NEW ORLEANS -- For at least the past two summers, high amounts of ground-level ozone -- a pollutant commonly called "smog" -- have seriously retarded the growth of ozone-sensitive white clover in agricultural areas of Long Island, N.Y., according to a plant pathologist at Cornell University's Horticultural Research and Extension Center in Riverhead, N.Y.

The effects of the pollutant on the clover appear to be a warning of a wider threat.

"We need to pay attention to the amount of ground-level or ambient ozone," says Margaret McGrath, Cornell plant pathologist at the station. "There used to be a lot of spinach grown on Long Island. Not anymore. Spinach is very sensitive to ozone, which causes spotting on leaves and making them unmarketable. Acute injury routinely occurs on other Long Island crops, including grapes, pumpkins, watermelons and tomatoes. It's difficult to assess the impact on yield and injury to the plant's photosynthetic tissue. That's why scientists are developing and using indicator systems such as white clover."

McGrath is presenting her research in a poster, "Impact on White Clover of Ambient Ozone at Long Island, New York," on Aug. 15 and 16 at the annual meeting of the American Phytopathological Society at the Hyatt Regency Hotel in New Orleans.

Most ozone occurs in the stratosphere and is called "good ozone" because it provides a shield against the harmful effects of the sun's ultraviolet light. Ozone pollution occurs when abnormally high concentrations of the gas accumulate near the ground, the result of the reaction of sunlight with man-made precursor chemicals, including nitrogen oxides and volatile organic compounds.

The gas enters leaves through tiny openings called stomata. McGrath says that environmental, biological and cultural factors -- such as irrigation -- promote the stomatal openings and increases the risk of ozone injury to plants. She also says that ozone causes deleterious effects on plant photosynthesis, as well as the rate of plant production, flowering and yield.

While the processes are not well understood, ozone also can influence the incidence of pathogens and pests. In addition to its effect on plant shoots, ozone is known to hurt carbon flow to the root and, consequently, reduce root growth.

Studying the response of cloned ozone-sensitive and ozone-tolerant white clover plants, McGrath found that ground-ozone levels were greater than 80 parts per billion (ppb) for 121 hours in 1998, and 184 hours in 1999. The highest ozone levels were reached on June 26, 1998, when concentrations peaked at 129 ppb, and on June 7, 1999 when they reached 123 ppb.

In the 1998 research, the ozone-sensitive and ozone-tolerant white clover plants grew at a similar rate though the spring until just before the start of summer. After that the researchers noticed the ozone-sensitive clover experienced a 24 percent reduction in the growth rate through late summer. The researchers correlated the clover's stunted growth with the increased ground-level ozone.

In 1999, the ground-level ozone reduced the growth of the sensitive clover by 27 percent throughout the Long Island summer.

"On hot summer days, the air just hangs over the island. Combine that with the intense ultraviolet rays and the greater New York City pollutants in the air, and you have a recipe for a lot of high ozone days," says McGrath. "These results document that ozone is high enough to greatly reduce growth and yield of sensitive plants on Long Island, where important agriculture in New York is located."