Effects Of Different Forms Of Zinc On Cadmium Uptake And Accumulation In Wheat

In recent years,due to the development of industry and human activities,heavy metal pollution of farmland soil has become increasingly serious.Wheat is one of the main food crops of human beings.Long-term consumption of wheat contaminated by heavy metal cadmium(Cd)will affect human health and lead to renal dysfunction and multiple organ damage.Zinc(Zn)and Cd have very similar chemical properties and can interact by competing for adsorption sites on the soil.When Zn and Cd coexist,Zn can significantly reduce the adsorption of Cd by soil minerals.As an essential nutrient element for plant growth and development,Zn is a component and activator of many enzymes.Zn plays an important role in reducing the absorption of Cd by plants and reducing the toxicity of Cd.However,the effect of Zn on the bioavailability of Cd is not consistent so far.The interaction between Zn and Cd is related to plant genotype,ecotype,metal ion concentration and morphology.In this study,two wheat varieties Zhoumai 12797(Z797)and Suimai 1216(S216)with different genotypes were selected and two zinc sources(Zn SO4·7H2O and Zn O NPs)were set up.Hydroponics and pot experiments were conducted to reveal the mechanism of Zn in different forms on different genotypes of wheat under Cd stress.In order to provide theoretical guidance and technical support for Cd toxicity in wheat.1.Zn application significantly affected the growth,development and photosynthesis of wheat under Cd stress.Under Cd stress,low Zn(2.5μM)application significantly decreased root dry weight,while high Zn application significantly increased plant height and biomass,alleviating Cd toxicity.The root length,root surface area and photosynthetic pigment content of the two wheat varieties were decreased under low and high concentrations of Zn SO4(2.5 μM and 25 μM).However,high concentrations of Zn SO4(25 μM)promoted the photosynthetic parameters of Z797 under Cd stress,but inhibited the photosynthetic parameters of S216 under the stress of Cd.The application of low concentration Zn O NPs(2.5 μM)significantly increased the number of root tips,photosynthetic pigment contents(chlorophyll a,chlorophyll b,carotenoid and total chlorophyll),photosynthetic rate and stomatal conductance of two wheat varieties.The application of high concentration Zn O NPs(25 μM)significantly increased the number of root tips and photosynthetic rate of the two wheat varieties by38.3%~41.8% and 76.7%~105.0%,respectively.Therefore,the application of Zn can alleviate the toxic stress of Cd on wheat,but attention should be paid to controlling the dosage to prevent excessive application of Zn.2.The application of Zn significantly affected the absorption,transport and accumulation of Cd and Zn in wheat under Cd stress.The application of high concentration of Zn(25 μM)significantly decreased the Cd content and accumulation in roots of Z797 and S216 by 42.3%~56.9% and 49.5%~58.2%.The application of low concentration of Zn(2.5 μM)significantly increased the content and accumulation of Cd in the above-ground parts of the two wheat varieties by 33.8%~107.5% and39.1%~131.7%.The application of Zn significantly promoted the transfer of Cd from wheat roots to aboveground parts,and the transfer coefficient of Cd increased with the increase of Zn application amount.At the same time,Zn application significantly increased the Zn content and accumulation in wheat roots and aboveground,and the Zn transport coefficient decreased with the increase of Zn application.3.The application of Zn significantly affected the distribution proportion and ultrastructure of Cd and Zn in wheat subcells under Cd stress.The application of Zn reduced the proportion of Cd in the cell wall of root cells in Z797,but the proportion of Cd in the cell wall of middle and upper ground cells in Z797 showed an increasing trend.Zn application increased the percentage of Cd in root cell wall in S216.The application of Zn increased the proportion of Cd in soluble component of root cells in Z797,while the soluble component of root cells in S216 showed the opposite result.The application of Zn increased the proportion of Zn in soluble components of roots and aboveground cells,but decreased the proportion of Zn in roots and aboveground cell walls and organelles.Scanning electron microscopy(SEM)results showed that,compared with CK,low Zn(2.5 μM)treatment significantly affected the root cell structure of the two wheat varieties,resulting in interference of nuclear materials,loss of nucleolar and thickening of cell walls.High Zn treatment alleviated the toxicity of Cd and maintained the relative integrity of the ultrastructure of root cells of two wheat seedlings.Meanwhile,compared with Zn SO4(25 μM)treatment,the structure of root cells treated with high Zn O NPs(25 μM)is more complete and regular,indicating that the addition of Zn O NPs can reduce the effect of oxidative damage on the cell structure to a greater extent.4.The application of Zn significantly affected the expressions of Cd and Zn transporter genes and the contents of trace elements in wheat under Cd stress.The application of Zn SO4 and Zn O NPs significantly increased the expression of Ta TM20 in the two wheat genotypes by 37.9%~231.7%,and significantly decreased the expression of Ta ZIP3,Ta ZIP5 and Ta ZIP7 in the two wheat genotypes by 67.4%~90.2%.After the application of Zn SO4,the expression of Ta Nramp5 in wheat roots was up-regulated,and after the application of Zn O NPs,the expression level of Ta HMA3 in wheat roots was up-regulated.The application of Zn increased the contents of Fe and Cu in the roots of the two wheat varieties,but significantly decreased the contents of Mn in the roots.With the increase of Zn SO4 and Zn O NPs concentrations,the contents of Fe,Mn and Cu in the aboveground part of Z797 showed an obvious decreasing trend,while the application of low concentration Zn SO4 and Zn O NPs(2.5 μM)significantly increased the contents of metal elements in the aboveground part of S216.The results of high concentration Zn SO4 and Zn O NPs(25μM)are contrary to that of Zn O NPs.5.Pot experiment showed that Zn application significantly increased the content of available Zn and decreased the content of available Cd in soil.Foliar application of Zn significantly reduced the absorption and transport of Cd by S797,but promoted the absorption and transport of Cd by Z797.Soil application of Zn reduced the absorption of Cd by wheat,increased the absorption of Zn,and reduced the redistribution of Cd from cob to grain.Compared with spraying,soil application of Zn can more effectively reduce wheat grain Cd content,and the effect of Zn O NPs is significantly better than Zn SO4,especially Zn O NPs at high concentration.