Impact of osmotic and matric water stress on germination, growth, mycelial water potentials and endogenous accumulation of sugars and sugar alcohols in Fusarium graminearum.

Studies were conducted to determine the effect of osmotic (NaCl, glycerol) and matric (PEG 8000) water stress on temporal germination and growth of two F. graminearum strains over the water potential range of -0.7 to -14.0 MPa at 15 and 25 C. The effect on endogenous water potentials and accumulation of sugars and sugar alcohols also were measured. For both strains, germination occurred rapidly over the same range of osmotic or matric potential of -0.7 to -5.6 MPa after 4-6 h incubation. At lower osmotic and matric potentials (-7.0 to -8.4 MPa), there was a lag of up to 24 h before germination. Optimum germ-tube extension occurred between -0.7 and -1.4 MPa for both strains but varied with the solute used. Growth was optimal at -1.4 MPa and 25 C in response to matric stress, with the minimum being about -8.0 and -11.2 MPa at 15 and 25 C, respectively. In contrast, F. graminearum grew fastest at -0.7 MPa and was more tolerant of solute stress modified with either glycerol or NaCl with a minimum of about -14.0 MPa at 15 and 25 C. A decrease in the osmotic/matric water potential of the media caused a large decrease in the mycelial water potential (Ψ(c)) as measured by thermocouple psychrometry. In general, the concentration of total sugar alcohols in mycelia increased as osmotic and matric potential were reduced to -1.2 MPa. However, this increase was more evident in mycelia from glycerol-amended media. The quality of the major sugar alcohol accumulated depended on the solute used to generate the water stress. The major compounds accumulated were glycerol and arabitol on osmotically modified media and arabitol on matrically modified media. In response to matric stress, the concentration of trehalose in colonies generally was higher in the case of osmotic stress. In each water-stress treatment there was a good correlation between Ψ(c) and total sugar alcohol content.