GRAND ISLAND, Neb. — To satisfy the world's growing demand for food, scientists are trying to pull off a genetic trick that nature itself has had trouble accomplishing in millions of years of evolution. They want to create varieties of corn, wheat and other crops that can thrive with little water.

As the world's population expands and global warming alters weather patterns, water shortages are expected to hold back efforts to grow more food. People drink only a quart or two of water every day, but the food they eat in a typical day, including plants and meat, requires 2,000 to 3,000 quarts to produce.

For companies that manage to get “more crop per drop,” the payoff could be huge, and scientists at many of the biggest agricultural companies are busy tweaking plant genes in search of the winning formula.

Monsanto, the biggest crop biotechnology company, says its first drought-tolerant corn will reach farmers in only four years and will provide a 10 percent increase in yields in states like Nebraska and Kansas that tend to get less rainfall than eastern parts of the Corn Belt.

At a recent farm show here called Husker Harvest Days, a few thousand farmers were guided past a small plot on which Monsanto had grown its drought-tolerant corn next to a similar variety without the “drought gene.” A transparent tent had shielded the plants from any rain through the hot Nebraska summer.

The results were, to be sure, less than miraculous. Both the drought-tolerant and the comparison plants were turning brown and shriveling, and they were about three feet shorter than the lush green irrigated corn growing nearby. But the drought-tolerant plants, which also contained a second gene to protect their roots from a pest, were a little greener and a few inches taller than the comparison plants, and their cobs were missing fewer kernels.

Monsanto said the improvement was significant. And the Nebraska and Kansas farmers who toured Monsanto’s plot, many of them facing water-use restrictions and soaring pumping costs for irrigation, said any improvement would be welcome.

“We pump water like there’s no end, and that’s not going to last forever,” said Tom Schuele, a farmer in Cedar Rapids, Neb. Monsanto’s competitors, including DuPont’s Pioneer Hi-Bred unit and Syngenta, say they also plan to introduce water-efficient corn in a few years. And companies are working on plants that can stand up to heat, cold, salty soils and other tough environments.

A small California company called Arcadia Biosciences is trying to develop crops that need only half as much nitrogen fertilizer as a conventional plant. Fertilizer is crucial to modern food production, but the large quantities used today damage the environment. And because fertilizer is made from natural gas, its costs have soared along with other energy costs.

Public sector scientists are also on the hunt. Researchers at the University of California and the International Rice Research Institute in the Philippines are developing rice that can survive flooding, which causes major crop losses for poor farmers in the lowlands of India and other countries. While rice is typically grown in standing water, the plants will die if submerged for more than a few days.

Many of these advanced crops are being developed using genetic engineering. The technology, already used to make crops that can resist weeds and insects, has spurred worldwide controversy. But in an era in which people are marching in the streets of many countries to demand more food at lower prices, low-water crops might win over areas that now shun biotech crops, such as most of Africa.

“Drought tolerance to me is the most critical entry point,” said Calestous Juma, a professor of international development at Harvard who has advised African governments on biotechnology. “This is kind of reopening the window for genetic modification.”

Critics accuse the biotechnology industry and its backers of exploiting the recent global food crisis to push a technology that has been oversold and that could have unanticipated health and environmental effects.

Indeed, many past predictions of how biotechnology would create novel crops have not come to fruition. And some experts say Monsanto and its peers have not published enough information to prove they can make drought-tolerant crops.

“I want to see more, I guess, from the Monsanto work before I’d be convinced they’ve got it,” said John S. Boyer, an emeritus professor at the University of Delaware.

Safety questions must also be answered. Changing the water needs of a plant requires a more fundamental alteration of its metabolism than adding a gene to make the plant resistant to insects. “The potential for unintended side effects is greater, so the testing has to be greater,” said David A. Lightfoot, a professor of genetics and genomics at Southern Illinois University.