Abstract: Acid stress lowered the water potential and inhibited the increase of seedling length, freshand dry weight of Black Locust seedlings, but increased the electrolyte leakage significately. The free proline contents of roots in Black locust seedling under low pH (pH 3. 0) showed no significant change,while putrescine titer increased dramatically. After 2 days of acid stress (pH 3. 0), putrescine contents rose to 4 folds as compared with the control (pH 6. 0). Putrescine accumulation in eight differenttree species also increased under acid stress. Therefore, putrescine accumulation seems to be a physiological indicator of acid stress.