Week 6 (Unit 11)
Maize (Zea mays L.)
Maize morphology and reproduction
Maize (Zea mays L.) is a member of the grass family, Poaceae (Gramineae). In the USA, we call it 'corn'. It is an annual species that is cross-pollinated by wind. It is unique among the cereal crops because it is monoecious (it bears separate male and female inflorescences on the same plant). The main stem terminates in a tassel (male inflorescence), and the grain is borne on lateral shoots (ears).
Source: Missouri Botanical Gardens
Most commercial varieties of field corn produce a single ear. Ear shoots grow in the leaf axils below the top ear, but generally do not develop unless the top ear is removed, or the plants are widely spaced in the field. Sweet corn and popcorn varieties are often prolific (have more than one ear per plant).
To make artificial crosses in maize, you need to cover the silks, which are the styles of the female flowers, to prevent pollination by wind-borne pollen. Pollen can then be collected in a bag from the tassel of another plant and poured onto the silks. To develop an inbred line, you would obtain the pollen from the tassel of the same plant or a closely related plant. Because maize is an outcrossing species, inbreeding results in a reduction in height and vigor and often greater sensitivity to diseases, insects, and environmental stresses.
Maize is a large-seeded plant that can be rapidly multiplied. A single cob of maize can produce about 800 seeds, which would be enough to plant about 100 m2 of corn in the US. The ease of crossing maize and its potential for rapid multiplication make it a model plant for genetic research.
The Origin of Maize
The origin of maize has been a matter of controversy over the years. It is presently classified in the same species as the Mexican annual teosintes, which grow in the wild in Mexico and Central America:
All members of the species are diploid and can be intermated to produce fertile offspring. They all have ten pairs of chromosomes which are remarkably similar in genetic makeup.
Among the world's major cereal crops, maize has the highest yield potential. The record yield for corn, as it is known in the USA, is about 21,000 lbs/acre (370 bushels/acre or 23 t/ha). There is tremendous diversity in the maize crop - it can be grown in many different environments and is used for human consumption, animal feed, and industrial purposes.
Following the domestication of maize in Mexico, Native Americans took the crop to new environments in North and South America. Maize is sensitive to photoperiod (daylength). Tropical varieties require short days to trigger flowering - if they are planted in temperate areas with long summer days they will grow to great heights before they flower. Varieties that moved north and south with the Native Americans were selected over time for daylength insensitivity. Selection for adaptation to temperate climates was also required. Maize was the primary grain crop grown throughout the Americas by the time the Europeans arrived. It was rapidly adopted by the European colonists, and spread to Europe, Africa and Asia with the Spanish and Portugese explorers of the 16th and 17th centuries.
Today, the crop is produced from 50° latitude N to 40° S, at elevations ranging from 0 to 4,000 meters above sea level. Maize may mature in 60 to 330 days from planting and vary from 0.5 to 5 meters in height, depending on the variety and growing conditions. Maize is an extraordinarily diverse crop in terms of its morphology and geographic distribution. This is a consequence of its cross-pollinating nature and the genetic diversity of traditional open-pollinated varieties. Despite this diversity, some generalizations can be made about the optimum conditions for maize production. As a C4 plant, it maintains a high level of productivity in warm climates in comparison to temperate cereals. It cannot withstand freezing temperatures.
Ideal conditions for maize production:
Maize has relatively high requirements for water and nitrogen in comparison to other cereal crops. It is particularly sensitive to environmental stresses at flowering. Although some varieties are more drought tolerant than others, most varieties that experience a severe drought at flowering will have barren ears and low yields. For a good discussion on fertilizer requirements of maize in the midwest, see NebGuide G174 from the University of Nebraska.
Maize is third behind wheat and rice in terms of total area of world production. About 2/3 of this area is in developing countries, where maize is widely grown for direct human consumption. Average grain yield of maize worldwide is 4474 kg/ha (3992 lbs/acre). Due to its high yield potential, maize grain production is comparable to wheat and rice (ranks of the three major crops vary from year to year). Total annual production is about 630 million Mt (metric tons).
†Upland rice is grown on an additional 19 million ha and contributes ~24 million Mt to the world production of rice.
The USA is the leading producer of maize in the world yielding about 38% of the total or 229 million Metric tons (Mt) annually. China, Brazil, and Mexico are the 2nd, 3rd, and 4th leading producers, respectively.
In the USA, maize is grown primarily in the midWestern states, where the soils are deep and fertile and summer rainfall and temperature regimes are ideal for maize growth. In general, maize in the US is grown where it has the highest yield potential.
