Effects Of Cadmium Stress And Supplement Of Exogenous Nitrogen On Plant Resistance Of Populus Yunnanensis Dode

The elemental defense hypothesis holds that heavy metals,as exogenous chemicals,can be absorbed and enriched by plants as a special form of chemical defense to enhance their resistance to insects.However,the current research on this hypothesis focuses on herbaceous plants,and so far no woody plant species has been reported.Since the absorption,transport and accumulation distribution of heavy metals after enrichment in trees are different from that in herbaceous plants,the applicability of this hypothesis urgently needs to be further verified in trees.On the other hand,the accumulation of heavy metals in plants beyond the tolerance limit will threaten the health of plants and increase their sensitivity to insect pests.Therefore,how to enhance the insect resistance of plants by the enrichment of heavy metals without affecting the growth and development of plants is an urgent problem in the application of this hypothesis.In previous studies,we found that the application of exogenous nitrogen to plants subjected to heavy metal stress can reduce the toxic symptoms of plants and increase the accumulation of heavy metals in plants.Therefore,this study proposed the hypothesis that a certain degree of exogenous nitrogen would enhance the insect resistance of forest trees under heavy metal stress.In order to test the Elemental defense hypothesis on woody plant species and whether exogenous nitrogen could enhance plant resistance to herbivores,this research used the Populus yunnanensis Dode as the research material,combined with the insect growth and feeding choice bioassy,to investigate the effects of sole cadmium treatment and the additional supplement of exogenous nitrogen on plant resistance to specialist and generalist herbivores.the results are as follows:（1）In the experiment of sole soil cadmium treatment,the concentration of cadmium enriched in the leaves of P.yunnanensis Dode increased linearly with the concentration of soil cadmium treatment,and the cadmium concentration of poplar leaves reached the maximum value under the soil cadmium treatment of 60mg·kg^-1.In the growth and development experiment of insect larvae,the weight growth of the two larvae feeding on the leaves of P.yunnanensis Dode decreased gradually with the increase of cadmium concentration in the leaves within 7 days,and the inhibition rate of insect growth was the highest under the soil cadmium treatment of 60mg·kg^-1.Therefore,the experiment of insect feeding selection was carried out with cadmium-treated plants in the soil at60mg.kg^-1,and the results showed that both insects tended to feed on the leaves of the control plants,and the leaves of the cadmium-treated plants had obvious chemoattractant effect on both insects.The experimental results support the element defense hypothesis,which proves that cadmium can inhibit the growth and feeding of the specialist insects Botyodes diniasalis Walker and the generalist insect Spodoptera exigua Hiibner.（2）In the experiment of exogenous nitrogen treatment,the concentration of cadmium enrichment in poplar leaves under cadmium stress increased linearly with the concentration of nitrogen applied,and reached the maximum value under high nitrogen treatment,indicating that the application of exogenous nitrogen could increase the concentration of cadmium in poplar leaves.In addition,the plant height,ground diameter and chlorophyll content of poplar leaves also increased significantly with the increase of the concentration of soil nitrogen,indicating that the application of nitrogen could alleviate the cadmium stress and promote the growth of plants.It was found that the larval weight growth of both B.diniasalis Walker and Spodoptera litura Fabricius decreased with the increase of soil nitrogen treatment concentration feeding with each treated poplar leaves.The paired selection experiment of insects showed that the leaves of poplar under high nitrogen treatment had obvious chemoattractant effect on the feeding selection of two insect larvae,and the chemoattractant effect was stronger than that of the leaves under single cadmium treatment.The results show that the application of exogenous nitrogen can significantly enhance the insect resistance of poplar under cadmium stress.The results of this study verify the applicability of the element defense hypothesis in poplar trees which further enrich and improve the application scope of the element defense hypothesis.In addition,this study also proved that the application of exogenous nitrogen can promote the growth of poplar trees and improve the insect resistance of the plants,which can provide a new idea and method for enhancing the insect resistance of the trees in the heavy metal contaminated areas.