Ray Fall

Professor (biochemistry). Ph.D., University of California, Los Angeles, 1970; National Research Council Fellow, 1987; Fellow of the Cooperative Institute for Research in Environmental Sciences.

Professor Fall's research program concerns biochemical reactions and pathways in plants and microorganisms that affect the earth's atmosphere. Current work centers around three questions concerning hydrocarbon emissions from vegetation to the atmosphere: What are the biochemical mechanisms for hydrocarbon formation? What is the physiological regulation of hydrocarbon synthesis and emission? And why do plants emit hydrocarbons to the atmosphere? The answers to these questions, obtained in laboratory and field experiments, will provide new understanding of the impact of plants on atmospheric chemistry.

Several different specific problems are under investigation. First, the role of plants in the production of tropospheric oxidants is of interest. It is puzzling that major forrest trees emit volatile hydrocarbons that result in the formation of ozone, a phytotoxin, in the boundary layer. The group is focusing on the biosynthesis and emission of isoprene (C5H8), the major nonmethane hydrocarbon in the atmosphere. Currently, it is not known why plants emit so much of their fixed carbon as isoprene. Typically, 1-2% of CO2 fixed in photosynthesis is emitted as isoprene, and globally it is estimated that 300-400 million tons of isoprene is emitted by forests each year. The laboratory is probing the metabolic regulation of isoprene synthesis in leaves and the linkage of isoprene synthesis to light-dependent reactions in the chloroplast. The researchers have recently discovered the enzyme isoprene synthase and are working to characterize it and the developmental regulation of the isoprene synthase gene. It now appears that the level of isoprene synthase activity in leaves regulates global isoprene emissions.

Second, Professor Fall's group has recently discovered that plants emit large amounts of oxygenated hydrocarbons, such as methanol and acetone, to the atmosphere. Virtually nothing is known about the biogenesis of these compounds in plants, and the group is working to establish the biochemical pathways leading to their production. Methanol production appears to result from demethylation of pectin in the cell walls of all plants, and the properties and regulation of the enzyme pectin methylesterase are of interest. The role of methylotrophic bacteria on plant surfaces as sinks for methanol is also being investigated; these bacterial grow on methanol as the sole carbon source and for some plants (e.g., soybean) they are the dominant bacterial species on leaves.

Third, various exploratory projects on biogenic emissions of formaldehyde, formic acid, acetaldehyde, ethanol and acetic acid are under way. To make such measurements the laboratory is developing sensitive enzymatic and fluorescence methods to measure these gas phase compounds. The figure diagrams the methods we are currently using to quantitate methanol flexes from leaves by gas chromatography and a specific enzymatic reaction.

Selected Publications

G.M. Silver and R. Fall, "Enzymatic Synthesis of Isoprene from Dimethylallyl Diphosphate in Aspen Leaf Extracts," Plant Physiol. 97, 1588-1591 (1991).

R. Fall and R. K. Monson. "Isoprene Emission Rate and Intercellular Isoprene Concentration as Influenced by Stomatal Distribution and Conductance," Plant Physiol. 100, 987-992 (1992).

J. Kuzma and R. Fall. "Leaf Isoprene Emission and the Level of Isoprene Synthase," PlantPhysiiol. 101, 435-440 (1993).

R. MacDonald and R. Fall. "Detection of Substantial Emissions of Methanol from Plants to the Atmosphere," Atmos. Environ. 27A, 1709-1713 (1993).

A. B. Guenther, P. R. Zimmerman, P. C. Harley, R. K. Monson, and R. Fall. "Isoprene and Monoterpene Emission Rate Variability: Model Evaluations and Sensitivity Analyses," J. Geophys. Res. 98, 12609-12617 (1993).

J. P. Greenberg, P. R. Zimmerman, B. E. Taylor, G. M. Silver, and R. Fall. "Sub-Parts per Billion Detection of Isoprene Using a Reduction Gas Detector with a Portable Gas Chromatograph," Atmos. Environ,. 27A, 2689-2692 (1993).