Effects Of Exogenous Nitrogen On Alleviating Cadmium Toxicity Of Trifolium Repens

With the rapid development of global economy,the pollution of human living environment is becoming more and more serious,which threatens human health.Heavy metal pollution is one of the main pollution problems faced by human beings.Phytoremediation has the advantages of friendly environment and low cost,but because of its low efficiency and long period,it can not be popularized and applied on a large scale at the present stage.so it is very important to improve the effect of restoration by manual intervention.Studies have shown that nitrogen can alleviate the toxicity of heavy metals to plants.Therefore,in the phytoremediation of heavy metal contaminated soil,the rational application of nitrogen fertilizer can improve the remediation efficiency.But how does nitrogen play a role in the detoxification of Cd in plants? In order to explore this problem,the following research contents were carried out by pot experiment with Trifolium repens:(1)six different concentrations of cadmium(0 mg / kg,12.5 mg / kg,25 mg / kg,50 mg / kg,75 mg / kg,100 mg / kg)were used to study the effects of cadmium on the growth,biomass and chlorophyll content of Trifolium repens,and to screen the cadmium concentration of 75 mg / kg,which caused the plant to be poisoned.(2)a total of8 treatments with different concentrations of nitrogen(0mg/kg?1mg/kg?2mg/kg?4mg/kg?8mg/kg?12mg/kg?18mg/kg?24mg/kg of N under Cd stress),and a blank control group(without cadmium and nitrogen treatment)were used to study the effect of nitrogen on the growth of Trifolium repens under cadmium stress,and to screen the nitrogen concentration of 18 mg/kg to alleviate cadmium toxicity in plants.(3)a total of 4 treatments were set up(0mg/kg Cd+0mg/kg N,75mg/kg Cd+0mg/kg N,0mg/kg Cd+18 mg/kg N and 75mg/kg Cd+18)to study the effects of increasing nitrogen fertilizer application on photosynthetic fluorescence characteristics,Cd content and PCs content of antioxidant enzymes in plants under Cd stress.The results are as follows:(1)The low concentration of Cd(?50 mg/kg)did not significantly affect the leaf morphology and promoted the plant growth to some extent,and its growth and chlorophyll content increased with the increase of cadmium concentration.When Cd concentration was75mg/kg,plant growth was affected,its growth,biomass and chlorophyll content decreased to the blank control level,even lower than the control.It indicated that cadmium caused heavy metal stress on the plant.And the plant growth was seriously affected by cadmium concentration of 100mg/kg,the leaves of the plant were seriously chlorotic,the length of the main root was significantly lower than that of the control,and the other indexes were lower than those of the control,and the difference was very significant.It was suggested that Cd treatment with low concentration(?50mg/kg)could promote the growth and development of Trifolium repens,and when the concentration of Cd reached 75mg/kg,the plant was inhibited from Cd stress and plant growth was inhibited.Therefore,I selected the cadmium concentration of 75mg/kg for the next experiment to study the effects of different concentrations of exogenous nitrogen on the growth of Trifolium repens under cadmium stress of 75mg/kg concentration.(2)Under cadmium stress(75 mg/kg Cd),exogenous nitrogen at a certain concentration of 0-24 mg / kg could alleviate the growth inhibition of Trifolium repens.Plant growth biomass and chlorophyll content increased with the increase of nitrogen concentration.After low concentration of nitrogen(?12 mg/kg),the cadmium toxicity in leaves was still obvious,the leaves were seriously chlorotic or had a large number of white spots,the plant growth could not return to normal growth level.When the nitrogen concentration was 18 mg/kg and 24 mg/kg,the plant growth returned to normal,and there was no significant difference in growth,biomass and chlorophyll content compared with CK,even better than the control group.Combined with the application of production practice,this experiment selected a relatively low dosage of 18 mg/kg of effective nitrogen concentration to be used in the next experiment to study the effect of nitrogen on growth physiology of Trifolium repens under cadmium stress and its role in alleviating cadmium toxicity.(3)The activities of antioxidant enzymes(POD SOD and CAT)in Trifolium repens were significantly decreased by 75mg/kg sole cadmium treatment.Compared with the control,they decreased 31.71% ?32.91% and 41.59%,respectively.In addition,cadmiumstress inhibited the photosynthetic fluorescence physiology,affected the activity of light system ? reaction center,reduced the photo-binding power,and affected its normal growth.The plant growth and development were promoted by nitrogen treatment alone,but there was no significant difference in the content of the synthesized glutathione(GSH)plant complex PCs to the control group.Application of nitrogen under cadmium stress can alleviate the inhibition of heavy metal cadmium on plant growth physiology,promote the effective utilization of light energy,improve the absorption capacity of cadmium,and increase the content of GSH and PCs.It is indicated that nitrogen plays an important role in the detoxification of heavy metal Cd in Trifolium repens,and sufficient nitrogen supply can not only guarantee the normal growth of plants,eliminate oxidative stress caused by heavy metal cadmium,but also synthesize chelating substances to chelate heavy metal cadmium and reduce the concentration of free cadmium,thus reducing the toxicity of heavy metal cadmium to plants.