THE BIOLOGY OF MYCORRHIZA IN THE ERICACEAE

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Abstract
SUMMARY: Plants with ericoid mycorrhizas successfully colonize acidic soils containing high levels of heavy metals. While mycorrhizal infection normally enhances uptake of mineral elements, such a function in metalliferous soils would be disadvantageous. The role of infection in heavy metal tolerance was therefore investigated in three ericaceous species. Plants of Calluna vulgaris, Vaccinium macrocarpon and Rhododendron ponticum were grown in the mycorrhizal and non‐mycorrhizal conditions in sand containing a dilute nutrient solution supplemented with Cu or Zn at 0, 10, 25, 50 and 75 and 0, 25, 50, 100, 150 mg−1 respectively. Fungal endophytes isolated from these host plants were also grown in pure culture over a wide range of concentrations of both metals. Mycorrhizal plants showed some growth in all metal treatments, whereas non‐mycorrhizal plants failed to grow in all but the lowest metal levels. The endophytes all grew vigorously up to 75 mg Cu 1−1 and 150 mg Zn 1−1. In all but one case, the mycorrhizal plants had a significantly lower concentration of metal in their shoots. Concentrations of metal in roots were higher in mycorrhizal plants. It is proposed that the hyphal complexes of the endophyte provide adsorptive surfaces within the cortical cells of the host roots, thus facilitating exclusion of metals from the shoot and avoidance of metal toxicity. The ecological significance of these observations is discussed.