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.