Corn seed

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Maize (/ˈmz/ MAYZ; Zea mays subsp. mays, from Spanish: maíz after Taíno mahiz), known in some English-speaking countries as corn, and in the United Kingdom as sweetcorn, is a large grain plant domesticated by indigenous peoples in Mexico[1] in prehistoric times about 10,000 years ago.
The leafy stalk produces separate pollen and ovuliferous inflorescences or ears which are fruits yielding kernels (often erroneously called seeds). Maize kernels are often used in cooking as a starch. The six major types of maize are dent, flint, pod, popcorn, flour, and sweet.[2]
Most historians believe maize was domesticated in the Tehuacan Valley of Mexico.[4] The Olmec and Mayans cultivated it in numerous varieties throughout Mesoamerica, cooked, ground or processed through nixtamalization. Beginning about 2500 BC, the crop spread through much of the Americas.[5] The region developed a trade network based on surplus and varieties of maize crops. After European contact with the Americas in the late 15th and early 16th centuries, explorers and traders carried maize back to Europe and introduced it to other countries. Maize spread to the rest of the world because of its ability to grow in diverse climates. Sugar-rich varieties called sweet corn are usually grown for human consumption as kernels, while field corn varieties are used for animal feed, various corn-based human food uses (including grinding into cornmeal or masa, pressing into corn oil, and fermentation and distillation into alcoholic beverages like bourbon whiskey), and as chemical feedstocks.
An influential 2002 study by Matsuoka et al. has demonstrated that, rather than the multiple independent domestications model, all maize arose from a single domestication in southern Mexico about 9,000 years ago.[6] The study also demonstrated that the oldest surviving maize types are those of the Mexican highlands. Later, maize spread from this region over the Americas along two major paths. This is consistent with a model based on the archaeological record suggesting that maize diversified in the highlands of Mexico before spreading to the lowlands.[7]
Before they were domesticated, maize plants only grew small, 25 millimetres (1 in) long corn cobs, and only one per plant. Many centuries of artificial selection by the indigenous people of the Americas resulted in the development of maize plants capable of growing several cobs per plant that were usually several centimetres/inches long each.[8]
Maize is the most widely grown grain crop throughout the Americas,[9] with 332 million metric tons grown annually in the United States alone. Approximately 40% of the crop—130 million tons—is used for corn ethanol.[10] Genetically modified maize made up 85% of the maize planted in the United States in 2009.[11]


Many small male flowers make up the male inflorescence, called the tassel.
The word maize derives from the Spanish form of the indigenous Taíno word for the plant, maiz.[12] It is known by other names around the world.
The word "corn" outside North America, Australia, and New Zealand refers to any cereal crop, its meaning understood to vary geographically to refer to the local staple.[13][14] In the United States,[13] Canada,[15] Australia, and New Zealand,[citation needed] corn primarily means maize; this usage started as a shortening of "Indian corn".[13] "Indian corn" primarily means maize (the staple grain of indigenous Americans), but can refer more specifically to multicolored "flint corn" used for decoration.[16]
In places outside North America, Australia, and New Zealand, corn often refers to maize in culinary contexts. The narrower meaning is usually indicated by some additional word, as in sweet corn, sweetcorn, corn on the cob, baby corn, the puffed confection known as popcorn and the breakfast cereal known as corn flakes.
In Southern Africa, maize is commonly called mielie (Afrikaans) or mealie (English),[17] words derived from the Portuguese word for maize, milho.[18]
Maize is preferred in formal, scientific, and international usage because it refers specifically to this one grain, unlike corn, which has a complex variety of meanings that vary by context and geographic region.[14] Maize is used by agricultural bodies and research institutes such as the FAO and CSIRO. National agricultural and industry associations often include the word maize in their name even in English-speaking countries where the local, informal word is something other than maize; for example, the Maize Association of Australia, the Indian Maize Development Association, the Kenya Maize Consortium and Maize Breeders Network, the National Maize Association of Nigeria, the Zimbabwe Seed Maize Association. However, in commodities trading, corn consistently refers to maize and not other grains.[citation needed]

