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.