branes (23), interference with stomatal regulation of transpiration (31), and reduction of flow through the stem by either vascular plugging (4) or an increase in the viscosity of the xylem sap (5).The purpose of this investigation was to study the effect of the glycopeptide toxin, isolated from C. ullni cultures, on the water balance and stem conductance of elm seedlings in order to elucidate the possible mode of action of the toxin in elm. We also tested whether dextrans similar in molecular weights to the toxin can mimic its action.
MATERIALS AND METHODSCeratocystis ulmi (Buisman) C. Moreau, freshly isolated from infected Ulmus americana L. trees, was used in all experiments. Cultures were grown in liter amounts of medium (18) in Fernbach flasks at 23 C on a rotary shaker at 110 rpm. Cultures, harvested after 2 to 4 weeks, were filtered through Whatman No. 42 paper, then through a 0.45-,um Millipore filter and concentrated approximately 20-fold under vacuum at 40 C. One volume 95% (v/v) ethanol was added and the precipitate was discarded. The ethanol was then removed and the filtrate was concentrated another 2-fold using vacuum evaporation at 40 C. The concentrated filtrate was dialyzed against distilled water at 4 C for 24 hr with 2 changes of H20. To prevent microbial growth, 0.5% (v/v) chloroform was added to the H,O. The dialysate was discarded and 5 volumes of 95% (v/v) ethanol were added to the dialyzed solution. It was then placed in a freezer for at least 24 hr. The precipitate, i.e., the crude toxin, was collected by centrifugation at 10,000g for 10 min. The yield of crude toxin was approximately 100 mg liter-'. These methods were similar to those of Salemink et al. (18)