Allelopathic Effects Of Mirabilis Jalapa Alkaloids On Radish And Lettuce Under Cadmium Stres

As an important type of plant interspecies relationship,allelopathy widely exists in natural ecosystems.The alien invasive plant Mirabilis jalapa L.has a strong ability to enrich cadmium（Cd）,if it can be intercropped with native species,it is expected to achieve“repair while producing”Cd-contaminated farmland.However,whether M.jalapa has an allelopathic effect on native species is a key scientific question that must be paid attention to when using M.jalapa for plant restoration.Although there have been many studies on the allelopathic effects of the extracts of various parts of M.jalapa,there are few studies on the allelopathic effects of root exudates and alkaloid allelopathic substances under heavy metal conditions.Therefore,in this study,using M.jalapa as the test plant,firstly,under indoor hydroponics conditions,the effect of adding different concentrations of Cd on the secretion of alkaloids in the roots of M.jalapa was studied,and according to the alkaloid concentration in the root exudates,the exogenous addition of Cd was studied.The effects of the root exudates of M.jalapa and alkaloids on seed germination and seedling growth of two native species（radish and lettuce）were investigated,and the alkaloid species with the strongest allelopathic effect were found.Then,through indoor pot experiments,the effect of the soil that had been planted with nonsterile and sterile treatment on seed germination and seedling growth of native species under the stress of different concentrations of Cd in the soil was further studied,and explored the degree of soil microorganism’s influence on allelopathic effects.Furthermore,to provide a theoretical basis for evaluating the ecological risk of using M.jalapa in Cd-contaminated farmland soil remediation and studying the feasibility of invasive plants in phytoremediation practice.The main findings of the paper are as follows:1.Under the hydroponic condition,trigonelline,berberine,palmatine and jatrorrhizine were detected in the plants and root exudates of M.jalapa.The contents of trigonelline,berberine,and palmatine in root exudates of M.jalapa under 1～3mg·L^-1Cd stress were significantly increased（P<0.05）.Under pot experiments,trigonelline,berberine,palmatine,jatrorrhizine and coptisine were all detected in the plants and rhizosphere soil.The contents of trigonelline and berberine in the rhizosphere soil of M.jalapa under 5 mg?kg^-1Cd stress were significantly increased（P<0.05）.2.Allelopathic effects of M.jalapa root exudates on radish and lettuce under Cd stress.Under the stress of 3 mg·L^-1Cd,the root exudates had the strongest inhibitory effect on seed germination and seedling growth in radish and lettuce,with inhibition rates of 25.17%and 44.23%in seed germination and root length in radish,respectively,and of 10%and 40.44%,respectively,in lettuce.The inhibitory effect of root exudate from radish was higher than that from lettuce.The allelopathic effects of the exogenous alkaloids on the seedling growth of radish and lettuce were different.The lowest inhibitory concentrations of berberine were 0.1 and 0.05 mg·L^-1,showing the highest allelopathic effect.Palmatine and rhizobine showed a moderate allelopathic effect,with a minimum inhibitory concentration of 0.25 mg·L^-1.The weakest allelopathic effect was observed for trigonelline,with minimum inhibitory concentrations of 100 and 50 mg·L^-1.3.The allelopathic effect of M.jalapa-growing soil under Cd stress also differed between non-sterile and sterile soils.Except for 25 mg·kg^-1Cd stress,the non-sterile soil promoted seed germination and seedling growth of radish and lettuce to different degrees,and lettuce showed better promotion.However,an inhibitory effect was observed in sterile soil.Under a stress of 5 mg?kg^-1Cd,the maximum inhibition rates of seed germination and seedling growth in radish were 41.23%and 48.54%,respectively,and those in lettuce were 21.07%and 15.25%,respectively.Except for25 mg?kg^-1Cd stress,radish and lettuce peroxidase（POD）,catalase（CAT）,superoxide dismutase（SOD）activities and the content of proline in nonsterile soils planted with M.jalapa was significantly increased in different degrees,and the content of malondialdehyde（MDA）was significantly decreased;In the sterile soil of M.jalapa,the activities of POD,CAT,SOD and proline content of the two plants were significantly reduced to varying degrees,malondialdehyde content in radish and lettuce increased significantly under 5～25 mg?kg^-1Cd stress.To sum up,under Cd stress conditions,the allelopathic effects of M.jalapa on the local species in hydroponic and pot experiments differed,and soil culture and non-sterile conditions mitigated the allelopathic effects of alkaloids.Moreover,lettuce is a suitable species for intercropping with M.jalapa for the remediation of Cd-polluted agricultural soil owing to its low allelopathic inhibition.