POSSIBLE MECHANISMS OF INJURY TO SUGAR-BEET SEEDLINGS BY DAMPING-OFF PATHOGENS

Potassium-deficient seedlings of disease-resistant and susceptible lin es of sugar beets (Beta vulgaris L. altissima D.C.) were treated with a fungal-pathogen complex causing the development of damping-off; the effects of the pathogen on K+ uptake, cell membrane potential, and the fatty acid composition of root lipids were examined. The day after th e inoculation, potassium absorption by roots and membrane potentials o f root cells were strongly suppressed in susceptible seedlings. At the same time, the roots of these seedlings retained high membrane permea bility to K+ and showed no destructive changes in the fatty acid compo sition of root lipids. These results suggest that the pathogen-induced decrease in K+ uptake, concomitant with cell depolarization, is due t o a suppression of electrogenic ion transport and the electrogenic com ponent of membrane potential. Possible mechanisms of damaging effects produced by pathogenic fungi on sugar beet seedlings are discussed.