| Heavy metals are widely distributed in various natural environments,but human beings often neglect sustainable development and pay little attention to pollution control when exploiting and utilizing the natural resources,so that a large number of heavy metals flow into the environment and threaten the normal operation of the whole ecosystem.In recent years,the absorption,transfer and enrichment of heavy metals in plants have been studied extensively,but mainly in aquatic and herbaceous plants and limited to the toxic effects of heavy metal stress on plant growth or tolerance mechanism of plants to heavy metal stress.Little is known about the growth fitness and insect resistance in woody plants exposed to heavy metal stress,as well as the detoxification mechanism of their harmful pests.In this study,we analyzed the enrichment characteristics of heavy metals and the response trends of growth fitness and chemical defense to heavy metal stress in Populus alba×P.berolinensis seedlings grown in non-contaminated soil,Cd-contaminated soil,Zn-contaminated soil or Pb-contaminated soil,plus the detoxification mechanism of their harmful pest gypsy moth(Lymantria dispar).The objective of this study was to reveal the physiological and ecological response mechanism of Populus alba×P.berolinensis and the phytophagous insect gypsy moth under heavy metal stress,and to provide a theoretical basis for controlling forest pests in heavy metal polluted areas.The results were as follows:(1)Accumulation characteristics to heavy metals.The bio-accumulation amounts of Cd?Zn or Pb in roots,stems and leaves were all significantly positively correlated with corresponding heavy metal content in the soil,and the concentrations of Cd,Zn and Pb in soil caused different,specific bio-concentration factor(BCF)of these three heavy metals in Populus alba×P.berolinensis roots,stems,and leaves.The bio-accumulation amount of Cd in all plant parts did not show any clear trends.The BCF of Cd between all parts showed no differences in response to the lowest concentration,but the BCF of the roots was significantly lower than those of the stems and leaves under the highest concentration.The bio-accumulation amount and BCF of leaves,roots,stems for Zn in the low concentration treatment group,and of roots,leaves,stems in the moderate or high concentrations treatment groups were decreased in turn.The bio-accumulation amounts and BCF of Pb in roots for all Pb concentrations applied were higher than those of the stems and leaves.(2)Effects on growth of Populus alba×P.berolinensis.Growth parameters(plant height,root length and ground diameter)and biomass parameters(the fresh and dry weights of leaves,stems and roots)of the Populus alba×P.berolinensis seedlings in all the Cd,Zn or Pb-treated groups were significantly lower than those in the control group in a near negative dose-dependent manner.(3)Effects on chemical defense of Populus alba×P.berolinensis.On the 30th,40th and 50th days after treatment,the contents of proteins and soluble sugars in leaves in all the Cd,Zn or Pb-treated groups were significantly lower than those in the control group.Furthermore,the contents of protein and soluble sugar were negatively correlated with the treatment concentration in Cd or Pb treatment group.However,under Zn stress,there was no significant difference in protein content between treatment groups,and the content of soluble sugar increased with the increase of the treatment concentration.Cd,Zn or Pb stress,regardless of its concentration level,significantly increased the contents of flavonoids and lignin in the leaves on all three sampling dates compared to the controls.In the Zn and Pb treatment groups,flavonoids and lignin contents under low concentration treatment were higher than those under medium and high concentration treatment,while in the Cd treatment group,flavonoids and lignin contents increased with the increase of treatment concentration.Compared to control group,the activity of phenylalnine ammonialyase(PAL)and polyphenol oxidase(PPO)in leaves were significantly increased at 30,40 and 50 days after Cd stress.However,under Zn or Pb stress,PAL activity presented a tendency of "promoting first and then restraining" with the increase of exposure duration,and PPO activity showed a significant decrease and a concentration effect of low promotion and high inhibition,respectively.Furthermore,significantly decrease in the activities of trypsin inhibitor(TI)and chymotrypsin inhibitor(CI)have been detected in all Cd or Zn treatment levels,but compared to control group,TI and CI activities presented a tendency of"promoting first and then restraining" under Pb stress.(4)Effects on growth and development of gypsy moth.Feeding on Cd or Pb stressed poplar leaves significantly decreased L.dispar larval body weights,body lengths and head capsule widths,pupal weights and female fecundity,and delayed the duration of larval development.Similar effects from the Zn stressed poplar leaves were also observed on all above mentioned variables except the male pupal weights and larval development durations that showed no differences from the control.Cd,Zn,or Pb stressed poplar leaves had no significant effects on L dispar larval survival,pupation and emergence rates;in fact,both larval survival and pupation rates all reached 100%.(5)Effects on enzyme activities of gypsy moth larvae.Compared to the untreated control,superoxide dismutase(SOD)and catalase(CAT)activities in Cd or Zn treatment group were gradually suppressed with the increases of larval ages,but Pb treatment had no significantly effects on SOD activities and significantly increased the CAT activities both in the 4th and 5th instar larvae;acid phosphatase(ACP)activities were gradually activated and alkaline phosphatase(AKP)activities were gradually inhibited with the increases of larval ages in Cd or Pb treatment group,but Zn treatment significantly increased the activities of ACP and AKP both in 4th and 5th instar larvae.All three heavy metals tested did not show any significantly effects on the amylase and protease activity in the 4th instar larvae,but increased their activities in 5th instar larvae.(6)Excretion mechanism of gypsy moth larvae to heavy metals.After feeding on Cd,Zn or Pb-stressed poplar leaves,the corresponding heavy metal concentrations and contents in the 3rd-6th larvae were significantly higher than that of control.Cd,Zn or Pb concentrations of larvae in the corresponding treatment group showed a significantly decrease with increasing larval age while their contents increased with the larval age.Furthermore,under Cd,Zn or Pb stress,the corresponding heavy metal concentrations in larval faces and exuvia were significantly higher than that of control.(7)Effects on food utilization of gypsy moth larvae.The larval food digestion rate of 3rd-5th instar larvae in the Cd,Zn or Pb-stressed group was higher than that in the control group,but their food conversion rate were lower.The larval food utilization rate of 3rd-4th instar larvae fed with Cd,Zn or Pb-stressed poplar leaves were not significantly different from those of the larvae fed with the control leaves;however,the food utilization rate of 5th instar larvae in these treatment groups were significantly lower than that in the control group.In conclusion,Cd,Zn or Pb can transfer from soils contaminated with corresponding heavy metals through Populus alba×P.berolinensis seedlings and gypsy moth,and after accumulation,Cd,Zn or Pb can significantly affect the growth and chemical defense characteristics of Populus alba×P.berolinensis,as well as disturb the growth,development and reproductive of gypsy moth.However,gypsy moth may have an effective detoxification mechanism for lessening the effects of plant-mediated defenses and heavy metals in leaves on larval survival,pupation and eclosion,such as the activation of antioxidant and detoxification enzymes,excretion of heavy metals and increase of food consumption rate. |
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