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CASE 6 AGRICULTURE: FEEDING A GROWING POPULATION

Human civilization and plant reproduction are closely intertwined. Not only do seeds and fruits make up a significant portion of our diet, the direct manipulation of plant reproduction plays a critical role in agriculture. Plant breeding began as the artificial selection for plants with seeds that were easy to harvest and has today become a highly quantitative field in which controlled crosses between plants are used to combine favorable traits into a single variety. Because many of the traits that affect crop productivity are the product of multiple genes, crop breeding remains a powerful tool for increasing crop yields. The spectacular increases in productivity that are often referred to as the Green Revolution resulted, in part, from controlling the movement of pollen between plants.

Norman Borlaug, an American agronomist, has been called “the father of the Green Revolution” and “the man who saved a billion lives.” In the mid-twentieth century, Borlaug led a project to make Mexico self-sufficient in bread wheat. Borlaug’s aim was to breed wheat that could resist infection by a devastating class of fungal pathogens known as rusts. To do this, he and his group hand-pollinated plants that exhibited resistance to rusts with pollen from plants that grew well under field conditions or had seeds that produced good flour.

Under natural conditions, wheat usually self-pollinates. To introduce new sources of pollen, Borlaug and his coworkers removed the anthers of each flower with tweezers before the flowers opened. They had to cover each flower to prevent unwanted pollen from coming into contact with the stigma. Finally, when the stigma was receptive, they dusted it with pollen collected from a plant with the desired characteristics. Under the hot sun, this is backbreaking work, but over a series of years the pollinations carried out by Borlaug and his coworkers achieved their goal of breeding wheat varieties that were disease resistant.

Yet another problem remained to be solved. To obtain high yields requires large amounts of fertilizer (Chapter 29), but under these conditions, the new wheat varieties produced so many seeds that stalks became top heavy, falling over in the wind. Borlaug knew that an answer to this problem was to cross his plants with wheat varieties that had shorter, sturdier stems. But given the selective advantage of height in natural populations, where was he to find such a plant? Japanese wheat breeders had identified and preserved a mutant dwarf plant that had arisen spontaneously, and Borlaug obtained seeds of this dwarf variety.

Much hard work and many thousands of hand pollinations lay ahead. But by 1963, more than 95% of Mexican wheat cultivation made use of Borlaug’s high-yielding semidwarf wheat varieties, resulting in wheat yields six times greater than in 1944, the year he began work in Mexico. In 1965, the seeds were exported in large numbers, first to India and Pakistan and soon to the rest of the world. Used in combination with greater investments in fertilizer and irrigation, these varieties prevented the famines that had been predicted by many as a result of rising human populations. For his work in alleviating world hunger, Borlaug was awarded the Nobel Peace Prize in 1970.

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