ABSTRACT: The effect of toxin preparations from culture filtrates of Alternaria solani (TPA) on the growth of cell suspensions of A. solani host and non-host plants was examined. Cells of host species were more sensitive to TPA than cells of non-host species. Both the inoculum density and the stage of growth of cells used for inoculum affected the tolerance of the cells to TPA. The inhibitory effects of TPA on cell growth are reproducible when the cells are obtained from a defined stage of growth and are inoculated at a predetermined inoculum density. Thus cultured cells provide an accurate assay for quantification of TPA phytotoxicity. This standardized growth assay was used to measure changes in production of phytotoxic activity of TPA during the growth of A. solani cultures. Results indicated that maximum production of extracellular phytotoxic activity occurred in the early log phase of fungal growth. The growth assay was used also to measure changes in tolerance of potato cells to TPA after selection. By repeated exposure to increasing amounts of TPA, potato cells became successively more tolerant to TPA. At least a 4-fold increase in tolerance was observed.
Physiological Plant Pathology.