Structure and physiology

The maize plant is often 3 m (10 ft) in height,[19] though some natural strains can grow 12 m (39 ft).[20] The stem is commonly composed of 20 internodes [21] of 18 cm (7.1 in) length.[22] A leaf grows from each node, which is generally 9 cm (4 in) in width and 120 cm (4 ft) in length.
Ears develop above a few of the leaves in the midsection of the plant, between the stem and leaf sheath, elongating by ~3 mm/day, to a length of 18 cm (7 in) [23] with 60 cm (24 in) being the maximum alleged in the subspecies.[24] They are female inflorescences, tightly enveloped by several layers of ear leaves commonly called husks. Certain varieties of maize have been bred to produce many additional developed ears. These are the source of the "baby corn" used as a vegetable in Asian cuisine.
The apex of the stem ends in the tassel, an inflorescence of male flowers. When the tassel is mature and conditions are suitably warm and dry, anthers on the tassel dehisce and release pollen. Maize pollen is anemophilous (dispersed by wind), and because of its large settling velocity, most pollen falls within a few meters of the tassel.
Elongated stigmas, called silks, emerge from the whorl of husk leaves at the end of the ear. They are often pale yellow and 18 cm (7 in) in length, like tufts of hair in appearance. At the end of each is a carpel, which may develop into a "kernel" if fertilized by a pollen grain. The pericarp of the fruit is fused with the seed coat referred to as "caryopsis", typical of the grasses, and the entire kernel is often referred to as the "seed". The cob 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 around a white, pithy substance, which forms the ear- maximum size of kernel in subspecies is reputedly 2.5 cm (1 in).[25] An ear commonly holds 600 kernels. They are of various colors: blackish, bluish-gray, purple, green, red, white and yellow. When ground into flour, maize yields more flour with much less bran than wheat does. It lacks the protein gluten of wheat and, therefore, makes baked goods with poor rising capability. A genetic variant that accumulates more sugar and less starch in the ear is consumed as a vegetable and is called sweet 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 the end of the growing season, the kernels dry out and become difficult to chew without cooking them tender first in boiling water.
Planting density affects multiple aspects of maize. Modern farming techniques in developed countries usually rely on dense planting, which produces one ear per stalk.[26] Stands of silage maize are yet denser,[27] and achieve a lower percentage of ears and more plant matter.
Maize is a facultative short-day plant [28] and flowers in a certain number of growing degree days > 10 °C (50 °F) in the environment to which it is adapted.[29] The magnitude of the influence that long nights have on the number of days that must pass before maize flowers is genetically prescribed[30] and regulated by the phytochrome system.[31] Photoperiodicity can be eccentric in tropical cultivars such that the long days characteristic of higher latitudes allow the plants to grow so tall that they do not have enough time to produce seed before being killed by frost. These attributes, however, may prove useful in using tropical maize for biofuels.[32]
Immature maize shoots accumulate a powerful antibiotic substance, 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA). 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.
Because of its shallow roots, maize is susceptible to droughts, intolerant of nutrient-deficient soils, and prone to be uprooted by severe winds.[33]
Zea mays "strawberry"—MHNT
Zea mays "Oaxacan Green" MHNT
Variegated maize ears
Multicolored corn kernels (CSIRO)
While yellow maizes derive their color from lutein and zeaxanthin, in red-colored maizes, the kernel coloration is due to anthocyanins and phlobaphenes. These latter substances are synthesized in the flavonoids synthetic pathway[34] from polymerisation of flavan-4-ols[35] by the expression of maize pericarp color1 (p1) gene[36] which encodes an R2R3 myb-like transcriptional activator[37] of the A1 gene encoding for the dihydroflavonol 4-reductase (reducing dihydroflavonols into flavan-4-ols)[38] while another gene (Suppressor of Pericarp Pigmentation 1 or SPP1) acts as a suppressor.[39] The p1 gene encodes an Myb-homologous transcriptional activator of genes required for biosynthesis of red phlobaphene pigments, while the P1-wr allele specifies colorless kernel pericarp and red cobs, and unstable factor for orange1 (Ufo1) modifies P1-wr expression to confer pigmentation in kernel pericarp, as well as vegetative tissues, which normally do not accumulate significant amounts of phlobaphene pigments.[36] The maize P gene encodes a Myb homolog that recognizes the sequence CCT/AACC, in sharp contrast with the C/TAACGG bound by vertebrate Myb proteins.[40]

Abnormal flowers

Sometimes in maize, inflorescences are found containing both male and female flowers, or hermaphrodite flowers.[41]


