Grain Crops

Categories: Agriculture; economic botany and plant uses; food

Grains were the first domesticated crops and allowed the development of all of the great early civilizations. Several factors contribute to the importance of grains in agriculture: ease of growth, storage, and preparation; high yields; and high-energy, easily digestible content (starch). The wild relatives of cereal grains all disperse their seeds by the shattering, or breaking apart, of mature fruiting stalks. Harvesting these wild species is a problem because the seeds are flung everywhere when the fruiting stalk is disturbed. A first step in the domestication of all grains was the elimination of shattering so that inflorescences could be harvested. For grasses, such as wheat, that produce many stems, or tillers, arising from the base of the plant, selection led to synchrony in the production of the tillers so that all the inflorescences of a plant would set fruit at the same time. For grasses, such as corn, that had a thick main stem, selection led to the elimination of secondary branches and a concentration of seeds in one or a few large inflorescences. The second half of the twentieth century saw selection for shorter stature that allows grains to grow better in tropical regions. While thirty-five species of grasses have been domesticated, only five are major crops today: wheat, rice, corn, sorghum, and barley.

Wheat

Wheat is the most widely cultivated grain in the world and was among the earliest grains to be domesticated. Archaeological deposits from the Middle East, the native home of wheat, containing domesticated wheat seeds have been dated to ten thousand years ago. The first species of wheat domesticated was the diploid einkorn wheat (Triticum monococcum), soon followed by the tetraploid, free-threshing emmer wheat (T. turgidum), which made it easier for people to separate the fruits from the papery tissues, or chaff, in which they are enclosed. Today, emmer wheats are grown throughout the world and are especially well suited to making pasta and pastries. Bread wheat (T. aestivum) was the last to be domesticated. This species is a hexaploid, and its increased cell size has as an important secondary effect: the high production of proteins, known as glutens, that allow bread wheat to form an elastic dough that produces light, spongy bread.

While some wheat is eaten as a grain in such dishes as tabbouli, most wheat is used for flour: whole wheat if the bran and germ are ground along with the starchy endosperm and white if the bran and embryo are removed. Because white flour (even organic flour) lacks the vitamins present in whole wheat flour, federal law in the United States requires that it be enriched with five nutrients: riboflavin, niacin, folic acid, thiamin mononitrate, and iron.

Rice

The acreage devoted to rice (Oryza sativa) is less than that of wheat, but more rice is produced annually than wheat, and more people in the world depend on rice as their primary food. Rice was domesticated in the Yangtze River region of China, probably more than nine thousand years ago. In most of the world, rice is grown by germinating seeds and growing seedlings in a nursery. Seedlings are then planted by hand in fields covered with water. Rice does not need to grow in standing water, but it needs high rainfall if the fields are not irrigated. Because rice contains no gluten, it is not used for leavened bread, but it is well suited for cooking because the seeds retain their shape and have a soft, chewy consistency. The two major types of rice are long-grained Indica, preferred in India, and short-grained Japonica, preferred in China and Japan because of the sticky grains that adhere to one another upon cooking. Removing the bran produces white rice, which lacks the vitamins and fiber of brown rice. Consequently, rice is often enriched with vitamin B₁.

Corn

Corn (Zea mays) is the only major grain native to the New World. It was domesticated in southern Mexico about eight thousand years ago from an annual grass known as teosinte. Corn plants are monoecious (having both pistillate and staminate flowers on one plant), with male flowers forming the tassel on the top of a corn plant and the female flowers packed inside the ear. The silks of an ear are the styles, one leading to each kernel of corn. Much of the U.S. crop is used for animal feed, but a large portion is converted to cornstarch or corn syrup to be used in the brewing, paper making, and processed food industries. A by-product of the cornstarch industry is corn oil, extracted from the germs.

Sorghum

Sorghum (Sorghum bicolor) is native to sub-Saharan Africa, where it was domesticated by five thousand years ago. Grain sorghums are a major source of food for millions of people in Africa and India, but in the New World sweet sorghum (sorgo) is grown primarily for animal feed. The plants are robust, with modern cultivars having a single, thick stem bearing a mass of seeds at the apex. Sorghum is the most drought-tolerant of the major grains and is therefore widely grown in arid regions. In addition to the grain sorghums and sorgos, other varieties yield rough fiber for brushes and booms.

Barley

Barley (Hordeum vulgare) was probably the first grain to be domesticated; ancient cultivated fruits found in the Near East have been dated to 10,500 years ago. Initially, barley was preferred over wheat and was used to make flat breads, pastes, gruel, and beer. Barley became less popular after the domestication of emmer and then bread wheat. However, it has remained the grain of choice for brewing beer because of its superior flavor after malting. Malting consists of germinating the grain just enough for it to produce enzymes that break down the starch into simple sugars that yeast can then ferment.

Bibliography

Chapman, G. P., ed. Grass Evolution and Domestication. Cambridge, England: Cambridge University Press, 1992. An edited volume that treats grass evolution in general as well as the domestication of selected grains.

Iltis, H. H. “Homoeotic Sexual Translocations and the Origin of Maize (Zea mays, Poaceae): A New Look at an Old Problem.” Economic Botany 54 (2000): 7-42. A discussion of a possible manner in which modern corn arose from its closest ancestor.

Pringle, H. “The Slow Birth of Agriculture.” Science 282 (1998): 1446-1450. Covers the role of grains as agriculture emerged in various parts of the world.

Simpson, Beryl B., and Molly C. Ogorzaly. Economic Botany: Plants in Our World. 3d ed. Boston: McGraw-Hill, 2001. In this text, an entire chapter is devoted to grains. A lively and well-illustrated account of the major plant crops of the world.

Smartt, J. J., and N. W. Simmonds, eds. Evolution of Crop Plants. 2d ed. London: Longman Scientific & Technical, 1995. This volume provides an account of almost every crop species, with each article written by an authority on that species and its relatives.

Zohary, D., and M. Hopf. Domestication of Plants in the Old World. Oxford, England: Clarendon Press, 1993. Most cultivated plants were domesticated in the Old World, and Zohary and Hopf provide a summary of when, where, and how domestication of each occurred.