Zea mays (Maize, Corn)

Mielie [Afrikaans]

Life > eukaryotes > Archaeoplastida > Chloroplastida > Charophyta > Streptophytina > Plantae (land plants) > Tracheophyta (vascular plants) > Euphyllophyta > Lignophyta (woody plants) > Spermatophyta (seed plants) > Angiospermae (flowering plants) > Monocotyledons > Order: Poales > Family: Poaceae

Zea mays under cultivation. [photo H.G. Robertson, Iziko ©]

Maize was domesticated in Mexico about 7000 years ago and by the time Columbus arrived in the New World, there were already many varieties. It was introduced to Africa in the 16th century and over time came to replace sorghum as the staple food in all but the drier areas.

History of domestication

There are four wild species in the genus Zea, all of which are native to Mexico and northern Central America. One of these, Zea mexicana, commonly called teosinte, gave rise to maize Zea mays. Genetic evidence suggests that maize originated mainly from the Balsas race of teosinte which is found in the Balsas River basin in the Michoacan-Guerrero border region of western Mexico. Zea mays is thought to have speciated from Z. mexicana into a separate gene pool many thousands of years ago afterwhich it diversified into a number of different races.

Maize is similar to teosinte in that it is a quick growing annual with C₄-type photosynthesis, giving it the ability to grow well in bright sunlight with limited water, and it has unisexual inflorescences: the tassel (male) and the ear (female). 

Maize is different from teosinte in a number of features including: 

• loss of the hard case around the grain which in teosinte helps grain to survive going through an animal's digestive system;
• doubling and redoubling of the two rows of grain in the teosinte ear; and
• the maize ear is covered by husks, with elongated styles sticking out the tip of the ear for pollination.
• larger grain size, loss of dormancy and retention of ripe grain on the ear that does not shatter (all typical features of grass domestication)

Archaeological evidence from the Tehuacan caves in Puebla, Mexico, suggests that people were using Z. mays rather that Z. mexicana from about 5000 BC. The remains of Z. mays from these caves still bare quite a close resemblance to Z. mexicana in that the ears are small and slender and the grains are tiny and hard. However, the cobs were non-shattering and there were mostly eight row of kernels although there were a few four rowed types. They were probably used to produce popcorn. By the time Columbus arrived in the Americas, people had developed numerous forms of maize and were often growing them in close proximity to one another. Although maize is wind-pollinated, people were able to keep races genetically distinct because (1) different races were grown in different fields with forest in between; (2) pollen of the same race as the plant tends to grow down the long styles faster than pollen of different races; and (3) farmer can spot a cob with pollination by different races of pollen because grains are often differently coloured - cobs like this would be rejected for planting. 

Columbus brought maize grains back to the Spanish court, originating from the Greater Antilles in the Caribean, and these were grown in Spain in 1493. Basque companions of Pizarro brought maize grains back from Peru and introduced maize growing to the Pyrenees. Maize growing spread rapidly in Europe although only in southern Europe did it become a major crop. The popularity of maize in this region stemmed from the increased yield it provided over other spring crops such as wheat. It soon became the staple diet of poor people which led to malnutrition because maize is deficient in the amino acids lysine and niacin and white maize is deficient in carotene which is converted to Vitamin A. The disease pellagra became common, caused by a deficiency of niacin.

Maize was introduced to Africa in the 16th and 17th centuries and was readily accepted by African farmers, partly because it was grown and used in a similar way to their traditional crop of grain sorghum. Maize displaced sorghum as the staple grain in all but the drier regions. The Portuguese are thought to have introduced maize to Asian regions where it became widely grown but in most cases did not replace rice and wheat as the major crops. 

In North America, the Red Indian tribes were growing maize as far back as 200 AD, but it was only in the 19th Century, with the aid of draft animals and ploughs, that European settlers rapidly developed the prairie grasslands of the Eastern US into what is now referred to as the Cornbelt. It was in this region that new, higher yielding maize varieties were developed, some of which were adopted in other parts of the world.

Corn on the cob is cooked and eaten as a vegetable. [photo H. Robertson, Iziko ©]	Popcorn kernels. Popcorn is a variety of Zea mays with seeds that puff out into popcorn when heated. [photo H. Robertson, Iziko ©]

References

• Sauer, J.D. 1993. Historical geography of crop plants - a select roster. CRC Press, Boca Raton, Florida.