Exotic varieties of maize are collected to add genetic diversity when selectively breeding new domestic strains
Many forms of maize are used for food, sometimes classified as various subspecies related to the amount of starch each has:
  • Flour corn: Zea mays var. amylacea
  • Popcorn: Zea mays var. everta
  • Dent corn : Zea mays var. indentata
  • Flint corn: Zea mays var. indurata
  • Sweet corn: Zea mays var. saccharata and Zea mays var. rugosa
  • Waxy corn: Zea mays var. ceratina
  • Amylomaize: Zea mays
  • Pod corn: Zea mays var. tunicata Larrañaga ex A. St. Hil.
  • Striped maize: Zea mays var. japonica
This system has been replaced (though not entirely displaced) over the last 60 years by multivariable classifications based on ever more data. Agronomic data were 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 is a diploid with 20 chromosomes (n=10). The combined length of the chromosomes is 1500 cM. Some of the maize chromosomes have what are known as "chromosomal knobs": 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 validate her transposon theory of "jumping genes", for which she won the 1983 Nobel Prize in Physiology or Medicine. Maize is still an important model organism for genetics and developmental biology today.[42]
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, is a stock center of maize mutants. 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.[43]
In 2005, the US National Science Foundation (NSF), Department of Agriculture (USDA) and the Department of Energy (DOE) formed a consortium to sequence the B73 maize genome. The resulting DNA sequence data was deposited immediately into GenBank, a public repository for genome-sequence data. Sequences and genome annotations have also been made available throughout the project's lifetime at the project's official site.[44]
Primary sequencing of the maize genome was completed in 2008.[45] On November 20, 2009, the consortium published results of its sequencing effort in Science.[46] The genome, 85% of which is composed of transposons, was found to contain 32,540 genes (By comparison, the human genome contains about 2.9 billion bases and 26,000 genes). Much of the maize genome has been duplicated and reshuffled by helitrons—group of rolling circle transposons.[47]
According to a genetic study by Embrapa, corn cultivation was introduced in South America from Mexico, in two great waves: the first, 5000 years ago, spread through the Andes; the second, about 2000 years ago, through the lowlands of South America.[48]


Field of maize in Liechtenstein
Maize reproduces sexually each year. This randomly selects half the genes from a given plant to propagate to the next generation, meaning that desirable traits found in the crop (like high yield or good nutrition) can be lost in subsequent generations unless certain techniques are used.
Maize breeding in prehistory resulted in large plants producing large ears. Modern breeding began with individuals who selected highly productive varieties in their fields and then sold seed to other farmers. James L. Reid was one of the earliest and most successful developing Reid's Yellow Dent in the 1860s. These early efforts were based on mass selection. Later breeding efforts included ear to row selection, (C. G. Hopkins ca. 1896), hybrids made from selected inbred lines (G. H. Shull, 1909), and the highly successful double cross hybrids using 4 inbred lines (D. F. Jones ca. 1918, 1922). University supported breeding programs were especially important in developing and introducing modern hybrids. (Ref Jugenheimer Hybrid Maize Breeding and Seed Production pub. 1958) by the 1930s, companies such as Pioneer devoted to production of hybrid maize had begun to influence long term development. Internationally important seed banks such as International Maize and Wheat Improvement Center (CIMMYT) and the US bank at Maize Genetics Cooperation Stock Center University of Illinois at Urbana-Champaign maintain germplasm important for future crop development.
Since the 1940s the best strains of maize have been first-generation hybrids made from inbred strains that have been optimized for specific traits, such as yield, nutrition, drought, pest and disease tolerance. Both conventional cross-breeding and genetic modification have succeeded in increasing output and reducing the need for cropland, pesticides, water and fertilizer.[49]

Global maize program

CIMMYT operates a conventional breeding program to provide optimized strains. The program began in the 1980s. Hybrid seeds are distributed in Africa by the Drought Tolerant Maize for Africa project.[49]

Genetic modification

Main article: Transgenic maize
Genetically modified (GM) maize is one of the 25 GM crops grown commercially in 2011.[50] Grown since 1997 in the United States and Canada, 86% of the US maize crop was genetically modified in 2010[51] and 32% of the worldwide maize crop was GM in 2011.[52] As of 2011, Herbicide-tolerant maize varieties are grown in Argentina, Australia, Brazil, Canada, China, Colombia, El Salvador, the EU, Honduras, Japan, Korea, Malaysia, Mexico, New Zealand, Philippines, the Russian Federation, Singapore, South Africa, Taiwan, Thailand, and USA, and insect-resistant corn is grown in Argentina, Australia, Brazil, Canada, Chile, China, Colombia, Czech Republic, Egypt, the EU, Honduras, Japan, Korea, Malaysia, Mexico, Netherlands, New Zealand, Philippines, Romania, Russian Federation, South Africa, Switzerland, Taiwan, USA, and Uruguay.[53]
In September 2000, up to $50 million worth of food products were recalled due to contamination with Starlink genetically modified corn, which had been approved only for animal consumption and had not been approved for human consumption, and was subsequently withdrawn from the market.[54]
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