Osmotic and matric potential effects on growth, sclerotia and partitioning of polyols and sugars in colonies and spores of Aspergillus ochraceus

Growth of A. ochraceus at 30 degrees C was significantly influenced by changes in both solute (NaCl, glycerol) and matric potential (PEG 8000). Optimum growth rates were at approx. - 10.0 MPa with the non-ionic solute, and between -3.5 (= 0.975 a(w)) and 14.0 MPa (= 0.90 a(w)) with the ionic solute in a sucrose-based minimal salts medium. The optimum matric potential for growth was at - 7.0 MPa (= 0.95 a(w)). Growth rates were influenced by whether colonies were grown directly on the agar medium or on cellophane overlays. Sclerotia were profusely formed on media > - 1.5 MPa(= 0.99 a(w)), only initials at -3.5 MPa, and none under optimum conditions for growth (- 7.0 to - 10.0 MPa). The contents of low and high mel, wt polyols (glycerol, erythritol, arabitol and mannitol) and sugars (glucose and trehalose) were quantified in whole colonies and conidia grown under different osmotic and matric potential conditions for the first time. There was a higher total amount of polyols (mu g mg(-1) D.W.) in conidia than in whole colonies. In glycerol-amended osmotic media maximum total polyols were present at -4.5 (= 0.965 a(w)) and - 17.0 MPa (= 0.88 a(w)) in mycelial colonies and conidia respectively, with glycerol and mannitol being predominant in both, with small amounts of erythritol also present in conidia. By contrast, in matrically-modified media, the high mel. wt mannitol was the major component (> 75% of the total) with small amounts of arabitol and the low mel. wt erythritol and glycerol, regardless of matric potential. For sugars, trehalose was predominant in conidia (> 90 %) from osmotically-modified media with low levels of glucose present in both conidia and mycelial colonies at all potentials tested. By contrast, in matric media trehalose was the predominant sugar in both conidia and whole colonies. This study suggests that xerophilic fungi may use different mechanisms for overcoming osmotic and matric stress.