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

Growth of A. ochraceus at 30 degrees C was significantly influenced by chan ges in both solute (NaCl, glycerol) and matric potential (PEG 8000). Optimu m growth rates were at approx. - 10.0 MPa with the non-ionic solute, and be tween -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 g rowth was at - 7.0 MPa (= 0.95 a(w)). Growth rates were influenced by wheth er colonies were grown directly on the agar medium or on cellophane overlay s. 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 t o - 10.0 MPa). The contents of low and high mel, wt polyols (glycerol, eryt hritol, arabitol and mannitol) and sugars (glucose and trehalose) were quan tified in whole colonies and conidia grown under different osmotic and matr ic 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 glycer ol-amended osmotic media maximum total polyols were present at -4.5 (= 0.96 5 a(w)) and - 17.0 MPa (= 0.88 a(w)) in mycelial colonies and conidia respe ctively, with glycerol and mannitol being predominant in both, with small a mounts of erythritol also present in conidia. By contrast, in matrically-mo dified media, the high mel. wt mannitol was the major component (> 75% of t he total) with small amounts of arabitol and the low mel. wt erythritol and glycerol, regardless of matric potential. For sugars, trehalose was predom inant in conidia (> 90 %) from osmotically-modified media with low levels o f glucose present in both conidia and mycelial colonies at all potentials t ested. 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.