Plant hormone: key to ozone toxicity?

Plant hormone: Key to ozone toxicity?

Ozone, a photochemical oxidant insmog, is considered by the government's National Crop Loss Assessment Network to account for about 90 percent of U.S. crop losses from air pollution. According to Walter W. Heck, a scientist with the Agricultural Research Service in Raleigh, N.C., which oversees the network, the pollutant's economic toll on U.S. corn, soybean, wheat and cotton producers is estimated at between $1 billion and $5 billion annually. What has remained a mystery is how the pollutant exacts its toll on plants.

Now a pair of biologists in Englandreport stumbling onto what may be an important clue: that dramatically increased production of a hormone, in response to stress, appears to increase a plant's ozone vulnerability. Moreover, the hormone-triggering stress in this case was a one-shot dose of ozone; long-term ozone exposure actually had the opposite effect on young seedlings, reducing hormone levels.

The researchers found that pea seedlingsexposed to between 50 and 150 parts per billion of ozone for seven hours daily throughout their first three weeks of growth showed no visible leaf injury. However, when seedlings grown in the absence of ozone for three weeks were given just one similar seven-hour exposure on day 21, they immediately developed severe leaf-tissue death.

The researchers, Horst Mehlhorn andAlan R. Wellburn of the University of Lancaster, wondered why the two sets of plants were responding so differently. They measured the plants' production of the hormone ethylene and discovered that the single-exposure group produced double the amount of ethylene produced by unstressed (control) plants. Plants in the three-week-exposure group, on the other hand, seemed to combat the ozone by producing 92 percent less ethylene than the controls.

"These two quite different responses toozone suggest that the rate of ethylene production may have an influence on and modify the extent of visible leaf injury caused by ozone,' the researchers write in the June 6 NATURE. As a further test of ethylene's role, they pretreated plants with an ethylene inhibitor on the day before the 21-day-old seedlings' single seven-hour ozone exposure. Not only did the treatment reduce by 85 percent the plants' ethylene production during ozone treatment, but it "also almost abolished the visible leaf injury normally caused by this short ozone fumigation,' they report.

Mehlhorn says it's not clear how or whythe seedlings exposed to three weeks of ozone reduce their ethylene production. But from the study, he says, this accommodation appears to be permanent.

"Since they both perform similar functionsin terms of making [leaf] membranes leaky, it seems both reasonable and interesting' that ozone and ethylene could interact, according to botanist Joseph Sullivan of the University of Maryland in College Park. Heck says he is unconvinced, largely because his research with other plants indicates that many days of exposure produce more damage than one-day exposures.