The Enhancing Tolerance Mechanism Of Maize To Pb/Cd Stress And Root Regulation By Streptomyces Rochei

The farmland and some surrounding industrialization area have different degrees of heavey metal Pb/Cd pollution,due to the rapid development of urbanization and industrialization,the indiscriminate accumulation of mining,smelting,and other wastes,as well as agricultural production such as fertilizer and sewage irrigation.The polluted soil accounts for 16.1%of total area,among which low and middle-degree farmland pollution accounts for 93.17%of the polluted soil.Due to non-degradation and strong migration,Pb/Cd can reduce the soil enzyme and microbial activities lead to affecting the growth and development of crops,which is not conducive to the growth of root crops.Even the higher enrichment in edible parts of crops deteriorate food safety and other problems.The streptomyces has some functions such as improving plant growth,biological controlling,and soil improvement.However,there is less in the remediation farmland contaminated with low or medium levels of heavy metals Pb/Cd,as well as in food safety production.It has significant theoretical and practical significance to use the Streptomyces improving contaminated soils.In this dissertation,we used the Streptomyces rochei（Str.）multifunctional screened from an extreme environment as materials,and used hydroponic experiments to research the effects of Streptomyces on the physiological and biochemical characteristics of maize under Pb/Cd stress,as well as the transport mechanism of Pb and Cd in maize.The main research results are as follows:（1）The maximum tolerance of the Streptomyces rochei（Str.）to Pb/Cd reached 1200mg L^-1 and 20 mg·L^-1.There exist amino acids and their secondary metabolites,carbohydrates,enzymes,and vitamins in the fermentation broth of Streptomyces.Therefore,the streptomyces strains not only has strong tolerance to Pb/Cd,but also has plant growth promoting properties.（2）Hydroponic experiments were conducted to investigate the effects of Streptomyces on the growth promotion and stress resistance of maize under Pb and Cd stress.The results showed that:（1）Inoculation of Streptomyces could significantly promote the growth of two maize varieties under Pb and Cd stress（P<0.05）.Under Pb stress,plant height,biomass,total root length and leaf area of Zhengdan 958 increased by15.64%～168.9%,49.08%～162.03%,12.05%～133.43%and 98.17%～124.34%,respectively.The price of Huanong 658 increased by 10.7%～153%,25%～129.02%,48.07%～52.43%,and 25.51%～134.40%,respectively.Under Cd stress,the dry weight and leaf area of Zhengdan 958 increased by 27.17%～46.34%and 28.22%～64.52%,respectively.The dry weight and leaf area of Huanong 658 increased by 26.92%～122%and 10.60～62.48%.（2）Adding Streptomyces could significantly improve the physiological and biochemical characteristics of maize resistance（P<0.05）.Under Pb stress,the root activity,proline and POD of Zhengdan 958 were significantly increased by 100%～220%,6.81～70.62%and 8.54%～93.86%,respectively（P<0.05）.The root activity,proline and SOD of Huanong 658 were increased by 120%～200%,2.65%～16.24%and 4.24%～144.62%,respectively.Under Cd stress,the root activity and proline of Zhengdan 958 increased by 177.6%～289%and 5%～159.53%,and that of Huanong 658increased by 105%-360%and 40%～167.47%.The result shows that:the growth of maize under Pb and Cd stresscould be inproved through inoculating Streptomyces,such as increasing root activity and antioxidant enzyme active,thus promote maize growth and high biomass.（3）Streptomyces could reduce the absorbption of Pb and Cd by maize,which is attributed to changing the existing forms of Pb and Cd,increasing the content of Pb and Cd in vacuoles,changing the transport pathways of Pb and Cd in the root system,and reducing the toxic effects of Pb and Cd on maize roots.（4）Under Pb and Cd stress,streptomyces reduces the levels of fulvic acid,lysine,soluble microbial byproducts,humic acid,and tryptophan in maiz root exudates.Among them,the root exudates are mainly zeaxanthin(C₄₀H₅₆O₂),dodecylcyclohexane(C₁₂H₃₆O₆Si₆),and 4-methyl-6-tert-butylphenol(C₂₃H₃₂O₂)under Cd stress;under Pb stress,they are 5-methyl-1,10-phenanthroline(C₁₃H₁₀N₂)and dispersed yellow C-4G(C₈H₁₀N₂).That materilas and mechanism reduce the bioavailability of Pb and Cd.