But the potential was clear. Soon, by exposing large numbers of seeds and young plants, scientists produced many more mutations and found a few hidden beneficial ones. Peanuts got tougher hulls. Barley, oats and wheat got better yields. Black currants grew.

The process worked because the radiation had randomly mixed up the genetic material of the plants. The scientists could control the intensity of the radiation and thus the extent of the disturbance, but not the outcome. To know the repercussions, they had to plant the radiated material, let it grow and examine the results. Often, the gene scrambling killed the seeds and plants, or left them with odd mutations. But in a few instances, the process made beneficial traits.

In the 1950s and 1960s, the United States government promoted the method as part of its “atoms for peace” program and had notable successes. In 1960, disease heavily damaged the bean crop in Michigan — except for a promising new variety that had been made by radiation breeding. It and its offspring quickly replaced the old bean.

In the early 1970s, Dr. Rutger, then in Davis, Calif., fired gamma rays at rice. He and his colleagues found a semi-dwarf mutant that gave much higher yields, partly because it produced more grain. Its short size also meant it fell over less often, reducing spoilage. Known as Calrose 76, it was released publicly in 1976.

Today, Dr. Rutger said, about half the rice grown in California derives from this dwarf. Now retired in Woodland, Calif., he lives just a few miles from where the descendants grow, he said.

A similar story unfolded in Texas. In 1929, farmers stumbled on the Ruby Red grapefruit, a natural mutant. Its flesh eventually faded to pink, however, and scientists fired radiation to produce mutants of deeper color — Star Ruby, released in 1971, and Rio Red, released in 1985. The mutant offspring now account for about 75 percent of all grapefruit grown in Texas.

Though the innovations began in the United States, the method is now used mostly overseas, with Asia and Europe the leading regions. Experts cited two main reasons: domestic plant researchers over the decades have already made many, perhaps most of the easiest improvements that can be achieved with radiation, and they now focus on highly popular fields like gene splicing.