Ozone resistance sought in crop varieties to combat pollution damage Scientists completed field re­search this summer that could eventually help farmers select crop seeds that are less suscepti­ble to damage from ground-level ozone. Studies have shown that crops exposed to excessive ozone levels generally produce yields of at least 10% below those grown in clean-air regions.

Because much of the East faces elevated ozone levels for the foreseeable future, scientists have been studying plant physiology to better understand how farmers can compensate for the crop damage now being inflicted by the pervasive pollutant. One goal is the identification of hardy plants that resist ozone damage.

"Over the past decade, there have been more than 50 scientific studies showing ozone's signifi­cant adverse impacts on yields of wheat, corn, barley, beans, soy­beans, cotton, and tomatoes," EPA Administrator Carol Browner told the Senate agriculture committee on July 22. The agency has estimated that crop damage from ozone approaches $3 billion annually.

At the U.S. Department of Ag­riculture's (USDA) research center in Beltsville, Md., scientists say the study of physiology has helped them identify seed varieties

that are resistant to ozone dam­age. "Essentially, we are trying to determine what differentiates sensitivity to ozone," said Steven Britz, head of USDA's Climate Stress Laboratory.

The findings should ultimately help farmers select seeds that are less susceptible to ozone damage, Britz said. Yields in eastern states with high ozone levels are consis­tently lower than those found in midwestern states with cleaner air. In 1995, for example, the highest soybean yields in the

ozone-affected states of Maryland and New Jersey were 24 bushels per acre. Although there are other variables such as soil quality and weather, midwestern yields were consistently higher—notably in Iowa and Wisconsin, where aver­ages were 43 bushels per acre, according to USDA statistics.

The research, begun in 1995 and completed this summer, in­volved two popular but geneti­cally different varieties of soybean seeds, each grown under field conditions and exposed to differ­ent ozone levels. Researchers used eight 10-foot-diameter cylin­drical chambers, covered with plastic and open at the top, to control ozone levels for each group of 100 plants by using air pumps connected to the base of each cylinder. Some plants were exposed to ozone between 10% and 20% above ambient levels while charcoal filters were used to purify the air for other plants. Vis­ible damage in the form of leaf discoloration was evident even at 60 parts per billion (ppb), a level below the current 80-ppb limit stipulated by the Clean Air Act.

The approach differed from earlier research by Britz and others

because it exposed plants to ozone levels that were roughly equivalent to the expo­sures occurring on farms throughout the East. Earlier, assuming that biological/bio­chemical effects would be more pronounced and easier to identify, scientists had exposed plants to extremely high concentrations of ozone.

Britz, a plant physi­ologist, and his col­leagues decided that plants react differently to low-level ozone ex­posure than to unreal-istically high levels. "We were concerned that responses observed at high-end treatments may not be reflective of physiology at ozone levels near ambient. We think our approach will give us much better

data," he said. The soybeans were harvested in August.

"We are finding that significant morphological effects—changes in leaf growth and development —as well as reductions in yield in sensitive lines of soybean can be observed under conditions in which photosynthesis and photo-synthetic carbon metabolism are not affected," Britz said. "The bot­tom line is, ozone affected growth but not photosynthesis."

Although they are still discuss­ing the implications of this sum­mer's research, Britz and his col­leagues were surprised that levels of ascorbic acid remained con­stant in all varieties. The findings are significant, because research­ers previously thought that ascor­bic acid may have actually helped repair ozone damage. If so, resis­tant varieties would have had higher levels of ascorbic acid than nonresistant seeds. But Britz and others now believe that the key is the ratio of ascorbic acid to oxi­dized dehydroascorbic acid rather than the level of ascorbic acid alone. The resistant soybean vari­eties all had high ratios between the two types of acids. —RAE TYSON

EPA has estimated that U.S. crop damage from ozone approaches $3 billion a year.

Scientists at the U.S. Department of Agriculture's Belts­ville, Md., research center are studying the effects of ozone on crops. Plants are grown in special cylinders to allow control of ozone exposure levels. (Courtesy USDA)

5 0 8 A • VOL. 31, NO. 11, 1997 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS