ObjectiveThis study aims to investigate the effects of Cd-Zn combined stress on chemical defense of Populus alba ‘Berolinensis ’ (PAB).

MethodThree concentrations of Cd (0.5, 1.5 and 2.5 mg/kg) and Zn (300, 500 and 700 mg/kg) were combined and applied in an orthogonal experimental design (3 × 3) into the soil with one-year-old PAB seedlings. The activities of defense proteins and the contents of secondary metabolites in the PAB leaves were analyzed at 30, 40 and 50 days after Cd-Zn applications, respectively.

ResultThe results showed that both the activities of defense proteins and the contents of secondary metabolites in the leaves of PAB seedlings decreased with the increase of Cd-Zn concentrations. Both parameters were significantly lower under the high Cd-Zn stresses than those under the medium-low Cd-Zn stresses (P < 0.05). The activities of PAL, PPO, TI, CI, and the contents of flavonoids and lignin in the PAB leaves under the high-medium Cd-Zn (combined) stresses were significantly lower than those of control (P < 0.05) for all three sampling dates (30, 40 and 50 days after the Cd-Zn application). Under the low Cd-Zn combined stresses, the flavonoid contents at 30 and 40 days after the Cd-Zn application and the PAL activity at 50 days after the Cd-Zn application were also significantly lower than those of control (P < 0.05).

ConclusionThe above results illustrate that Cd-Zn combined stress could inhibit the chemical defense levels of the PAB leaves, and such inhibitory effects vary, to certain degrees, with Cd-Zn concentrations and exposure durations. Further studies are need to determine how the Cd-Zn combined stresses affect PAB defoliators.