While it has long been known that plants absorb carbon dioxide for use in photosynthesis, a new study has found that plants play a larger role than previously thought in absorbing certain types of air pollutants from the atmosphere.
A study led by Thomas Karl, a scientist with the National Center for Atmospheric Research in Colorado, used direct measurements, gene expression studies, and computer modeling to show that deciduous plants absorb about a third more oxygenated volatile organic compounds (oVOCs) than were previously accounted for in studies of atmospheric chemistry. These compounds form in abundance in the atmosphere, primarily from automobile and industrial emissions of hydrocarbons and other chemicals, and they can have long-term effects on the environment and human health.
According to Karl, plants are highly efficient at absorbing these pollutants and using enzymes to convert them to compounds the plants can use. By measuring oVOC levels in various ecosystems around the world, Karl and his research partners determined that deciduous plants absorb the pollutants as much as four times faster, in particular cases, than previously believed. Conifers absorb about half the amount of pollutants absorbed by deciduous trees.
Karl said that all deciduous plant leaves absorb pollutants at similar rates, so trees or forests with more leaves absorb more pollutants. “The uptake rates seem to follow leaf area index,” he said. “The more leaf area, the more uptake you have.” The absorption of the pollutants was most evident in the forest canopy, which in some forests accounted for 97 percent of the total oVOC uptake they measured. The researchers then conducted a series of laboratory experiments on a poplar hybrid and found that when the tree was under stress due to a physical wound or exposure to an irritant like ozone pollution, its uptake of oVOCs increased sharply.
“We saw that when you stressed a plant, the plant started producing more of the enzymes associated with stress and cell repair,” Karl said. “The biochemical machinery of the plant was able to sense what it needed to get rid of [the compound that was causing the stress]. Over the long run, we saw that the plant was able to chew up more of the pollutant because the enzymes start working just a couple hours after [being exposed]. It’s a central part of the plant’s ability to detoxify.”
“Our results show that plants can adjust their metabolism and increase their uptake of atmospheric chemicals in response to various types of stress,” added Chhandak Basu of the University of Northern Colorado, a collaborator with Karl on the project. “This complex metabolic process within plants has the side effect of cleaning our atmosphere.”