Encyclopedia
Maize , also known as
corn, is a
cereal grain that was domesticated in
Mesoamerica. It spread to the rest of the world after European contact with the Americas in the late
15th century and early
16th century. It is called corn in the
United States, English Canada ,
New Zealand, and
Australia, but in other countries that term may refer to other 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 . 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 tall at certain locations, commercial maize has been bred for a height of 2.5 m .
Sweetcorn is usually shorter than field-corn varieties.
Genetics
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 , races, racial complexes, and recently branches.
Maize has 10 chromosomes . 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, 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 and stocks with abnormal chromosome numbers . Genetic data describing the maize mutant stocks as well as myriad other data about maize genetics can be accessed at , the Maize Genetics and Genomics Database.
In 2005 the U.S.
National Science Foundation , Department of Agriculture and the
Department of Energy 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.
Origin
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 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. 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 in the origins of maize has been refuted by modern genetic analysis, negating Mangelsdorf’s model and the fourth listed above.
The third model 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 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 . 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 ; 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 , 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 .
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.
Cultivation
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 , 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 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 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 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.
Pellagra
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 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 -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.
- Corn earworm
- Fall armyworm
- Common armyworm
- Stalk borer
- Corn leaf aphid
- European corn borer
- Corn silkfly
- Lesser cornstalk borer
- Corn delphacid
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.
Diseases
- Corn smut or common smut : 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
- Common Rust
- Goss's Wilt
- Grey Leaf Spot
- Mal de Río Cuarto Virus
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 as an additive in
gasoline 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
mamaliga 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 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 tall [Evening Journal; Washington, Iowa; 1946], or with cobs 24 inches long [Journal of Heredity, 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 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.
See also
- Protein per unit area
- Detasseling
- Moonshine
- Zein
References
- Ferro, D.N. and Weber, D.C.
- as of 22 September 2002
- This list is of historical interest to taxonomists. It is largely of no practical use because many or most are based on single-gene mutations and if completed would be thousands of entries long. Modern classifications are available that are of great utility.
External links
Food | List of fruits | List of vegetables
