The Responses Of Two Contrasting Populus Clones To Cadmium Stress Under Different Nitrogen Forms

Cadmium as a highly toxic and harmful heavy metal endangers the health of animals and humans through the enrichment of the food chain.Soil cadmium pollution has become increasingly serious due to the intensification of human activities in recent years.Therefore,restoring cadmium-contaminated land and improving plant growth and crop safety in mildly/moderately cadmium-contaminated soils have become urgent issues for agricultural and forestry development.Phytoremediation is considered to be an effective biotechnology method for remediation of heavy metals in the soil due to the advantages of low cost,wide adaptability and no secondary pollution.The woody plant poplar with well-developed root system,rapid growth,and large above-ground biomass has been selected as an excellent candidate for phytoremediation of heavy metal contaminated soil.Previous studies have found that poplars can enrich a mass of heavy metals and show strong metal tolerance.However,due to genetic differences within species,there may be differences in enrichment and tolerance between different clones.Nitrogen is a macro-element constituting plants.With the development of agriculture and agronomy,nitrogen fertilizer utilization is getting more efficient than before.NO₃^--N and NH₄⁺-N as inorganic nitrogen sources have been absorbed and used by plants,and meanwhile different nitrogen forms may affect plant physiology and biochemistry.Some pre-work reports the mitigation effect of nitrogen on cadmium stress in plants,but only few focuses on the regulatory mechanism of nitrogen form on cadmium tolerance.In this study,two poplar clones?Nanlin 1388 and Nanlin 895?with different nitrogen deficiency sensitivity are used as plant materials.Physiological and biochemical techniques are used to explore the differences in cadmium tolerance and mechanisms of two poplar clones under different nitrogen forms,as well as the effects of root and leaf cell wall characteristics,carbon and nitrogen metabolism under the interaction of different nitrogen forms and cadmium stress were studied.The main results obtained are as follows:1)Differential responses of two poplar clones to cadmium stress under different nitrogen formsThe differential responses of two poplar clones to cadmium stress under different nitrogen supply forms were measured and analyzed using physiological and biochemical indicators combined with scanning electron microscopy-spectrum analysis.The results showed that Nanlin 1388?clones which is not sensitive to low nitrogen stress?has stronger cadmium tolerance under both NO₃^--N and NH₄⁺-N than Nanlin 895?clones which is sensitive to low nitrogen stress?.The cadmium distribution,transport,ionic homeostasis,and antioxidant capacity of the two clones are different under cadmium stress,and have different tolerance mechanisms with different nitrogen supply forms?NO₃^--N and NH₄⁺-N?.Under NO₃^--N,Nanlin 1388reduced the transfer of cadmium roots to the ground,improved oxidation resistance and effective cadmium chelation ability compared with Nanlin 895,and NO₃^--N further promoted Nanlin 1388 the distribution of epidermal cadmium on the leaves and leaf veins which has a certain protective effect on plant photosynthetic systems.NH₄⁺-N significantly increased the stress tolerance of the two clones through promoting the cadmium accumulation and the transportation from root to the up-ground;specifically,Nanlin 1388 increased the cadmium content in the bark and the higher P/S ratio in the leaf and bark;NO₃^--N promotes the distribution of cadmium in the epidermis and vascular tissue of leaves,especially in Nanlin 895.2)Cell wall regulation mechanisms of two poplar clones in response to cadmium stress under different nitrogen formsThe cell wall regulation mechanism of cadmium stress in two poplar clones under different nitrogen supply forms was studied by measuring cell wall cadmium content,cell wall composition changes,nutrient element content,and Fourier infrared spectroscopy?FTIR?changes of each cell wall component.The results showed that the content of cadmium in the leaf and root cell walls of Nanlin 1388 and Nanlin 895under NH₄⁺-N significantly increased,which may be an effective strategy to reduce cadmium toxicity in two poplar clones;Nanlin 1388 mainly enriched cadmium in the cell wall in the root system while Nanlin 895 has a stronger transport to the leaves which is much greater than Nanlin 1388.The enrichment of cadmium by the two poplar clones under cadmium stress may be related to the content of cell wall polysaccharides.The cell wall polysaccharides in Nanlin 1388 were higher in NO₃^--N,but higher in Nanlin 895 after added NH₄⁺-N.FTIR was used to characterize the functional group information of cadmium adsorption sites on leaves,root cell walls and its components of Nanlin 1388 and Nanlin 895.The results show that cadmium mainly binds to carboxyl functional groups in cell wall cellulose,hemicellulose and lignin.Under cadmium stress,the binding force of hydrogen bonds between peptide bonds in the secondary structure of the protein on the leaf,root cell wall and its components was enhanced,which promoted the synthesis of some polysaccharides,thus made Nanlin 1388 and Nanlin 895 show some adaptability under cadmium stress.3)Effects of different nitrogen forms on carbon and nitrogen metabolism of two poplar clones under cadmium stressThe carbon and nitrogen metabolism response mechanisms of two poplar clones under different nitrogen supply forms to cadmium stress were studied through carbon and nitrogen isotope measurement,carbon and nitrogen compound measurement,and transmission electron microscope observation and analysis.The results showed that different low nitrogen-sensitive clones under different cadmium stress had different nutrient adaptation strategies in different nitrogen supply forms?NO₃^--N and NH₄⁺-N?.The utilization efficiency of photosynthetic nitrogen,the true photosynthetic efficiency and glucose content of Photosystem II under NO₃^--N were significantly higher than those of Nanlin 895.It can be explained that Nanlin 1388 diluted cadmium poison through increasing its own growth rate and thus improved its own cadmium tolerance;while Nanlin 895 may improve its cadmium tolerance by synthesizing more carbon and nitrogen metabolites?such as sucrose and certain functional amino acids?.The content of starch and sucrose in Nanlin 895 under NH₄⁺-N was significantly higher than that in Nanlin 1388.The photosynthetic nitrogen utilization efficiency,the true photosynthetic efficiency of photosystem II,glucose,and some functional amino acid contents were significantly lower than those in Nanlin 1388;and both clones showed significant accumulation characteristics of starch granules under NH₄⁺-N.The chloroplasts of Nanlin 895 were almost occupied by starch granules and the photosynthetic organs were severely damaged.This may be due to the cadmium stress damage caused was greater under NH₄⁺-N on two poplar clones.Nanlin 1388 increased the storage of carbon and nitrogen compounds to buffer cadmium stress while Nanlin 895 was severely stressed.