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.