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Ragweed Research Is Nothing to Sneeze At

September 25, 2007
To a person with a pollen allergy, an 18-acre ragweed field sounds like a sneezy, red-eyed zone of misery. But to two environmental engineering researchers at Johns Hopkins, the parcel presented a rare and valuable opportunity to learn how the troublesome weeds grow, reproduce and scatter their pollen under varying weather conditions.

Their findings, gathered with a mix of high-tech and low-tech tools, could lead to better ways to track the pollen's travel and control the pesky plant's spread, discoveries that could aid the 15 million people with ragweed allergies in the United States and Canada alone. And although the plant is native to North America, the nuisance appears to be spreading. Researchers say the plant has invaded China, Japan and parts of Australia, and is now moving rapidly across Europe as well. To address this problem, the Johns Hopkins team is using data from the 18- acre field to develop a computer model of ragweed pollen behavior. The model also could someday help to predict the spread of bioengineered corn pollen before it contaminates natural crops.

Under the guidance of several faculty advisers, the ragweed research is being carried out by Mike Martin, 23, and Marcelo Chamecki, 29, two doctoral students in the Department of Geography and Environmental Engineering in the university's Whiting School of Engineering. At the onset of ragweed pollen season last year, the students set out to find a real-world lab site in which to collect data. Just outside of Washington, D.C., they stumbled upon an 18-acre piece of vacant land that was covered by a dense growth of the plant. With the property owner's permission, they set up camera and computer equipment, meteorological gauges and pollen-collecting instruments to gather information about ragweed. They have spent the past year analyzing these data and hope to publish some of their findings soon in a scientific journal. The research will also serve as the foundation for their doctoral theses.

Although neither of them is allergic to ragweed, the students know how easily it can trigger a response among those who are. "Concentrations of fewer than 10 pollen grains per cubic meter can cause an allergic reaction in people who are sensitive to ragweed," said Martin, who is from Lafayette, La. "During our field research, we found concentrations of 10,000 grains per cubic meter in the air above the plants. My clothes were stained yellow with pollen."

Although ragweed is not new to North America, other scientists have determined that it has spread significantly throughout the continent since the arrival of European settlers. The newcomers cleared shady forest areas to create farmland, enabling ragweed to flourish in the sunny new open spaces. "I'm trying to develop a detailed description of the recent evolution of ragweed populations," Martin said. "I want to know how the plant's structure and behavior have influenced its success as an invasive weed. If we can understand how ragweed was adapted to its prehistoric environment, we may find better ways to control its harmful effects in the present day by predicting when the pollen will be released and where it will end up."

His fellow researcher has somewhat different aims. "My main interest is learning how the wind spreads the pollen under different turbulence and temperature conditions," said Chamecki, who is from Curitiba, Brazil. "I want to use the data from our field experiments to develop and calibrate a computer model. This model could be used to predict how pollen grains are likely to spread under different topographic and atmospheric conditions. If the computer model works for ragweed, it should also work for other types of pollen and other tiny airborne particles and organisms like bacteria, soot and even snowflakes."

In the immense ragweed field, the graduate students gathered data by first marking off 25 randomly selected study sites, each measuring one square meter. Based on a survey of the plants in each of these samples, the students concluded that the field contained about 90 ragweed plants per square meter, a figure that included full-grown plants as well as seedlings. When they considered that one small plant is capable of releasing 1 billion grains of pollen per season, the young researchers realized that this single field probably caused a lot of suffering for allergic people living nearby.

The students knew that ragweed reproduces in the late summer and early fall when male flowers release pollen to fertilize female portions of the plant, which release seeds. To study this process, they aimed a video camera at male flowers, using a close-up lens. The camera captured microscopic images of pollen being released in clumps of about 500 grains each. The student researchers later were able to document the way such clumps begin to fall apart and disperse as they move through the air.

To find out how the airborne pollen concentration changes as the clumps move away from the plants, the researchers set up an 18-foot-tall pole equipped with six pollen samplers mounted at different heights. Each sampler spun a rod coated with sticky silicone to capture pollen clumps moving through the air. The students have been able to count and study the pollen grains by placing the samples under a microscope.

To document weather conditions at the time of each sampling, the students assembled a meteorological tower measuring 6 meters tall. The tower was equipped with instruments to measure solar radiation, air temperature, humidity, wind direction, wind speed and turbulence. This information was collected by a datalogger device and stored on a memory card that could be uploaded regularly to a laptop computer. The tower's devices were powered by a car battery that was recharged by solar cells.

The students left the ragweed field with two week's worth of pollen behavior data that they are continuing to organize and analyze, including 3,000 pollen samples and many gigabytes of computer files. Johns Hopkins faculty members involved in the project include professors Grace Brush and Marc Parlange, both professors in the Department of Geography and Environmental Engineering, and Charles Meneveau, a professor in the Mechanical Department of Mechanical Engineering. Brush said Martin and Chamecki's work is important because "it combines the biological characteristics of the plant and the mechanisms of pollen release and transport in the atmosphere, providing insight into the adaptation of the species to its environment."

The research was supported in part by funding from the National Science Foundation.

Johns Hopkins University


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