Effect of osmotic and matrix potential on Saprolegnia diclina and S. ferax

Growth of biomass and sporulation of pathogenic and non-pathogenic Saprolegnia species was markedly decreased at reduced water potentials. Oogonium and zoosporangium formation were more sensitive to reduced osmotic and matrix potentials than growth in biomass. Although little difference was observed between the effects of matrix and osmotic potentials, the Saprolegnia species investigated responded differently to those solutes utilized in control of osmotic potential. Biomas, oogonium and zoosporangium formation were greater in the presence of reduced osmotic potentials mediated by mannitol than equivalent potentials mediated by potassium chloride. Endogenous potassium levels varied little with reduced matrix or osmotic potentials. Conversly, mannitol content of colonies exposed to reduced osmotic potentials mediated by mannitol initailly increased while endogenous amino acid levels were observed to rise in response to moderately reduced water potentials. Sensitivity of Saprolegnia species to reduced potantials and effects on substrate colonization are discussed in the light of these observations.