Influence of pH on copper and zinc sensitivity of ericoid mycobionts in vitro.

The effect of pH on growth, metal uptake and toxicity in four isolates of ericoid mycobionts (two Hymenoscyphus ericae from unpolluted heathland sites and two H. ericae-type mycobionts from metal-contaminated mine spoil) was assessed in vitro. These isolates were incubated in liquid medium (10% Rorison's medium, glucose at 10 g l(-1)) containing either 0.25 mM Cu or 2.0 mM Zn and adjusted to pH 2, 3, 4, 5 or 6. After 30 days incubation, dry mass and mycelial metal content were determined and growth was expressed as tolerance index, i.e. dry mass in the presence of metal as a percentage of dry mass in the absence of metal. Initial medium pH had a significant effect on both tolerance index and metal accumulation. Tolerance indices were highest at pH 2, with several isolates showing a stimulation of growth (i.e. tolerance index >100%) at this pH. Tolerance index decreased at higher initial pH values and growth of two mycobionts was completely inhibited (tolerance index=0) in the Cu-supplemented media at pH 6. Reduction in tolerance index coincided with an increase in mycelial accumulation of Cu and Zn. Practical and environmental implications of these results are discussed.

Nutritional influences on the solubilization of metal phosphate by ericoid mycorrhizal fungi.

Four ericoid mycobionts (two isolates of Hymenoscyphus ericae, and two dark, sterile ericoid mycobionts isolated from metal-contaminated mine sites) were grown on solid agar plates supplemented with zinc phosphate (0.25 %) containing different forms of nitrogen (nitrate, ammonium or alanine) and different concentrations of carbon (glucose) and phosphorus (K2HPO4). The influence of nutrient variation on solubilizing ability of the fungi was assessed by measuring the zones of solubilization appearing beneath the growing colonies. All four mycobionts were capable of zinc phosphate solubilization in the presence of all three nitrogen sources and in media containing no nitrogen. No solubilization was observed at 0 mM glucose-C but was observed with increasing glucose concentration from 300 to 600 mM C. Increasing phosphorus concentration (0-5 mM P) had no effect on the solubilizing ability of the isolates. All but one of the mycobionts were capable of solubilizing calcium phosphate (CaHPO4), while no solubilization was observed in media containing aluminium phosphate (AlPO4), iron phosphate (FePO4 x 4H2O) or copper phosphate (Cu3O8P2 x 2H2O) under conditions which were found to be optimal for zinc phosphate solubilization. Under conditions of glucose at 300 mM C and alanine as the N source in the zinc phosphate-amended agar medium, one of the mycobionts produced new crystals, which were morphologically distinct from the original zinc phosphate crystals. It is concluded that medium composition influences the metal-phosphate solubilizing ability of ericoid mycobionts. The results are discussed in relation to the possible mechanisms involved in solubilization and the potential benefits of metal-phosphate solubilization to ericoid mycobionts and their host plants.