The Mechanism And Application Of Azospirillum Brasilense And Hemin In Resistancing The Accumulation Of Cd In Brassica Chinensis L.

Vegetables provide an important component of mineral nutrition for humans.The substances and phytochemicals contained in healthy vegetables are closely related to food safety.However,industrialization and urbanization,as well as the widespread use of agrochemicals,have led to increased emissions of heavy metals in the air and soil,thus allowing excess trace elements and heavy metals to be enriched in humans through the food chain.Heavy metals have the characteristics of toxicity,persistence and bioaccumulation.In these types of agroecological zones,vegetable planting poses a serious threat to soil function,agricultural production and human health.Compared to other food crops,leafy vegetables can accumulate heavy metals excessively,and consumption of leafy vegetables contaminated with heavy metals is the main pathway of heavy metal enrichment in humans.In general,70%of the total cadmium（Cd）accumulated by humans comes from leafy vegetables.Therefore,it is a vital task to reduce the excessive amount of toxic heavy metals in leafy vegetables.In this study,the leafy vegetable,Brassica chinensis L.,which was widely distributed in China,was selected.We investigated the effects of different treatment modes of Azospirillum brasilense and hemin on the growth,Cd accumulation and physiological characteristics of Brassica chinensis L.in Cd-contaminated soil,and revealed the mechanisms of the effects of different treatments on the alleviation of Cd toxicity in plant.The main conclusions were as follows.1.With the increase of Cd stress,the biomass of shoot and root parts of Brassica chinensis L.gradually decreased,and the greatest decrease was observed at 10 mg kg^-1 of Cd stress,in which the shoot part biomass and root biomass decreased by 59.44%and50.73%,respectively（P<0.05）.2.Inoculation with Azospirillum brasilense,application of hemin,and both together were effective in increasing shoot and root biomass of plants subjected to Cd stress;their shoot biomass increased by 29.10%,25.40%,and 32.56%under the three different treatments at 10 mg kg^-1 Cd,respectively.Compared with the uncontaminated soil,the tolerance of plants shoots and root decreased under Cd-contaminated,and the tolerance of root was stronger than that of shoot.Compared with the control,the tolerance of Brassica chinensis L.was the highest under 3 mg kg^-1 Cd,and its shoot tolerance was further enhanced by the application of hemin,reaching 82.24%;the tolerance of roots was most significantly enhanced by the co-application of Azospirillum brasilense and hemin,reaching 105%.3.With the increase of Cd stress,the chlorophyll content of Brassica chinensis L.leaves gradually decreased.Inoculation with Azospirillum brasilense,application of hemin and co-application significantly increased the chlorophyll content of plants.In the 5 mg kg^-1 Cd,the application of hemin treatment significantly increased the chlorophyll content of Brassica chinensis L.leaves by 17.93%compared with the unapplied treatment.In contrast,in 10 mg kg^-1 Cd-contaminated soil,the application of hemin and inoculation with the Azospirillum brasilense and hemin significantly increased the chlorophyll content of plants by 18.16%and 18.09%,respectively.4.The MDA,O₂·-and H₂O₂ contents of plants increased in Cd stress.Inoculation with Azospirillum brasilense,application of hemin and both together significantly increased the activities of antioxidant enzymes such as POD and SOD in plants for scavenging excess reactive oxygen species（ROS）in plants as a way to alleviate oxidative damage caused by Cd stress and promote plant growth.