Nickel toxicity in mycorrhizal birch seedlings infected with Lactarius rufus or Scleroderma flavidum I. Effects on growth, photosynthesis, respiration and transpiration

SUMMARY Growth, photosynthesis, transpiration and respiration were measured in birch (Betula papyrifera Marsh.) seedlings exposed to 85mUM nickel. The seedlings were inoculated with either Scleroderma flavidum E. & E., a fungus known from previous work to increase Ni tolerance, or Lactarius rufus (Scop, ex Fr.) Fr., another mycorrhizal associate, but one which did not increase Ni tolerance, or else they were not inoculated. Decreased water uptake per seedling was used as an indicator of the progression of Ni toxicity in the seedlings over 16 weeks. These measurements indicated that 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 weights 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 weight. The weight of S. flavidum tissue per root was significantly greater than that of L. rufus in both the Ni‐treated and control (non‐Ni‐treated) seedlings, and this amount increased with time. Lactarius rufus did not grow once the roots were exposed to Ni. Thus, 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. Thus, S. flavidum increased Ni tolerance in the host, but not by preventing Ni‐induced reductions in photosynthetic rates or by affecting shoot respiration rates.