Effects Of Exogenous Addition Of Pseudomonas Fluorescens On Cadmium Enrichment Characteristics And Antioxidant Metabolism Of Ginkgo Biloba L Under Nitrogen Deposition And Cadmium Pollution

With the rapid development of industrialization and urbanization,atmospheric nitrogen deposition and soil cadmium pollution have become global environmental problems.In the southeastern and northeastern regions of China,nitrogen deposition and cadmium pollution often overlap on a large scale,and plants in these areas often suffer from more severe environmental stresses.Pseudomonas fluorescens is one of the most studied and widely used plant growth promoting rhizobacteria in recent years,which can improve the soil and promote plant growth,but its mitigating effect on plant growth under the stress conditions of nitrogen deposition and cadmium pollution has been little studied.In this experiment,Ginkgo biloba L were used as the material,and eight treatments were set up:no addition（CK）,nitrogen deposition(N,10g·m^-2·y^-1),cadmium pollution(Cd,100 mg^-1kg^-1),cadmium pollution+nitrogen deposition（Cd+N）,Pseudomonas fluorescens(Pf,6×10⁸cfu·m L^-1),Pseudomonas fluorescens+nitrogen deposition（Pf+N）,Pseudomonas fluorescens+cadmium pollution（Pf+N）,and Pseudomonas fluorescens+cadmium pollution（Pf+N）.Pseudomonas fluorescens+cadmium pollution（Pf+Cd）and Pseudomonas fluorescens+cadmium pollution+nitrogen deposition（Pf+Cd+N）to investigate the effects of nitrogen deposition,cadmium pollution and combined stress on cadmium enrichment characteristics and antioxidant metabolism of Ginkgo biloba L.The exogenous addition of Pseudomonas fluorescens was used to investigate how microorganisms affect the tolerance of Ginkgo biloba L to Cd by affecting the polysaccharide fraction of the cell wall and antioxidant capacity under nitrogen deposition and cadmium pollution.The main results of this experiment are as follows:（1）Nitrogen deposition,cadmium pollution and combined stress all resulted in the inhibition of plant growth and the processes of physiological and biochemical.The plant height and diameter increments were significantly lower than those of the control group（P<0.05）,photosynthetic pigments（chlorophyll a,chlorophyll b and carotenoids）content was significantly lower（P<0.05）,reactive oxygen species（ROS）level increased,malondialdehyde（MDA）content increased,membrane lipid peroxidation deepened,and oxidative damage to Ginkgo biloba L increased.Meanwhile,nitrogen deposition would increase the effectiveness of soil cadmium ions,Ginkgo biloba L would absorb more cadmium ions under combined stress,and the ROS level and MDA content were higher under combined stress than single stress,causing more serious damage to the plant.（2）Specific defense strategies exist in Ginkgo biloba L to cope with abiotic stresses.The cell wall polysaccharide（pectin,hemicellulose 1 and hemicellulose 2）contents of Ginkgo biloba L roots were significantly increased（P<0.05）under nitrogen deposition and cadmium pollution,and cell wall polysaccharides play a very important role in root defense against external stresses.The contents of phytochelatins（PCs）and glutathione（GSH）in Ginkgo biloba L increased to different degrees under any stress conditions,they can be converted into non-toxic compounds by chelating with cadmium ions entering the protoplasts.Meanwhile,the activation of antioxidant system in Ginkgo biloba L alleviated the oxidative stress in the plant by enhancing antioxidant enzyme activities（SOD,CAT,APX,GR）.（3）In this experiment,Pseudomonas fluorescens was tested for cadmium tolerance,the normal growth and development of the bacterium could be maintained at a high cadmium concentration.Cadmium treatment had no significant effect on the growth curve of the strain,but it inhibited the growth of the strain.In addition,Pseudomonas fluorescens had the ability to produce siderophore and indole-3-acetic acid（IAA）.（4）Pseudomonas fluorescens significantly alleviated the inhibitory effects of the growth of Ginkgo biloba L under nitrogen deposition and cadmium stress,mainly as follows:exogenous addition of Pseudomonas fluorescens increased the photosynthetic pigment content under stress,cell wall pectin,hemicellulose 1 and hemicellulose 2 content of Ginkgo biloba L root system increased significantly（P<0.05）,PME activity increased,causing more low methyl esterified pectin generating,so root cell wall cadmium enrichment capacity was significantly enhanced.The exogenous addition of Pseudomonas fluorescens enhanced the activity of enzymes related to nitrogen metabolism,reduced the content of ROS and MDA in vivo,increased the content of macromolecules involved in plant chelation such as PCs and GSH,while the enzymatic activities of antioxidant systems（SOD,CAT,APX,GR）were increased to different degrees,the expression levels of antioxidant enzyme-related genes were up-regulated,so the resistance of plants to stress was enhanced.In summary,nitrogen deposition and cadmium pollution reduced the photosynthetic pigment content and increased the ROS content in Ginkgo biloba L leaves,and the damage suffered by Ginkgo biloba L under combined stress was greater than that under single stress.Ginkgo biloba L could alleviate the inhibitory effects of nitrogen deposition and cadmium pollution on the growth of the trees by increasing the polysaccharide content of the root cell wall and the activity of antioxidant enzymes in vivo.As plant growth promoting rhizobacteria,Pseudomonas fluorescens can alleviate the damage caused by nitrogen deposition and cadmium pollution on Ginkgo biloba L by enhancing the enrichment ability of plant cell walls,reducing the degree of oxidative damage,regulating the activity of antioxidant enzymes and the expression of stress-resistant genes.