidence was added. Now the categories are forms (little used), races, racial complexes, and recently branches.
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 centre of maize mutants, The Maize Genetics Cooperation - Stock Centre, funded by the USDA Agricultural Research Service and located in the Department of Crop Sciences at the University of Illinois at 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). Genetic data describing the maize mutant stocks as well as myriad other data about maize genetics can be accessed at MaizeGDB, the Maize Genetics and Genomics Database.
In 2005, the U.S. 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.)
There are several theories about the specific origin of maize in Mesoamerica:
- 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;
- 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;
- It underwent two or more domestications either of a wild maize or of a teosinte;
- 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 a number of problems, among them:
- how the immense diversity of the species of sect. Zea originated,
- how the tiny archaeological specimens of 3500–2700 BC (uncorrected) could have been selected from a teosinte, and
- how domestication could have proceeded without leaving remains of teosinte or maize with teosintoid traits until ca. 1100 BC.
The domestication of maize is of particular interest to researchers— archaeologists, geneticists, ethnobotanists, geographers, etc. The process is thought by some to have started 7,500 to 12,000 years ago (corrected for solar variations). Recent genetic evidence suggests that maize domestication occurred 9000 years ago in central Mexico, perhaps in the highlands between Oaxaca and Jalisco. The wild teosinte most similar to modern maize grows in the area of the Balsas River. Archaeological remains of early maize cobs, found at Guila Naquitz Cave in the Oaxaca Valley, 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 to 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 U.S. Southwest and a millennium later into northeastern U.S. and southeast Canada, transforming the landscape as Native Americans cleared large forest and grassland areas for the new crop. Gavin Menzies, in his book 1421 - The Year China Discovered the World, claims to show that Maize was most likely transplanted from the Americas by the Chinese during their great voyages of the 15th century (although this claim is widely disputed ).
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.
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.
|Top Maize Producers
|(million metric tons)|
UN Food & Agriculture Organisation
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, 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 $23 billion.
Because it is cold-intolerant, in the temperate zones maize must be planted in the spring. 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 C3 plants 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 traditionally predicted if the corn was "knee-high by the Fourth of July", although modern hybrids generally exceed this growth rate. Maize used for silage is harvested while the plant is green and the fruit immature. Sweet corn is harvested in the "milk stage", after pollination but before starch has formed, between late summer and early to mid-autumn. Field corn is left in the field very late in the autumn in order to thoroughly dry the grain, and may, in fact, sometimes not be harvested until winter or even early spring. The importance of sufficient soil moisture is shown in many parts of Africa, where periodic drought regularly causes famine by causing maize crop failure.
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 home gardeners. 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 seeds 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 alfalfa in cooler climates and soybeans in regions with longer summers. 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 was not 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.
When maize was first introduced outside of the Americas it was typically welcomed enthusiastically by farmers everywhere for its productivity. However, a widespread problem of malnutrition soon arose wherever maize was introduced. This was a mystery since these types of malnutrition were not seen among the indigenous Americans under normal circumstances.
It was eventually discovered that the indigenous Americans learned long ago to add alkali — in the form of ashes among North Americans and lime ( calcium carbonate) among Mesoamericans — to corn meal to liberate the B-vitamin niacin, the lack of which was the underlying cause of the condition known as pellagra. This alkali process is known by its Nahuatl (Aztec)-derived name: nixtamalization.
Besides the lack of niacin, pellagra was also characterized by protein deficiency, a result of the inherent lack of two key amino acids in pre-modern maize, lysine and tryptophan. Nixtamalization was also found to increase the lysine and tryptophan content of maize to some extent, but more importantly, the indigenous Americans had learned long ago to balance their consumption of maize with beans and other protein sources such as amaranth and chia, as well as meat and fish, in order to acquire the complete range of amino acids for normal protein synthesis.
Since maize had been introduced into the diet of non-indigenous Americans without the necessary cultural knowledge acquired over thousands of years in the Americas, the reliance on maize elsewhere was often tragic. Once alkali processing and dietary variety was understood and applied, pellagra disappeared. The development of high lysine maize and the promotion of a more balanced diet has also contributed to its demise.
Pests of maize
- Corn earworm (Helicoverpa 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 expressing the Bacillus thuringiensis toxin. "Bt corn" is widely grown in the United States and has been approved for release in Europe.
- 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)
- Goss's Wilt (Clavibacter michiganese)
- Grey Leaf Spot
- Mal de Río Cuarto Virus (MRCV)
Uses for maize
In the United States and Canada, the primary use for maize is as a feed for livestock, forage, silage or grain. Silage is made by fermentation of chopped green cornstalks. The grain also has many industrial uses, including transformation into plastics and fabrics. Some is hydrolyzed and enzymatically treated to produce syrups, particularly high fructose corn syrup, a sweetener, and some is fermented and distilled to produce grain alcohol. Grain alcohol from maize is traditionally the source of bourbon whiskey. Increasingly ethanol is being used at low concentrations (10% or less) as an additive in gasoline ( gasohol) for motor fuels to increase the octane rating, lower pollutants, and reduce petroleum use.
Human consumption of corn and cornmeal 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 U.S. 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. Popcorn is kernels of certain varieties that explode when heated, forming fluffy pieces that are eaten as a snack.
Maize can also be prepared as hominy, in which the kernels are bleached with lye; or grits, which are bleached coarsely ground corn. These are commonly eaten in U.S. Southern States, foods handed down from Native Americans. Another common food made from maize is corn flakes. The floury meal of maize ( cornmeal or masa) is used to make cornbread and Mexican tortillas. Teosinte is used as fodder, and can also be popped as popcorn.
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. Additionally, size-superlative varieties, having reached 31 ft (9m) tall [Evening Journal, Washington Iowa 1946], or with cobs 24 inches long [Kempton 1924], have been popular for at least a century .
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 use 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 is to create a Maize Maze as a tourist attraction. This is a maze cut into a field of maize. The idea of a Maize Maze was introduced by Adrian Fisher, one of the most prolific designer of modern mazes, with The American Maze Company who created a maze in Pennsylvania in 1993. Traditional mazes are most commonly grown using yew hedges, but these take several years to mature. The rapid growth of a field of maize allows a maze to be laid out using GPS at the start of a growing season and for the maize to grow tall enough to obstruct a visitor's line of sight by the start of the summer. In Canada and the U.S., these are called "corn mazes" and are popular in many farming communities.
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.
Maize is sometimes used as a biomass fuel, such as ethanol . A biomass gasification power plant in Strem near Güssing, Burgenland, Austria was begun in 2005. Research is being done to make diesel out of the biogas by the Fischer Tropsch method.
Maize is also used as fish bait. It is particularly popular in Europe for coarse fishing.
Stigmas from female corn flowers, known popularly as corn silk, are sold as herbal supplements.