| Mirabilis jalapa L.,belonging to the genus Mirabilis,is an annual or perennial ornamental plant,which is easy to grow,highly adaptable and widely grown in Asia,Europe and America with good ornamental value.M.jalapa is also a new plant with high accumulation and tolerance to cadmium（Cd）.In addition,M.jalapa is also an alien invasive plant.Alien invasive plants are usually characterized by high tolerance and resistance,rapid growth and reproductive spread,high biomass,and high competition.The possibility of screening and using some hyperaccumulator alien invasive plants to remediate soil heavy metal pollution,in order to turn harm into benefit,turn waste into treasure.However,when using these alien invasive plants in phytoremediation,we cannot ignore its possible impact on environmental safety.Allelopathic effects are an important type of plant interspecific relationships,and organic acids are an important class of allelopathic substances in plants.Most of the current studies have focused on the allelopathic effects of the extracts of M.jalapa plants on other plants under natural growth.However,there are few reports on whether their roots secrete the allelopathic substance organic acid to produce allelopathic effects on native species under Cd stress and the effect of the secreted organic acid on their own Cd accumulation.In this study,the organic acid species and contents of various plant parts,root secretions and inter-root soil of M.jalapa were determined by hydroponic and potting experiments with different concentrations of Cd.Secondly,to study the effect of root secretion of M.jalapa on seed germination and seedling growth of two native species（lettuce and maize）under stress with different Cd concentrations,and the allelopathic effects of exogenous added organic acids on lettuce and maize were studied,based on the concentration of organic acids in root secretions under different concentrations of Cd stress,and the most potent organic acid species and effect concentrations were obtained.Again,pot experiments in greenhouse was conducted to further verify the effects of sterilized and unsterilized soil planted with M.jalapa on the germination and seedling growth of lettuce and maize seeds at different soil Cd treatment concentrations,and to investigate the extent of the allelopathic effects of soil microorganisms on M.jalapa.Finally,orthogonal soil culture experiments with different exogenous organic acids were conducted to study their effects on Cd accumulation in M.jalapa,and the best organic acid combination to assist Cd accumulation in M.jalapa was screened,which in turn provided a basis for applying alien invasive plants in the phytoremediation practice of contaminated soil.The main findings of the paper show that:1.Oxalic acid and citric acid were detected in the root secretions of M.jalapa under Cd stree both hydroponic and soil culture,in addition,glutaric acid was newly detected in the inter-root soil.Oxalic acid,citric acid and glutaric acid were detected in M.jalapa plants under both cultures.Under the hydroponic condition,oxalic acid content in root secretions of M.jalapa significantly increased under 1～3 mg·L-1 Cd stress,the citric acid content decreased significantly with increasing Cd concentration.Under soil culture conditions,3 mg·kg-1 Cd stress caused the oxalic acid and citric acid contents in the inter-root soil of M.jalapa to be significantly higher than the control,and then gradually decreased with increasing Cd concentration,and the glutaric acid content decreased significantly with increasing Cd concentration.2.Allelopathic effects of root secretions of M.jalapa on lettuce and maize under Cd stress.The strongest inhibition of lettuce by root secretion of M.jalapa under 3mg·L-1 Cd stress,the strongest inhibitory effect on maize was at higher concentrations of 5 mg·L-1Cd stress.The synthesis allelopathic effect index for lettuce was-41.78%and for maize was-18.79%,lettuce was more strongly inhibited than maize.Different allelopathic effects of exogenous addition of organic acids on lettuce and maize,the minimum inhibitory concentrations of oxalic acid for lettuce were 30 mg·L-1 and 7.5mg·L-1 for citric acid,the minimum inhibitory concentration of oxalic acid for maize was 60 mg·L-1 and citric acid was 5.0 mg·L-1.Oxalic acid inhibited lettuce more than maize,while citric acid inhibited maize more than lettuce.3.In unsterilized soil under Cd stress,the effect of planting M.jalapa or not on lettuce and maize was different.Under 5 and 7 mg·kg-1 Cd stress,the germination and fresh weight of lettuce were inhibited compared with that under the same Cd concentration without M.jalapa.For maize,the germination potential and germination index were inhibited in the soil under 1,3 and 7 mg·kg-1 Cd stress,compared with that under the same Cd concentration without M.jalapa,but the root length and plant height of maize in the soil under Cd stress were significantly higher than that under the treatment without M.jalapa.Combined with the synthesis allelopathic effect indexs,under the stress of 0,1 and 3 mg·kg-1 Cd,the synthesis allelopathic effect indexs of the soil planted with M.jalapa to maize was greater than 0,indicating the promoting effect,while the synthesis allelopathic effect indexs of lettuce under the same concentration was less than 0,indicating the inhibiting effect.Moreover,under the stress of 5 and 7mg·kg-1 Cd,the synthesis allelopathic effect indexs of lettuce was-17.95%and-33.31%,and that of maize was-6.22%and-7.67%,indicating that the inhibition effect of lettuce was obviously stronger than that of maize.4.The allelopathic effects on lettuce and maize were different in the soil treated with and without sterilization under Cd stress.In unsterilized soil with Cd concentration of 7mg·kg-1,the germination potential and germination index of lettuce and maize were significantly lower than that of sterilized soil.Under Cd stress,the root length and plant height of lettuce in unsterilized soil that had been planted with M.jalapa were significantly lower than those under sterilization,while for maize,after planting M.jalapa,sterilization or not had no significant effect on its growth.Therefore,when Cd concentration was low,there was no significant difference in allelopathy between the two native species with or without microorganisms,but when there are microorganisms in the soil and Cd concentration is high,the allelopathic effect of M.jalapa on lettuce and maize will be increased.5.An orthogonal test based on the types and concentrations of organic acids secreted by the roots of M.jalapa under Cd stress revealed that only the addition of oxalic acid alone at a concentration of 50 mg·L-1 significantly increased the Cd content in M.jalapa leaves and 2.88 times compared to the control（no addition）treatment.In addition,at this concentration,soil p H decreased significantly and activated Cd in the rhizosphere soil,it promoted the absorption of Cd by the root system of M.jalapa,the increase of chlorophyll a content in the leaves of M.jalapa promoted photosynthesis and enhanced metabolism.However,the toxicity of Cd in the body of M.jalapa was mitigated by the increase of CAT and POD enzyme activities,thus ensuring the normal growth of M.jalapa.Through this series of processes,the increase of Cd accumulation of M.jalapa can be realized.Summarize the above,the root system of M.jalapa will secrete allelopathic substances-organic acids under the stress of Cd,and low concentration of Cd will promote the secretion of organic acids,and the organic acids secreted by M.jalapa will produce allelopathic effects on tested crops.However,combined with the synthesis allelopathic effect indexs,it can be concluded that the allelopathic effects on maize are relatively weak.Therefore,it is suggested that low accumulation of maize and M.jalapa intercropping should be used to achieve restoration and production at the same time,so as to achieve restoration of polluted farmland and safe production of crops.In addition,the concentration of 50 mg·L-1 oxalic acid can promote the accumulation of Cd in M.jalapa,which could be added artificially to speed up the restoration process and reduce the restoration cost during the specific application. |
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