In the 1940's, Anderson and Cutler recognized the importance of characterizing the genetic diversity in maize, and defined a race as "a group of related individuals with enough characteristics in common to permit their recognition as a group." Morphological characteristics that were thought to reflect genetic relationships were used to make a preliminary classification of the races of maize in Mexico, Central and South America, and the USA. Based on these studies and additional molecular evidence, there are currently thought to be at least 42 races in 3 great racial complexes. In the USA, mixing of Northern Flint varieties with Southern Dent types by colonial farmers led to development of the Corn Belt Dents. These were the dominant open-pollinated varieties grown in the corn belt before hybrids were introduced. They also provided the source germplasm for nearly all of the hybrids that are now grown in the US and in most temperate areas of the world. Regrettably, most of the indigenous North American varieties of maize have been lost.
Hybrids have been widely adopted in some developing countries, but in other parts of the world, open-pollinated varieties are still widely grown. These varieties have often been selected for particular attributes and represent important sources of genetic diversity for maize.
Improvements attained through plant breeding
Nearly 100% of the maize area planted in the US is planted with hybrid varieties. The principle of inbreeding and hybridization was first proposed in 1908 by G.H. Shull. However, the inbreds at that time were very low yielding, and production of single-crosses was not commercially viable.
In 1922, D. F. Jones proposed using double-cross hybrids to facilitate seed production. The hybrid seed is borne on a high-yielding single-cross parent, which made the operation profitable, leading to development and marketing of commercial double crosses in the 1930’s. Hybrid vigor is only maintained if new seed is purchased every year. The seed produced by a hybrid should not be replanted.
The method used to generate double crosses
There are many disease and insect pests of maize, many of which are common to other cereal crops. The predominant species vary from one geographic location to another and depend on prevailing weather conditions. Disease and insect pressure are particularly high in tropical climates, where warm temperatures year-round and high relative humidity favor their reproduction.
The ear rot pathogen, Aspergillus flavus, is particularly problematic, because it can produce a mycotoxin called aflatoxin that is a health hazard to humans and animals.
For a good online reference for maize diseases in the temperate zone, see http://www.btny.purdue.edu/Extension/Pathology/CropDiseases/Corn
Prominent insect pests
Another important pest of maize that is becoming increasingly important in tropical Africa is the parasitic weed Striga spp. Striga is traditionally a pest on sorghum, but has adapted to maize as its production has expanded in the African savannas. Traditional farming systems relied on rotations with nonhost crops and long fallow periods, which kept the Striga problem at a tolerable level. As the human population in Africa increases and land use intensifies, fallow periods are reduced. Soils become degraded and prone to drought, which favors reproduction of Striga. Each Striga plant can produce thousands of seeds which persist for years in the soil. Annual losses in the savannas of Africa due to Striga are estimated at $7 billion (FAO).
Maize can be used for many purposes – every part of the plant has commercial value. Maize is an excellent source of carbohydrates. From a nutritional standpoint, it is somewhat deficient in the amino acids lysine and tryptophan. Grain legumes are a good source of these amino acids and can be used to complement maize in the diet. Quality Protein Maize (QPM) has been bred to be high in lysine. For more information about CIMMYT's work to promote adoption of QPM maize in developing countries see http://www.cimmyt.cgiar.org/research/maize/world_food_prize_qpm/qpm_wfp.htm
The great diversity that typifies maize germplasm is also evident in kernel characteristics. Kernels may be colorless (white) or solid yellow, red, blue or mottled in color.
Classification of maize based on endosperm characteristics
Endosperm characteristics are often controlled by one or a few genes. These traits have a tremendous impact on the suitability of varieties for various end-uses, but are not reliable indicators of genetic diversity among varieties.
The endosperm consists of starch granules embedded in a protein matrix. Flinty endosperm has a more rigid protein structure and is also higher in protein content than floury endosperm. Floury granules are also surrounded by a protein matrix, but the matrix is thinner and tends to rupture upon drying, leaving air pockets. With further drying, floury endosperm shrinks to some extent.
Flint - In flint varieties, the flinty endosperm forms a shell around the softer, floury endosperm. In Africa, these varieties are often preferred where the maize kernels are dehulled before they are processed into food products. They may also have an advantage where storage conditions are poor, as they may be more resistant to weevil attack.
Dent - In dent varieties, the sides of the kernel consist of flinty endosperm, but the inner core and top of the kernel are floury. When the kernel dries the floury endosperm shrinks, causing the characteristic dent on top. These varieties account for 73% of commercial production worldwide, and are used as livestock feed and for industrial purposes.
Floury - In floury varieties, the entire endosperm is floury. These types are often preferred where the entire maize kernel is ground for food uses, such as for tortillas. Ease of processing and high recovery of a fine flour are desirable characteristics.
Popcorn - This type has a small, spherical grain, with a hard, flinty endosperm shell. Moisture trapped in the core, floury starch expands upon heating and bursts through the hard shell, causing the kernel to pop.
