The Screening Of Cadmium Barrier Agents For Wheat Sprying And The Mechanism Research Of Mannose On Reducing Cadmium Accumulation In Wheat(Triticum Aestivum L.)

Cadmium(Cd),is one of the most toxic heavy metals which harm is extremely serious.Excessive Cd in the soil will inhibit the normal growth and development of plants.The residue of Cd in the edible part will enter the human body through the food chain,causing great harm to human health.Varying degrees Cd pollution exists all over our country,Cd pollution of soil area has reached 2 x 105 km2,accounting for 1/6 of the total farmland area.Wheat(Triticum aestivum l.)is one of the most important grain crops in the world.It is the third largest grain crop globally after corn and rice,with an area of about 218 million hectares,ranking the second in dietary intake.Spraying barrier material to reduce Cd accumulation in wheat is quite an important measure.In this study,pot and field methods were used to screen out three reagents that could effectively reduce the content of Cd in wheat seeds,then the effects of single spraying and compound spraying on the transport and bioconcentration of Cd in various parts of wheat,the quality of wheat and the content of other elements were studied.Finally,the mechanism of Cd barrier in wheat was studied by hydroponic experiment.The main research results are as follows:1、Wheat growing in Cd contaminated soil accumulated a large amount of Cd in its seeds.Under 23 kinds of spraying treatment,the content of Cd in the wheat grain in both pot and field can be significantly reduced,including mannose,MgSO4 and MnSO4.In the pot,the wheat seeds decreased by 25.6%,25.0%and 26.2%respectively.In the field,the wheat seeds were reduced by 21.4%,24.8%and 19.8%,respectively.2、Foliar sprayong with mannose+MgSO4 and mannose+MnSO4 reduced the content of Cd in wheat grain of pot by 42.3%and 43.6%respectively,the reduction rate is higher than that of individual treatment.Foliar sprayong with mannose+MgSO4 and mannose+MnSO4 reduced the content of Cd in wheat grain of field by 42.3%and 43.6%respectively.3、After spraying mannose+MgSO4 and mannose+MnSO4 at heading、booting and filling stage respectively,it was found that the decrease of Cd in wheat grain by spraying treatment at different stages was different.In addition,the Cd lowering effect of the two compound spraying reagents in different growth periods was relatively consistent,which was successively:filling stage>heading stage>booting stage.4、The values of TFroot-stem、TFstem-leaf、TFleaf-grain、BCFstems、BCFleaves and BCFgrain all decreased to different degrees under separate spraying and compound spraying,and the values of TFleaf-gain was significantly reduced under all treatments.Treatment with mannose,mannose+magnesium sulfate and mannose+manganese sulfate increased BCFRoot to varying degrees,among which the increase reached a significant level when treated with mannose alone,indicating that mannose spraying could increase the accumulation of Cd in the underground part of wheat.5、Separate spraying of mannose,MgSO4、MnSO4 and compound spraying of mannose+MgSO4 and mannose+MnSO4 can increase the 1000-grain weight of pot and field wheat,but there is no significant change.The gluten value and crude protein content of wheat in pot and field under all treatments increased.The results of pot and field experiments showed that Mannose as a barrier agent could effectively reduce the content of Cd in wheat seeds.Compared with MgSO4 and MnSO4,previous studies paid less attention to mannose.At present,the specific mechanism of mannose reducing the content of Cd in wheat remains unclear.Then,a hydroponic experiment was carried out to examine the effect of mannose(0,40,80,160,320,640μmol·L-1)on the growth index,root morphology and Cd content of wheat seedlings(Triticum aestivum L.)under Cd stress(20μmol·L-1 of CdCl2)was studied.The optimal concentration of 160μmol·L-1 was selected to further study the effect of foliar spraying of mannose on the subcellular distribution,cell wall components and chemical forms of Cd in wheat root.The main research results are as follows:1、Cd stress severely inhibited the growth and development of wheat seedlings.Compared with the control,plant height,root length,dry and fresh weight of root and shoot decreased significantly,and a large amount of Cd was accumulated in bothroot and shoot.With the application of mannose with different concentrations except for the maximum concentration of 640 μmol·L-1,the root length,the dry and fresh weight of the root,and the Cd content in the root all increased significantly compared with that of the Cd control group,and the Cd content in the shoot decreased to different degrees.When the concentration was 160 μmol·L-1,the effect was the best,significantly alleviating the damage of Cd stress on the growth of wheat seedlings,and the Cd content in the shoot decreased the most.160 μmol·L-1 mannose significantly reduced the chlorophyll content,net photosynthetic rate(Pn)、stomatal conductance(Gs)and transpiration rate(Tr)induced by Cd stress.2、The subcellular distribution of Cd in root cells was further investigated.With mannose the accumulation of Cd in the root wall increased by 124.6%,and the content of Cd in soluble fraction,plastids,mitochondria and nuclei decreased by 9.1%,5.3%,10.4%and 9.4%,respectively,and the proportion of Cd in cell walls increased by 15.5%。3、The increase of hemicellulose content in root cell wall increased Cd accumulation by 78.4%,which resulted in the increase of Cd content in cell wall.4、Chemical forms analysis showed that the content of CdE and CdW decreased by 13.3%and 6.4%respectively,the content of CdHAc and CdHCl increased by 51.3%and 54.3%respectively,and there was no significant change in the content of CdNaCl and CdR.In conclusion,the application of barrier agent is an important measure to reduce the accumulation of Cd in wheat.Separate foliar spraying of mannose,MgSO4、MnSO4 and compound spraying of mannose+MgSO4 and mannose+MnSO4 all can effectively reduce the content of Cd in wheat grain.Foliar spraying with mannose increased the Cd accumulation in the root cell wall of wheat,and reduced the content of ethanol-Cd and water-Cd which have strong migration activity,thus the transport of Cd to overground was inhibited and the Cd accumulation in shoot of wheat was reduced.