The Effects Of Exogenous Silicon On Alleviation Of Cadmium Toxicity In Raspberry (Rubus Idaeus L.)

Cadmium（Cd）is considered as one of the most toxic heavy metals in the environment.Plants grown in cadmium-contaminated soil absorb Cd,and their growth and development are inhibited when Cd content in their tissues and organs accumulates to a certain extent.At the same time,Cd into the food chain also cause irreversible damage to human health.Silicon（Si）has been shown to improve the resistance of many plants to stress,especially in alleviating heavy metal toxicity in plants.Red raspberries（Rubus idaeus L.）has a long history of planting in our country,which is an important economic tree species.However,the effects and mechanisms of Si on alleviating Cd stress in red raspberry has not been cleared.In this study,red raspberries were used as the research object,and two-factors test design and potted control soil culture experiment test methods were carried out to evaluate the effects of exogenous Si（0,100,200 mg/kg）on growth,physiological and biochemical properties,nutrient absorption and distribution,cell ultrastructure and Cd uptake and transfer characteristics of red raspberry under different concentrations of Cd（0,5,10mg/kg）.The main results are as follows:（1）With the increase of Cd stress,dry weight of leaves,stems and roots,biomass and root length in red raspberry showed a decreasing trend.Exogenously application Si increased the biomass and root length of red raspberry,and the effect of high concentration Si（200 mg/kg）treatment was more significant.（2）With the increase of soil Cd concentration,chlorophyll content,net photosynthetic rate（P_n）,stomatal conductance（G_s）and transpiration rate（T_r）significantly decreased,while intercellular CO₂concentration（C_i）significantly increased,which suggested that photosynthesis in red raspberry was inhibited.However,the application of Si under Cd stress increased the chlorophyll content,P_n,G_sand T_rin red raspberry,and decreased the concentration of C_i,and the 200 mg/kg Si treatment was more significant.（3）With the increase of Cd stress,the activities of superoxide dismutase（SOD）,peroxidase（POD）and catalase（CAT）in leaves and roots of red raspberry increased gradually.And the content of malondialdehyde（MDA）was also increased with the increase of soil Cd concentration.Exogenously applied Si additionally further increased the activity of antioxidant enzymes under Cd stress,while the content of MDA was gradually decreased.Meanwhile,the contents of proline（Pro）,soluble protein（SP）and soluble sugar（SS）in red raspberry showed a decreasing trend with the increase of Cd stress.However,exogenously applicated Si enhanced the osmotic regulation ability in red raspberry by further increased the osmotic regulation substances content.（4）With the increase of soil Cd concentration,the contents of N,P and K in leaves,steams and roots in red raspberry decreased significantly.The nutrient elements of each organ showed little change under single Si treatment.However,the contents of N,P and K in all organs were increased compared with that under single Cd stress when exogenously applicated Si.（5）With the increase of Cd stress,the Cd content of each organ in red raspberry increased gradually,and there were significant differences compared with CK.Under the same level of Cd treatment,the order of Cd content was root>stem>leaf.The soil available Cd content also increased significantly,and the above-ground translocation factor（TF）was less than 1.Exogenously applicated Si reduced the Cd content of leaves,stems and roots in red raspberry and the available Cd content in soil.Meanwhile,the TF also decreased gradually under treatment.（6）With the increase of soil Cd concentration,the ultrastructure of mesophyll cells in red raspberry obviously changed,the number and volume of starch granules and osmophilic granules increased,the chloroplast became swollen and even deformed,and the nuclear membrane was ruptured under 10 mg/kg Cd treatment.Meanwhile,the root tip cells were damaged under Cd stress,including round mitochondria,disappearance of crista and even degeneration of cell membrane structure.At the same time,a large amount of electron dense materials was deposited on the cell wall.In contrast,exogenously added Si altered the ultrastructure in red raspberry mesophyll cells under Cd stress,the number and volume of starch granules and osmophilic granules decreased,and the integrity of nuclear membrane was improved.The application of Si significantly reduced the electron dense material deposited in the root tip cells and maintained the structural integrity of the cells.In conclusion,exogenous application of Si alleviated the adverse effects of Cd in red raspberry by enhancing the growth,increasing photosynthesis and antioxidant defense system,reducing soil available Cd meanwhile reducing the absorption and translocation of Cd to amend the ultrastructural changes induced by Cd.Therefore,Si could be applied to Cd-contaminated soil to inhibit and alleviate the Cd toxic on red raspberry and reduce the risk of planting.