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Birch seedlings (Betula papyrifera) were inoculated with either S. flavidum, a fungus known to increase Ni tolerance, or L. rufus, which does not increase Ni tolerance, or were not inoculated, and were exposed to 85 µM Ni. Decreased water uptake/seedling was used as an indicator of the progression of Ni toxicity in the seedlings over 16 weeks. L. rufus provided some initial protection against Ni toxicity. However, water uptake in S. flavidum-infected seedlings was least affected by Ni, and was significantly higher than that in L. rufus-infected and uninoculated seedlings after week 10. Total dry wt. of the S. flavidum-infected seedlings were higher than those of other seedlings at weeks 12 and 21. Chitin analysis was used to determine the fungal component of root wt. The wt. of S. flavidum tissue/root was significantly greater than that of L. rufus in both the Ni-treated and control seedlings, and this amount increased with time. L. rufus did not grow once the roots were exposed to Ni. The ability of S. flavidum to continue to grow following Ni exposure may be an important characteristic distinguishing between fungi which are effective or ineffective in enhancing metal tolerance. Nickel reduced photosynthetic rates in seedlings infected with S. flavidum relative to S. flavidum-infected seedlings not treated with Ni, but did not have this effect in uninoculated seedlings. Respiration rates were not affected by metal treatment, or by mycorrhizal formation. S. flavidum increased Ni tolerance in the host, but not by preventing Ni-induced reductions in photosynthetic rates or by affecting shoot respiration rates.
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