Kimani Mustafa, Peter Hartel, and Bruce Haines are planning a project to identify mutant plants that do not release the toxic gas, carbon disulfide (CS2) from their roots. To discover the concentrations of CS2 at the plant root surface that inhibit or kill various soil inhabiting pathogens, they need to quantify the growth and survival of these pathogens exposed to CS2-emitting roots (experimental group) versus those exposed to roots that do not emit CS2 (control group).

Why are Mustafa, Haines, and Hartel interested in CS2 emissions from plants? Some years ago, Haines was quantifying emissions of the sulfur gases dimethyl sulfide and ethyl mercaptan from leaf litter and soils in a rain forest in Costa Rica. Laurie McHargue, a scientist from the University of Miami, found that some of Haines' reference sulfur gas standards smelled like many of the legume plants she had in her living plant collection. (McHargue was studying nitrogen-fixing microbes associated with roots of legumes.) Most of these emissions were coming from plants in the subfamily Mimosoideae. Haines determined that CS2 was the principal sulfur gas. Emissions were stronger from roots than from leaves, stems, seeds, or wood. How do CS2 emissions help the plant? Could they help other plants? It was while discussing these questions with Hartel that the proverbial lightbulb went off for both scientists. "My expertise is in plant-soil-microbe interactions, and we are always looking for the effects of plants on the soil microorganisms," said Hartel. "I was looking for a plant that did something, and then I heard Haines talking about the stinky plants. I thought, now there's a plant that does something!"

Hartel's interests include both basic and applied science. The interaction of sulfur and soil has always interested researchers. Could the sulfur-emitting properties of these rain forest plants be harnessed to help agricultural crops? CS2 is known to inhibit root rot fungi, nematodes, insects, and nitrifying bacteria. Applying CS2 directly to the soil with a tractor is not practical because the compound is explosive. There were stories about tractors catching fire. Furthermore, one commonly used fumigant in agriculture is methyl bromide. It is toxic to humans and its use may contribute to loss of stratospheric ozone. The use of methyl bromide will soon be banned. Suddenly, a need for natural alternatives to this farm chemical is important. CS2, only this time from plants, is a possibility.

What is next? While Stryphnodendron excelsum (see photo) is a strong CS2 emitter, its size, reaching 30 m high when mature, makes it impractical in a search for mutants. Brassica rapa (Cruciferae), a sometime weak CS2 emitter, is too small to be photographed with three people looking at it, but can be grown in large numbers to be screened for mutants. In collaboration with Russell Malmberg, Brassica seeds have been exposed to a mutagen. With CS2- and non-CS2-emitting Brassica plants, we can explore the effects of CS2 on pests, pathogens, and nitrifiers living in the soil. Someday Mustafa may discover which native African sulfur-emitting plants can be grown in African agriculture to protect crops against pests, pathogens, and nitrogen losses.