Detached leaf sections (2 × 2 cm2) from transgenic poplar line 18-1 and its wild type (WT) (Populus × euramericana ‘Neva’) were used to test their salt tolerance and gene expression under controlled environment conditions. The sections from line 18-1 displayed better tolerance to NaCl stress, indicated by high chlorophyll retention and K+ content but low relative electrolyte leakage (REL). Transient overexpression of NTHK1 (Nicotiana tabacum histidine kinase 1) and V-H+-PPase was found in the detached young leaves from line 18-1 after they had been stressed for a few minutes. The activities of vacuolar-type H+-ATPase and H+-PPase in line 18-1 were boosted initially and then decreased to normal level as in unstressed leaves. After sections were stressed for 10 days, the maximal Na+ concentration in line 18-1 was much higher than that in the WT. The higher capacity for Na+ accumulation in line 18-1 may be due to stable Na+ sequestration into the vacuoles. Osmotic stress imposed little effect on REL and chlorophyll content of the sections. The capacity of detached leaf sections in NaCl solution to tolerate stress and to accumulate Na+ may be useful for identifying genotypes with good salt tolerance in poplar and other plants.