Effects of ectomycorrhizal (ECM) fungus Suillus tomentosus on water transport properties were studied in jack pine (Pinus banksiana) seedlings. The hydraulic conductivity of root cortical cells (L(pc)) and of the whole root system (L(pr)) in ECM plants was higher by twofold to fourfold compared with the non-ECM seedlings. HgCl2 had a greater inhibitory effect on L(pc) in ECM compared with non-ECM seedlings, suggesting that the mercury-sensitive, aquaporin (AQP)-mediated water transport was largely responsible for the differences in L(pc) between the two groups of plants. L(pc) was rapidly and drastically reduced by the 50 mM NaCl treatment. However, in ECM plants, the initial decline in L(pc) was followed by a quick recovery to the pre-treatment level, while the reduction of L(pc) in non-ECM seedlings progressed over time. Treatments with fluoride reduced L(pc) by about twofold in non-ECM seedlings and caused smaller reductions of L(pc) in ECM plants. When either 2 mM KF or 2 mM NaF were added to the 50 mM NaCl treatment solution, the inhibitory effect of NaCl on L(pc) was rapidly reversed in both groups of plants. The results suggest that AQP-mediated water transport may be linked to the enhancement of salt stress resistance reported for ECM plants.