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Plants see hormone as toxic pollutant.

Ethylene gas, a by-product of incomplete combustion, is a pollutant associated not only with industrial manufacturing but also with urban automotive exhaust. Ironically, the simple hydrocarbon is also a natural plant hormone. Because ethylene can encourage premature ripening, it is used agriculturally to make an entire crop ripen at once for a single harvesting or to put the color in early-picked fruit.

What George E. Taylor and his colleagues at Oak Ridge (Tenn.) National Laboratory were curious to find out was whether plants respond to chronic low-dose exposures of this chemicalas though it were a toxic pollutant -- regardless of any subtle, slow-acting hormonal action that might also be occurring. Their experiments with several important crops now suggest that some plants indeed respond immediately and adversely to ethylene gas.

The researchers worked with corn, soy-beans, peanuts, tobacco and seedlings of the green ash tree. Except for corn, which for unknown reasons showed no adverse reaction to any concentration of ethylene, the plants responded in a dose-dependent fashion by reducing both their photosynthesis and respiration.

These changes, says Taylor, suggest that the site of ethylene action is the stomata--the little openings on the underside of leaves through which gas exchange occurs. However, since stomata changes were not noticed in all of their adversely affected plants, Taylor says a closing down of stomata apertures may be secondary to changes in a more fundamental mechanism regulating photosynthesis, involving an unhealthy buildup of carbon dioxide concentrations in leaves.

Writing in the May ENVIRONMENTAL SCIENCE AND TECHNOLOGY, Taylor and co-workers report that after three- to six-hour exposures to ethylene at concentrations of 3.7 parts per million (ppm) in air, the photosynthetic rate fell in some of the more sensitive species, including soy-beans and peanuts, by more than 60 percent. A mere 0.9-ppm exposure cut their photosynthesis by almost a third. And such effects occur very rapidly, Taylor says, sometimes "in less than half an hour."

Although only a handful of published studies have quanitified ethylene concentrations in air, Taylor says, some of the reported values do approach levels that affected plants in his study. Moreover, ethylene levels tend to peak when photosynthetic activity is at its highest. If decreases in photosynthesis occurred for prolonged periods, plant growth and crop yields would be stunted, he says.
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Title Annotation:ethylene
Author:Raloff, Janet
Publication:Science News
Date:May 18, 1985
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