Inducible defense responses in Populus alba berolinensis to Pb stress

To better understand the effects of heavy metal stress on inducible defenses of woody plants, contents of nutritional substances and secondary metabolites, plus the activities of defense enzymes and protease inhibitors in the leaves of Populus alba berolinensis seedlings that were planted in either non-contaminated soil (control) or Pb-contaminated soil (at 300, 500 and 700 mg/kg) were studied. Our results showed that protein and soluble sugar contents in leaves 30-50 days after the Pb stress were significantly lower than those in the control group in a dose-dependent manner, whereas the lignin and flavonoid contents were significantly higher. Polyphenol oxidase (PPO) and phenylalnine ammonialyase (PAL) activities in leaves were significantly enhanced by the low Pb concentration stress (300 mg/kg) for all three sampling dates (30, 40 and 50 days after the Pb treatments), while at the two higher Pb concentrations (500 and 700 mg/kg), PPO activities were always inhibited and PAL activities presented a tendency of “promoting first and then restraining” with the increase of exposure duration. The response patterns of trypsin inhibitor (TI) and chymotrypsin inhibitor (CI) activities to Pb stress were similar to that of PPO, and presented a “low-promotion, high-inhibition” effect of Pb concentrations in leaves. These results suggest that the inducible defense responses in P. alba berolinensis leaves to Pb stress depend on both the Pb concentrations and the exposure duration, and the low Pb concentration at 300 mg/kg seemed to induce the strongest defense responses in the poplar seedlings, much stronger than those at the higher Pb concentrations.