Physics professor Calvin Howell and his students are working with Duke biologists on an interdisciplinary project to discover more about how plants absorb and use carbon dioxide—a question of particular relevance as levels of carbon dioxide in our atmosphere continue to climb. Howell and his colleagues use positron emission tomography (PET) to track molecules of carbon dioxide—tagged with radioisotopes—as they are absorbed by leaves.
Howell, director of the Triangle Universities Nuclear Laboratory (TUNL) which is housed at Duke, says, “Physics brings two types of perspectives to the project. One is the experimental techniques intrinsic to nuclear physics. The other thing I hope physics will bring is an analytical and quantitative approach for modeling plants,” particularly biological processes involving the plant and the environment. PET has long been used in medicine, but its use with plants is relatively new. One of the advantages of PET is that it provides “movies” of biological processes rather than the “still” images of an X-ray or CAT scan that show mostly structural details. Howell and graduate student Matt Kiser, PhD ’08, worked closely with Duke biologist Chantal Reid to set up a system using the resources of TUNL and the Phytotron—a controlled-environment facility for plant research. “You have to produce radioisotopes that can be traced in plants, and you have to make it in a compound that the plant can use,” Howell says. In TUNL’s tandem accelerator, the scientists bombard nitrogen-14 with a beam of protons to produce carbon dioxide molecules made with radioactive carbon-11. The process also produces carbon monoxide and nitrogen, which need to be removed from the gaseous mixture. With help from the chemistry department’s Richard A. Palmer, Howell and Kiser designed a process to separate the gases by sending the mixture through copper tubing submerged in a nitrogen and ethanol bath. This “ethanol slushy” is cold enough to freeze the carbon dioxide but not the carbon monoxide or the nitrogen. The radioactive carbon dioxide is pumped to the Phytotron via underground pipes, where it travels in a closed loop to a single leaf. “We have to be very careful we don’t leak any radioactive gas out,” Howell says. “The weak point is the seal around the leaf. As a physicist, I just wanted to put a rubber seal on the leaf and clamp down hard, but that would damage the plant.” The biologists said Howell couldn’t use vacuum grease either, because it would be toxic to the plant. The solution turned out to be a low-grade silicon that’s the main component of silicon-based compounds used in medicines. The team has begun to quantify rates at which carbon is taken up by a leaf, and how long it takes for the carbon to travel to the plant’s stem and roots. Information about the timing of carbon allocation and transport processes will help the scientists connect observable physiological responses to the underlying cell biology, which in turn may illuminate how plants will respond to the ever-increasing amounts of carbon dioxide in the atmosphere. Howell is working with a UNC student to refine the system so that it can be controlled remotely by computer. “We want to be able to point and click instead of having to adjust controls manually,” Howell says. “It reduces our exposure to radiation and makes the system more user-friendly for the biologists.” TUNL is also collaborating with the Jefferson National Lab on a project funded by the Department of Energy to develop techniques to improve the quality and resolution of PET images of plants. In addition to learning more about how plants use carbon dioxide, Howell would like to investigate how, and how fast, plants respond when insects attack their leaves. “With the temporal component of PET, we can see how long it takes the plant to respond,” Howell says, “and by using known fluid-flow analysis we can tell whether the response was active or passive, due to a concentration gradient.” Howell has enjoyed branching out into biology and chemistry for this project. “Whenever you try to do something that requires experience and knowledge that’s not specific to your discipline, you have to not be shy and call up people and ask questions,” he says. “Everyone I’ve talked to at Duke about this research has been enthusiastically helpful—this is a wonderful place for pursuing cross-disciplinary research.” Mary-Russell Roberson is a freelance science writer who lives in Durham.