Plant 92

Zea mays L. (Poaceae)



Golden-yellow maize cobs and kernels crushed and toasted for cornflakes or exploded for popcorn are familiar foods, whilst purified maize starch and syrup are frequent food additives. Yet when maize first arrived in the Britain, the sixteenth-century English herbalist John Gerard considered it 'a more conuenient foode for swine than for men'.

As the first explorers of the Americas island-hopped through the Caribbean and marched across Central and South America they found maize being consumed and used everywhere. In the Caribbean, the now-extinct Taino people called the plant mahiz. Traditional Latin American maize cobs are tremendously diverse in kernel colours and in the size and number of cobs per plant. In contrast, modern maize cultivars usually produce one or two large, yellow cobs per plant.

Maize is part of a small group of usually annual, Mexican and Central American grasses. Maize has only ever been found associated with humans; it is unknown in the wild. Furthermore, it looks very different to all other American grasses, and consequently close wild relatives are not obvious. Maize is one of a handful of crops whose widespread Old World distribution occurred after 1492. References to pre-1492 Old World maize are based on forged documents or misinterpreted manuscripts. In the Old World, maize has never been recovered from archaeological sites and there are no close maize relatives.

A chronological sequence of thousands of cob fragments was discovered in the caves of the Tehuacan Valley, southwestern Mexico. Some of the oldest cobs are little more than the length of the first joint of one's thumb, with eight rows of six to nine kernels. Significantly, each kernel is partially enclosed by a husk-like sheath.

The sole ancestor of maize is the Mexican grass teosinte. Flowerless, teosinte looks like branched maize but when the kernels mature, the differences are stark. Teosinte kernels are arranged in two-sided ears, with single kernels surrounded by hard shells, which break apart. In contrast, maize kernels are arranged in multiple-sided cobs of soft, paired kernels, which remain attached to the cob. Maize cannot disperse its kernels naturally but teosinte kernels are specialised for animal dispersal. Modification of three genes transforms teosinte into maize.

Maize took 400 years to cover the Old World; it took about 9,000 years to cover the New World. Today, hundreds of millions of tonnes of kernels are produced annually, mainly for the production of animal feed and, controversially, biofuels.

Further reading

Doebley J 2004. The genetics of maize evolution. Annual Review of Genetics 38: 37-59.

Jaenicke-Després, V et al. 2003. Early allelic selection in maize as revealed by ancient DNA. Science 302: 1206-1208.

Staller JE 2010. Maize cobs and cultures: history of Zea mays L. Springer-Verlag.

Stephen Harris