Allelopathic Effect Of Phenolic Acids In The Litters Of Mirabilis Jalapa L. Under Cadmium Stress

With the development of the chemical industry and non-ferrous metal smelting,China’s soil is seriously polluted by heavy metals,especially cadmium（Cd）.Cadmium pollution has become a very serious environmental problem,and its diffusion,translocation and dilution in soil is slow and accumulation in humans,animals and plants via food chain,endangering human health and food safety.Therefore,the remediation of Cd-contaminated soils is a matter of urgency.There are many remediation methods for Cd-contaminated soil,but phytoremediation technique has received closed attention due to its green,low cost,good efficiency and low-risk advantages.Yunnan is located in the border,and the biological invasion is serious.The alien invasive species will lead to the loss of biodiversity and the deterioration of the global ecological environment.The use of alien invasive species has the advantages of large biomass,strong resistance,and rapid growth and reproduction to repair heavy metal contaminated soil.The ecological risk of alien invasive plants is the biggest problem in phytoremediation.The application of Mirabilis jalapa L.in the remediation of Cd-contaminated soil should take into account its allelopathy on local species.Phenolic acid is an important allelochemical and the main reason for continuous cropping obstacles.Exploring the allelopathy of Mirabilis jalapa L.on local species under Cd stress provides a theoretical basis for assessing the ecological risk of exotic invasive plants in repairing heavy metal contaminated soil.Mirabilis jalapa L.has a strong ability to accumulate cadmium（Cd）,but it is also an invasive species.Whether it will produce allelopathic effects on native species under Cd stress is a question that must be answered when Mirabilis jalapa L.is applied to phytoremediation practice.To investigate whether phenolic acids in Mirabilis jalapa L.have an allelopathic effect on the native species under Cd stress.Firstly,ultra-performance liquid chromatography（UPLC）was used to determine the types and contents of phenolic acids in the resh leaf aqueous extracts,leave litters aqueous extracts,natural decomposition liquids and artificial decomposition liquids of leaf litters from Mirabilis jalapa L.under different concentrations of Cd(0,1,4,7,11 mg·kg^-1)stress.And explore the changes of types and contents of phenolic acids before and after the fresh leaves of Mirabilis jalapa L.Wither（resh leaf aqueous extracts,leave litters aqueous extracts）;before and after the decomposition of leaves litter（leave litters aqueous extracts and artificial decomposition liquids）.Secondly,add the collected two kinds of extracts and two kinds of decomposition liquids to the local species,and explore the effects of the four treatment solutions on the seed germination and seedling growth of the local species.Through the comprehensive allelopathic intensity and average sensitivity index of four treatment solutions on the germination and seedling growth of local species,the allelopathic inhibition effect of four treatment solutions on local species was determined.The four phenolic acid allelochemicals（chlorogenic acid,p-hydroxybenzoic acid,vanillic acid,ferulic acid）with the highest content detected in the four treatment solutions were exogenously added to the native species at different mass concentration gradients(0,0.1,1,10,50,100 mg·L^-1)to confirm the allelopathic effect of phenolic acids on the native species and screen out the phenolic acids with the strongest allelopathic effect.The main research results are as follows:1.The content of phenolic acid in different parts of Mirabilis jalapa L.under different Cd concentrations have significant different.The concentrations of phenolic acids showed a significant increase（P<0.05）at low Cd treatments(1,4 mg·kg^-1),it indicates that low concentration of Cd may stimulate the release of phenolic acids.The content of total phenolic acid in different tissues of Mirabilis jalapa L.is leaf>stem>root.Under Cd stress,phenolic acid is still released after the leaves withered,six phenolic acids,namely caffeic acid,chlorogenic acid,p-hydroxybenzoic acid,ferulic acid,vanillic acid,and cinnamic acid were detected in the aqueous extracts and decomposition liquids of leaf from Mirabilis jalapa L.The content of phenolic acids in the fresh leaves extract of Mirabilis jalapa L.is