Effect Of Zinc On Cadmium Absorption And Accumulation Characteristics Of Wheat And Its Mechanism

Recently,farmland soil was contaminated by heavy metal,and become increasingly serious,especially for excessive cadmium(Cd)content in agricultural products.Studies have shown that Zn can alleviate the toxicity caused by Cd,and meanwhile increase the content of Zn in crops,but the mechanism by which Zn could alleviate the toxicity induced by Cd in wheat was still unclear.In recent decades,the problem of excessive Cd content in wheat heavy metals has attracted most public attention.Many studies have shown that screening the wheat varieties with low accumulation of Cd could reduce the accumulation of heavy metal Cd in edible parts,but did not always meet the Cd limit in Food Security Standards.It requires a combination with other measures to enhance this effect.Therefore,this study conducted by hydroponics-and pot-method to explore how exogenous Zn could reduce Cd uptake and increase Zn content in grains in two wheat varieties differing in Cd accumulation.Growth status,photosynthesis,root morphology,root Cd2+ flux,antioxidant enzyme activity,Cd sub-cellular distribution and chemical forms of wheat under Cd stress were determined.The transport of Cd and Zn within the seedlings,during heading stage,grouting stage and mature period were explored,in order to provide a theoretical basis for the remediation of Cd contaminated farmland.The results of this experiment are listed as follows:1.Cd stress can reduce the biomass of wheat and cause toxic effects on wheat growth.Applying a certain amount of Zn(50 ?M)can alleviate biomass reduction caused by Cd.It significantly relieved the inhibition of Cd on wheat root growth and enhanced the photosynthesis,but Zn(100 ?M)decreased the photosynthetic rate of high-accumulation wheat varieties(H27),while have no effect on the low accumulated wheat varieties(L979).Results showed that the application of Zn could alleviate the inhibition of wheat roots by Cd,also increased the biomass of wheat,and enhanced its photosynthetic capacity.However,attention should be paid to the application amount of Zn for wheat with different Cd accumulation characteristics.2.Zn(50 ?M)treatment significantly reduced the absorption rate of Cd2+by the two wheat roots.And the content of Cd in roots and shoots of wheat decreased.Zn(50?M)significantly reduced the concentration of Cd in the above-ground of wheat H27,but not for L979.In addition,the mobility coefficient of Cd in H27 increased significantly with the addition of Zn.L979 showed a trend of an increase and then decreased.Therefore,for Cd high accumulation wheat,attention should be paid to the amount of Zn applied.For Cd low accumulation wheat,the amount of Zn should be appropriately increased.3.For sub-cellular distribution of Cd in wheat,it is mainly distributed in the cell wall and soluble fraction,and the distribution in cell organelle fraction is relatively low.The Cd in three fractions of the two wheat varieties decreased significantly with the addition of Zn.Cd in wheat leaves was mainly bounded to cell wall,followed by in cell fluid;while the Cd content in roots was mainly distributed in soluble fraction,followed by in cell wall.For chemical form,the addition of Zn significantly reduced the content of alcohol-extracted Cd,meanwhile increased the proportion of sodium chloride and acetic acid extracted Cd.The percentage of Cd extracted by acetic acid was increased in leaves of L979.The proportion of Cd in alcohol extracted decreased and that in sodium chloride extracted increased.Therefore,exogenous Zn can reduce the content of sub-cellular Cd in two wheat,increase the proportion of Cd in cell wall and soluble fraction,enhance the tolerance of cells to Cd,and Cd could be converted from a more active form to a less active form.4.Cd stress significantly increased the content of malondialdehyde(MDA)and hydrogen peroxide(H2O2),and decreased the activities of antioxidant enzymes SOD,POD and CAT.The addition of Zn(50 ?M)significantly reduced the content of MDA and H2O2 and enhanced the activities of antioxidant enzymes SOD,POD and CAT.In the treatment of Zn(100 ?M),except for the CAT enzyme activity in H27,the other enzyme activities did not change,and SOD and POD activities in L979 continued to increase.Results showed that the addition of Zn could significantly enhance the activity of wheat antioxidant enzymes,reduce the content of lipid peroxidation products,and improve the tolerance of wheat to Cd,but the application of Zn and enzyme activity for different Cd accumulation varieties is different.5.Pot-experiment showed that the SPAD value and biomass of wheat increased significantly with the addition of Zn,At different growth stages,Zn showed antagonistic or synergistic effect for the absorption of Cd in wheat.The addition of proper amount of Zn in Zn-deficient soil contributed to wheat growth and reduced Cd absorption.However,with the increase of Zn level in medium,Cd absorption firstly increases but then decreases.This indicates that when Zn is applied to wheat,attention should be paid to the content of Zn in the soil.The addition of Zn in the soil can help the wheat growth to reduce Cd absorption.