Is ozone giving acid rain a bad name.

Is ozone giving acid rain a bad name?

Ozone, the most plant-damaging gaseous pollutant, decreases photosynthesis and promotes premature leaf aging, a new study reports. The study also suggests that ambient levels of this pollutant, in all but high-elevation areas, may account for much of the U.S. forest damage previously attributed to acid rain.

The study, conducted by researchers at the Boyce Thompson Institute at Cornell University in Ithaca, N.Y., focused on measuring how rates of photosynthesis changed among crop plants (soybeans, wheat and clover) and trees (white pine, hybrid poplar, sugar maple and red oak) exposed to different levels of ozone. Pollutant levels, from 0.02 to 0.14 parts per million (ppm) in air, were "realistic'-- characteristic of mean, daylight concentrations actually observed in regions ranging from pristine areas to agricultural regions of the central United States to heavily polluted southern California. Plants were fumigated, either in the field or in controlled laboratory chambers.

Writing in the Nov. 1 SCIENCE, Peter Reich and Robert Amundson report that the ozone vulnerability of a plant species seems to be related to the rate at which gases can enter its leaves--a factor determined by their pores, called stoma. Species with high rates of growth and photosynthesis, such as crop plants, tend to have larger stomal openings--and therefore greater ozone uptake--explains Reich, who is now at the University of Wisconsin in Madison.

In the study, ozone-related declines in photosynthesis occurred among all species and at all concentrations. The rate of damage, however, was unique to each: Clover, wheat and soybeans were most vulnerable; red oak and white pine were least so. For instance, an internal ozone dose of 10 ppm-hr (ppm concentration multiplied by exposure time) brought a 50 percent reduction in wheat photosynthesis and a near 50 percent decline in yield. By contrast, a threefold higher dose to white pine brought only a 10 percent drop in photosynthesis and growth or yield. Finally, although there were no visible signs of acute ozone poisoning (mottled discoloration) in exposed leaves, the time it took a leaf to mature, discolor and drop decreased as ozone exposure increased --suggesting, Reich says, that the pollutant accelerates leaf aging.

When the tests were repeated using water with a pH comparable to that of acid rain, there was no additional decrease in photosynthesis, acceleration in leaf aging or change in plant growth and yield.

These findings came as no surprise to Allen Heagle, a plant pathologist in Raleigh, N.C., who is involved with the four-state, four-year-old National Crop Loss Assessment Network, the nation's largest program studying ozone's effects on plants. "Ozone is clearly the bad guy here,' Heagle says. What's more, he says, "In everything we've done with crops here, we find that at ambient levels ozone is much more of a factor [than acid rain].' The big unknown, he says, is how badly ozone is hurting trees, since "there are no studies that have looked at the long-term effects of ozone on trees.'

Curbing ozone will be no easy trick, Reich and Heagle point out, since the largest source of the pollutant's chemical precursors is auto exhaust.