higher than that in the leaf litters extract,phenolic acids content in natural decomposition liquids is higher than that in artificial decomposition liquids.Low Cd concentrations significantly promoted the release of phenolic acids in Mirabilis jalapa L.,while high Cd concentrations reduced the release of phenolic acids.This suggests that plant fade or residue decay of Mirabilis jalapa L.were the main pathways for the release of phenolic acids.2.Compared with adding Cd in two kinds of extracts or two kinds of decomposition liquids alone,the double stress of Cd+phenolic acids has stronger inhibition on lettuce and pakchoi.Under various Cd concentrations,the allelopathic inhibition of extracts or decomposition liquids on pakchoi was greater than that on lettuce.under the same Cd concentration treatment for lettuce and pakchoi,the inhibitory effect of fresh leaf extract was greater than that of leaf litters extract,the allelopathic inhibition of natural decomposition liquids is stronger than that of artificial decomposition liquids.During the germination and growth period of lettuce and pakchoi,the inhibitory effect of two extracts or two decomposition liquids on root length was higher than that of seedling height;The allelopathic inhibition of seedling growth was greater than that of seed germination.3.The exogenous addition of four phenolic acids had different allelopathic effects on lettuce.The lowest inhibitory concentrations of p-hydroxybenzoic acid,vanillic acid,ferulic acid and chlorogenic acid on seed germination were 1,10,10,and 10 mg·L^-1,and the lowest inhibitory concentrations on seedling growth were 0.1,1,1 and 10 mg·L^-1,respectively.The order of the inhibitory effects of the four phenolic acids on lettuce seed germination and seedling growth followed the order of vanillic acid>p-hydroxybenzoic acid>chlorogenic acid>ferulic acid.The inhibition of seedling growth was greater than that of seed germination,and the inhibition of root length was more pronounced than that of seedling height.The exogenous addition of four phenolic acids had different allelopathic effects on pakchoi.The lowest inhibitory concentrations of four phenolic acids on seed germination were 10 mg·L^-1.The minimum inhibitory concentration of vanillic acid on seedling growth was 1 mg·L^-1,and the minimum inhibitory concentration of other three phenolic acids on seedling growth were 10mg·L^-1.The order of the inhibitory effects of the four phenolic acids on pakchoi seed germination followed the order of vanillic acid>p-hydroxybenzoic acid>ferulic acid>chlorogenic acid,the inhibition effect on seeding of pakchoi followed the order of p-hydroxybenzoic acid>vanillic acid>chlorogenic acid>ferulic acid.The inhibition of seedling growth was greater than that of seed germination,and the inhibition of root length was more pronounced than that of seedling height.The allelopathic inhibition of four phenolic acids on pakchoi was greater than that on lettuce.Therefore,the allelopathic inhibition of lettuce was smaller than that of pakchoi when Mirabilis jalapa L.and local species were used for intercropping remediation of cadmium contaminated farmland soil,it is a more suitable intercropping type.4.The main factors affecting the release of phenolic acid are p H>decomposition time>decomposition temperature,the combination with p H of 5,decomposition time of 30 days and decomposition temperature of 35℃is the combination with the largest amount of phenolic acids release.In summary,the release of phenolic acids from Mirabilis jalapa L.under Cd stress showed a trend of“low promotion and high inhibition”;Phenolic acids also showed same trend on the germination and growth of native species.The phenolic acids in the extracts and decomposing liquids of Mirabilis jalapa L.leaves under Cd stress had allelopathic inhibitory effects on native species,low Cd concentration stimulated the release of phenolic acids,and the allelopathic inhibitory effect was strong,the double stress of Cd+phenolic acid was stronger than that of Cd alone.It was proved that the inhibitory effect of four treatment solutions on native species under Cd stress was mainly the allelopathic effect of phenolic.The results provide a suitable intercropping native species for the application of Mirabilis jalapa L.in Cd-contaminated soil.It provides a theoretical basis for evaluating the ecological risk of using invasive plants to restore heavy metal contaminated soil.