Effects Of Lead And Cadmium Interaction And Its Bioavailability On Rice Growth

In China,the problem of heavy metal pollution in soil is becoming increasingly serious and cannot be ignored.As the most important food crop in China,rice is mainly distributed in the south of the Yangtze River where Pb and Cd compound pollution is the main source.Pb and Cd in the soil can affect the normal growth of plants,and can accumulate in human and animals through the food chain,seriously threatening human production and life.In this study,rice was used as the object to analyze the dose-effect relationship between single and combined treatments of Pb and Cd and rice root elongation,predicted the interaction and combined toxicity of Pb and Cd with different concentration combinations by using the concentration addition model（CA）and independent action model（IA）,and analysis of biomass,growth indicators,physiological indicators and Pb and Cd enrichment and transport characteristics of rice during heading and mature stages of single and compound pollution treatment of Pb and Cd,in order to prove the interaction,biological effect and influence of Pb and Cd composite treatment on rice growth in soil environment.The main findings are as follows:（1）The dose-response curve of rice root elongation showed that the interaction of low concentration Pb and Cd was mainly antagonistic,the combined toxicity of Pb and Cd was weakened,and the high concentration was synergistic,and the combined toxicity was enhanced.In the hydroponic test,the interaction of 0～0.05 mg/L Cd and 0～0.6 mg/L Pb mainly showed antagonism,while Cd>0.05 mg/L and Pb>0.6 mg/L mainly showed synergy;With 0.1,0.3 mg/L Cd was added,and the toxicity threshold EC₅₀-Pb was increased from0.112 mg/L to 0.550 and 0.950 mg/L,respectively,while EC₁₀-Pb was increased from 0.004mg/L to 0.039,0.370 mg/L;Toxicity threshold under single treatment of Cd,with the addition of 0.6 mg/L Pb,EC₅₀-Cd and EC₁₀-Cd increased from 0.020 and 0.0002 mg/L to0.206 and 0.029 mg/L,but 80 mg/L Pb was added to decrease EC₅₀-Cd to 0.002 mg/L and increase EC₁₀-Cd to 0.0004 mg/L;CA model could simulate the interaction of Pb and Cd than IA model more accurately.（2）The interaction of Pb and Cd showed different degrees of influence on the physiological and biochemical indexes of rice during heading and mature stages.The interaction of different concentrations of Pb and Cd had no significant effect on the plant height and fresh weight of shoots at the heading and mature stages（P>0.05）;The interaction of 0.3,0.6 mg/kg Cd and 125,250 mg/kg Pb caused the fresh weight of roots,root length,root surface area and average root diameter to show an increasing trend compared to single treatment.Photosynthetic indicators such as chlorophyll content,net photosynthetic rate,stomatal conductance,transpiration rate,and intercellular concentration of CO₂ all increased to different degrees compared with single treatment.The interaction of Pb and Cd mainly showed antagonism,and the toxic effect on rice growth and development was weakened;However,the combined treatment of 0.6 mg/kg Cd and 250 mg/kg Pb reduced the number of root tips,bifurcations and total root length of rice,and the interaction of high concentrations of Pb and Cd adversely affected the growth of rice at later stages.（3）The contents of Pb and Cd in rice plants were positively correlated with the contents of Pb and Cd in soil.Pb content of rice organs:root>leaf>stalk>rice husk>brown rice,Cd content:root>stem>leaf>rice husk>brown rice.Compared with the single treatment of Pb and Cd,the contents of Pb and Cd in various parts of rice under the combined treatment of Pb and Cd showed different degrees of reduction,of which（0.3,125）,（0.6,125）and（0.3,250）treatments caused Pb in brown rice.Compared with the single treatment of Pb,the contents were significantly reduced by 57.86%,63.38%,46.94%,but there was no significant change under（0.6,250）,78%of the Pb content in the grain exceeded the national food safety standard value of 0.2 mg/kg;（0.3,250）,（0.6,125）and（0.6,250）treatments significantly reduced the Cd content in brown rice by 56.8%,24.77%and 45.68%,respectively,compared to the single treatment of Cd,while there was no significant change under the（0.3,125）,The Cd content in the grains was less than the national food safety standard value of 0.2 mg/kg（GB 2762-2017）.It showed that the addition of Cd would inhibit the enrichment of Pb in the edible parts of rice,and the addition of Pb would also inhibit the absorption of Cd in rice grains.The interaction of Pb and Cd in the accumulation of Pb and Cd in various organs of rice was mainly antagonistic.（4）The enrichment and transport coefficients of various parts of rice were BCF_Cd/TF_Cd>BCF_Pb/TF_Pb.The enrichment and transport capacity of Pb in various parts of rice:root>leaf>stalk>rice husk>brown rice,and Cd:root>stem>leaf>Rice husk>brown rice.The Cd and Pb enrichment coefficients of rice root,stem,leaf,rice husk and brown rice were26.756～101.592,4.656～9.893,1.311～3.457,0.082～0.346,0.061～0.408,0.928～3.006,0.062～0.363,0.334～0.850,0.035～0.270,0.001～0.011;Cd and Pb transfer coefficients of rice stem,leaf,husk and brown rice were 0.077～0.371,0.030～0.112,0.001～0.013,0.0007～0.009,0.066～0.146,0.167～0.438,0.015～0.239,0.0006～0.010,indicating that Cd was more likely to accumulate and migrate in rice than Pb.Pb and Cd in soil were absorbed into the rice by the roots,and were mainly distributed in the roots,followed by stems and leaves,Pb and Cd contents in the grains were the least.Pb and Cd interaction reduced the enrichment and transport of Pb and Cd in most organs of rice compared with the single treatment.The treatments of（0.3,125）,（0.6,125）,（0.3,250）and（0.6,250）caused the Pb enrichment coefficients in rice roots,stems,leaves,husks and brown rice were reduced by7.49%～50%,7.09%～75%,7.16%～33.33%and 17.63%～30.16%,respectively,while the Cd enrichment coefficients compared with Cd single treatment,it decreased by 1.36%,6.33%～19.65%,15.52%～57.64%and 11.98%～43.95%;meanwhile,Pb transport coefficients respectively compared with single treatment decreased by 23.81%～56.54%,52.38%～59.20%,4.35%～49.86%and 1.53%～34.78%,and the Cd transfer coefficients increased by 49.07%～83.36%compared with the single treatment under（0.3,125）treatment,other treatments decreased by 16.44%～27.21%,52.24%and 21.02%～49.85%;the addition of Pb inhibited the degree of bioconcentration and transport of Cd in rice,while the addition of Cd also inhibited the bioconcentration and transport capacity of Pb in rice.The interaction of Pb and Cd in the cumulative transport of Pb and Cd in rice was mainly antagonistic,which reduced the bioavailability of Pb and Cd,reduced the accumulation of Pb and Cd in rice,and thus alleviated the toxicity to rice.