ABSTRACT:To evaluate the lead tolerance of Populus euramericana (Dode) Guinier 561/41 and Populus nigra Linnaeus 63/135 clones in relation to photosynthetic efficiency, we measured physiological parameters of the two poplar clones exposed to Pb treatments. The pot experiment was established in a roofed place in a completely randomized design, with 5 Pb treatments (0, 0.5, 1, 1.5 and 2 g Pb·kg -1 ) per poplar clone. After 3 months, photosynthetic rate (A), transpiration rate (E), stomatal conductance, intercellular CO 2 concentration (C i ), leaf water potential were measured and water use efficiency (WUE) and mesophyll conductance (g m ) were computed. Results revealed that increasing Pb concentrations in soil reduced all physiological parameters of the two poplar clones, except C i . At all Pb treatments A and E, and at a concentration of 0.5 g Pb·kg -1 WUE and g m of P. euramericana plants were significantly higher than those of P. nigra plants. On the other hand, while a reduction in most physiological parameters of P. euramericana plants took place at a concentration of 1 g Pb·kg -1 and at higher concentrations, in P. nigra plants these reductions were from a concentration of 0.5 g Pb·kg -1 . With respect to physiological parameters, P. euramericana 561/41 clone was more tolerant of Pb than P. nigra 63/135 clone, therefore it can be considered as a suitable species in phytoremediation of lead-contaminated soils.