Effects of chromium and nickel on growth of the ectomycorrhizal fungus Pisolithus and formation of ectomycorrhizas on Eucalyptus urophylla S.T. Blake

Abstract

Effects of chromium (Cr 2+) (0–100 000 μmol Cr l −1 MMN agar, supplied as CrO 3) and nickel (Ni 2+) (0–100 000 μmol Ni l −1 MMN agar, supplied as NiCl 2) cations on growth of the ectomycorrhizal fungus Pisolithus, collected from under eucalypts growing in Western Australia, the Philippines and New Caledonia were studied in vivo. The effects of these heavy metals on mycorrhizal formation with Eucalyptus urophylla S.T. Blake were also examined in vitro. Nickel was more toxic to fungal growth and ectomycorrhiza formation than Cr. Substrate levels of Ni greater than 20 μmol l −1 agar and Cr levels greater than 200 μmol l −1 agar decreased mycelial dry weights except for the Australian isolate where mycelial dry weight at harvest was not affected by Cr. Nickel and Cr rates greater than 200 and 2000 μmol l −1, respectively, prevented fungal growth. The fungi differed in their tolerance to the heavy metals and isolates from the Philippines and New Caledonia grew at higher Ni levels than the Australian isolate. Media concentrations of 1000 μmol Cr l −1 and 80 μmol Ni l −1 reduced the percentage of root tips colonized by the fungi. Inoculation with the New Caledonian isolate prevented the appearance of foliar toxicity symptoms in seedlings grown at 80 μmol Ni l −1 agar. The effects of three rates of Ni (0, 30 and 60 μmol Ni kg −1 soil) on the formation of ectomycorrhizas and growth of E. urophylla seedlings in a yellow sand were examined in a glasshouse. The application of Ni reduced the percentage of root tips colonized at week 4, from 12% in the nil treatment to 3% at 60 μmol Ni kg −1 soil, but there was no effect at week 12. The New Caledonian isolate produced the greatest percentage of ectomycorrhizas in all Ni levels (85% at 0, 81% at 30 and 75% at 60 μmol Ni kg −1 soil) and was the only isolate that promoted an increase (5 times the uninoculated treatment) in seedling total biomass at 60 μmol kg −1 soil. Hence, the isolate from a heavy metal-contaminated site not only colonized a greater percentage of root tips in vitro, but it was more effective in promoting seedling total biomass in Ni-amended soils than the other isolates. By contrast, the good performance of the Philippine isolate in vitro, in terms of plant biomass and growth rate of mycelia, was not reflected in its capacity to form mycorrhizas and promote plant growth in vivo. This study indicates the importance of undertaking preliminary screening of ectomycorrhizal fungi for heavy metal tolerance under in vivo conditions before further testing in field soils of ultramafic origin.