Sweet - Sweet varieties have one or more genes which interfere with the conversion of sugar to starch. Most of the genes controlling sweetness are recessive, so production fields must be well isolated from normal (field) corn and other types of sweet corn (that have different recessive genes controlling sweetness). Fertilization by pollen from another variety could reduce quality.
In industrialized countries, an average of 76% of the maize produced is used for animal feed. In the US, about 10% of the maize area is used for silage.
Typical uses of maize grain in the USA
Food uses of maize
In developing countries, an average of 30% of maize produced is used for direct human consumption. These figures vary considerably from one region to another.
In subSaharan Africa, about 70% of the maize crop is used for human consumption. Green maize (fresh, on the cob) is an important food source early in the rainy season. The image on the right shows another common food use of maize in Nigeria, ‘Eko’, which is fermented, boiled and consumed with a spicy stew.
Industrial uses of maize
Maize is refined to generate a wide range of products including corn oil, sweeteners, corn starch, and ethanol. New bioproducts such as amino acids, antibiotics and degradable plastics are increasingly being synthesized using maize as a raw material.
Maize is wet-milled to separate the grain into components (starch, oil, protein and fiber) which are then converted into higher value products.
For more information about industrial processing of corn, see
Increased use of high fructose corn syrup (HFCS) as a sweetener, particularly in soft drinks, has led to claims that HFCS is a major contributor to the obesity epidemic in the US. The International Food Information Council has recently reviewed the science behind the controversy: http://ific.org/foodinsight/2004/ja/fructosefi404.cfm
For information about the potential of corn as a source of ethanol for fuel, see http://www.ontariocorn.org/ethahome.html
Cultural significance of maize
Maize was the primary energy source for the Mayan and Aztec Indian civilizations. In these and other Native American cultures, maize was revered as the “source of life”. Your assigned reading for this module provides a fascinating account of the history of maize in Mexico and the interrelationship of a crop and human society, which continues to this day.
Genetically modified maize
Maize continues to be at the forefront of the controversy surrounding the use of GMOs (genetically modified organisms).
Bt crops contain the gene from a soil bacterium, Bacillus thuringiensis (Bt). Corn varieties containing the Bt gene are able to produce a toxin that protects against the European corn borer and, to a lesser extent, the corn earworm, the southwestern corn borer, and the lesser cornstalk borer. The Environmental Protection Agency (EPA) approved Bt corn for use in August 1995. Its production peaked at about 26 percent of US corn acreage in 1999, but fell to 19 percent in 2000 and 2001.
Public concerns about GM maize
Herbicide Resistant Corn
For further information and a balanced discussion of the controversy
surrounding GMOs see htt://cls.casa.colostate.edu/TransgenicCrops/history.html
There will be three themes on the discussion board related to the use of Bt corn. One will be labelled "for", one will be "against", and the other will be "use with some restrictions". Add at least one argument to support any one of these positions that has not already been posted by one of your classmates.
Take the quiz on this Unit on the Blackboard.
Salvador, Ricardo J. Maize. Adaptation of an article published in “Encyclopedia of Mexico”, Fitzroy Dearborn Publishers, 1997. http://maize.agron.iastate.edu/maizearticle.html
Berner, D.K., J.G. Kling, and B.B. Singh. 1995. Striga research and control: a perspective from Africa. Plant Disease 79:652-660.
CIMMYT Maize Program. 2003. Maize Diseases: A Guide for Field Identification,
CIMMYT 1999/2000 Maize World Facts and Trends.
Corn Refiners Association. 2002. What is corn refining?
Doebley, John. 1994. Genetics and the morphological evolution of maize. pp. 66-77 In M. Freeling and V. Walbot (eds.) The maize handbook, New York : Springer-Verlag.
Doswell, C.R., R.L. Paliwal, and R.P. Cantrell. 1996. Maize in the Third World. Winrock Development-Oriented Literature Series. Westview Press, Boulder, CO. 268 p.
IITA research guides on maize (IRG 9, 33, 35).
Iowa State University. 2004. The Maize Page.
National Corn Growers Association Home Page.
Ontario Corn Producers' Association Home Page.
Purdue University, Department of Agronomy. 2004. KingCorn: The Corn
Ritchie et al. 1993. How a corn plant develops. Iowa State University
Cooperative Extension Service, Special Report No. 48.
Rooney, L. W. and S O. Serna-Saldívar. "Food Uses of Whole Corn and Dry-Milled Fractions." In Corn: Chemistry and Technology, ed. S. A. Watson and R. E. Ramstad. 399-429. St. Paul, MN: Amer. Assoc. Cereal Chemists, 1987.
Sprague, G.F. and J.W. Dudley. 1988. Corn and Corn Improvement. ASA, CSSA, and SSSA, Madison, WI.
University of Missouri. 2001. Interactive Maize Plant.
University of Nebraska production guides.
USDA/ARS. MaizeGDB: Maize Genetics and Genomics Database.